1.
Bulge (astronomy)
–
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the group of stars found in most spiral galaxies. It is now thought there are at least two types of bulges, bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have similar to those of elliptical galaxies are often called classical bulges due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars and these stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation, the distribution of light is described by de Vaucouleurs law. Classical bulges are thought to be the result of collisions of smaller structures and these disrupt the paths of stars, resulting in the randomness of bulge orbits. Following such a merger, gas clouds are more likely to be converted into stars, many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. They are often referred to as pseudobulges or disky-bulges and these bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies, subsequent studies show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a galaxy, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges, where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies, sometimes bulges contain nuclear rings that are forming stars at much higher rate than is typically found in outer disks, as shown in NGC4314. Properties such as structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies. Yet the theories for the formation of pseudobulges are less certain than those for classical bulges, pseudobulges may be the result of extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges. However, it is difficult for disks to survive the merging process, many astronomers suggest that bulges that appear similar to disks form outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas, the products of this process are often observed in such galaxies, both spiral disks and galactic bars can result from secular evolution of galaxy disks. Secular evolution is expected to send gas and stars to the center of a galaxy
Bulge (astronomy)
–
Artist's impression of the central bulge of the
Milky Way.
Bulge (astronomy)
–
An image of
Messier 81, a galaxy with a classical bulge. Notice that the spiral structure ends at the onset of the bulge.
Bulge (astronomy)
–
Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges.
Bulge (astronomy)
–
A Hubble Space Telescope image of the central region of
NGC 4314, a galaxy with a star-forming nuclear ring.
2.
Central massive object
–
Central massive object refers to a mass concentration at the center of a galaxy. It can be either a black hole or a nuclear star cluster. The most massive galaxies are thought to contain a supermassive black hole, these galaxies do not contain nuclear star clusters. Fainter galaxies usually contain a star cluster. In most of these galaxies it is not known whether a black hole is present. A few galaxies, for instance the Milky Way and NGC4395, are known to both a SBH and a NSC. The mass associated with CMOs is roughly 0. 1–0. 3% times the mass of the galactic bulge
Central massive object
–
The star-like nucleus at the center of this edge-on spiral galaxy,
NGC 4244, is a central massive object.
3.
VISTA (telescope)
–
The VISTA is a wide-field reflecting telescope with a 4.1 metre mirror, located at the Paranal Observatory in Chile. It is operated by the European Southern Observatory and started operations in December 2009. VISTA is a survey telescope working at infrared wavelengths, and is by far the largest telescope in the dedicated to surveying the sky at near-infrared wavelengths. The telescope has only one instrument, VIRCAM, the Vista InfraRed CAMera and this is a 3-tonne camera containing 16 special detectors sensitive to infrared light, with a combined total of 67 million pixels. A second-generation instrument is under development for installation around 2020, VISTA is carrying out surveys of the southern sky at near infrared wavelengths. Such surveys should both return direct scientific results and help select objects for further studies with larger telescopes, the project is valued at €46M. After considering several sites in Chile, the consortium chose the Paranal Observatory of the European Southern Observatory, the consortium selected the UK Astronomy Technology Centre to take technical responsibility for design and construction of the telescope. Two years later – in 2002 – the UK joined ESO and these will take up the majority of the observing time in the telescope’s first five years of operations. The surveys cover different areas of sky to different depths to attack a range of scientific questions. More information about each of the VISTA surveys can be found on the ESO — VISTA Surveys website, because VISTA is a large telescope that also has a large field of view it can both detect faint sources and also cover wide areas of sky quickly. VISTA observations will support research in many astronomical areas, within our galaxy, VISTA is expected to find many new brown dwarf stars and be able to test ideas about the nature of dark matter. One VISTA survey is designed to find and study huge numbers of stars in our galaxy by taking images of the same areas of sky at different times. Using VISTA data astronomers will be able to map the structure of our galaxy in much more detail than ever before, another VISTA survey will study our neighbouring small galaxies, the Magellanic Clouds, and their surroundings. VISTA data will also be used to create a 3D map of about 5% of the entire observable Universe, further out, VISTA will be a powerful tool for discovering remote quasars and studying the evolution of galaxies and clusters of galaxies. It will help to probe the nature of energy by finding very distant galaxy clusters. Infrared measurements from the VVV astronomical survey have been employed to bolster the cosmic distance ladder, namely by providing reliable distances to star clusters and Cepheid variable stars. The first released image shows the Flame Nebula, a spectacular star-forming cloud of gas and dust in the constellation of Orion. The wide field of view of the VISTA camera also captures the glow of NGC2023, other stunning nebula images include the VISTA views of the Orion Nebula and the Lagoon Nebula
VISTA (telescope)
–
VISTA (Credit:
ESO)
VISTA (telescope)
–
Early morning shot shows VISTA in front of the
Paranal summit.
VISTA (telescope)
–
VISTA at night (Credit:
ESO).
VISTA (telescope)
–
VISTA exposes high-speed antics of young stars in the Orion Nebula
4.
Milky Way
–
The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
Milky Way
–
The Milky Way's
Galactic Center in the night sky above
Paranal Observatory (the laser creates a
guide-star for the telescope).
Milky Way
–
A view of the Milky Way toward the constellation
Sagittarius (including the
Galactic Center) as seen from an area not polluted by light (the
Black Rock Desert, Nevada)
Milky Way
–
The Milky Way arching at a high inclination across the night sky (
fish-eye mosaic shot at
Paranal, Chile).
Milky Way
–
A photograph of galaxy
NGC 6744, which might resemble the Milky Way.
5.
Parsec
–
The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond
Parsec
–
The jet erupting from the
active galactic nucleus of
M87 is thought to be 7000150000000000000♠ 1.5 kiloparsecs (7019462629720109201♠ 4890
ly) long. (image from Hubble Space Telescope)
Parsec
–
A parsec is the distance from the
Sun to an
astronomical object that has a
parallax angle of one
arcsecond (the diagram is not to scale).
6.
Constellation
–
A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries. Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It also denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars also travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived. The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC
Constellation
Constellation
Constellation
–
Babylonian tablet recording
Halley's comet in 164 BC.
Constellation
–
Chinese star map with a cylindrical projection (
Su Song)
7.
Sagittarius (constellation)
–
Sagittarius is one of the constellations of the zodiac. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and its name is Latin for the archer, and its symbol is, a stylized arrow. Sagittarius is commonly represented as a centaur pulling-back a bow and it lies between Scorpius and Ophiuchus to the west and Capricornus to the east. The center of the Milky Way lies in the westernmost part of Sagittarius, as seen from the northern hemisphere, the constellations brighter stars form an easily recognizable asterism known as the Teapot. The stars δ Sgr, ε Sgr, ζ Sgr, and φ Sgr form the body of the pot, λ Sgr is the point of the lid, γ2 Sgr is the tip of the spout and these same stars originally formed the bow and arrow of Sagittarius. Marking the bottom of the teapots handle (or the shoulder area of the archer, are the bright star Zeta Sagittarii, named Ascella, and the fainter Tau Sagittarii. The constellation as a whole is often depicted as having the appearance of a stick-figure archer drawing its bow. Sagittarius famously points its arrow at the heart of Scorpius, represented by the reddish star Antares, following the direct line formed by Delta Sagittarii and Gamma2 Sagittarii leads nearly directly to Antares. Fittingly, Sagittarii is Alnasl, the Arabic word for arrowhead, and Delta Sagittarii is called Kaus Media, Kaus Media bisects Lambda Sagittarii and Epsilon Sagittarii, whose names Kaus Borealis and Kaus Australis refer to the northern and southern portions of the bow, respectively. α Sgr despite having the alpha appellation, is not the brightest star of the constellation, instead, the brightest star is Epsilon Sagittarii, at magnitude 1.85. Sigma Sagittarii is the constellations second-brightest star at magnitude 2.08, Nunki is a B2V star approximately 260 light years away. Nunki is a Babylonian name of origin, but thought to represent the sacred Babylonian city of Eridu on the Euphrates. Zeta Sagittarii, with apparent magnitude 2.61 of A2 spectra, is actually a star whose two components have magnitudes 3.3 and 3.5. Delta Sagittarii, is a K2 spectra star with magnitude 2.71 and only 85 light years from Earth. Eta Sagittarii is a star with component magnitudes of 3.18 and 10, while Pi Sagittarii is actually a triple system whose components have magnitudes 3.7,3.8. The Bayer designation Beta Sagittarii is shared by two systems, β¹ Sagittarii, with apparent magnitude 3.96, and β² Sagittarii. The two stars are separated by 0. 36° in the sky and are 378 light years from earth, Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name Arkab, meaning achilles tendon. Nova Sagittarii 2015 No.2 was discovered on March 15,2015, by John Seach of Chatsworth Island, NSW and it lies near the center of the constellation
Sagittarius (constellation)
–
The "Teapot" asterism is in Sagittarius. The Milky Way is the "steam" coming from the spout.
Sagittarius (constellation)
–
List of stars in Sagittarius
Sagittarius (constellation)
–
The constellation Sagittarius as it can be seen with the naked eye.
Sagittarius (constellation)
–
The Omega Nebula, also known as the Horseshoe or Swan Nebula.
8.
Ophiuchus
–
Ophiuchus /ɒfiˈjuːkəs/ is a large constellation located around the celestial equator. Its name is from the Greek Ὀφιοῦχος Ophioukhos, serpent-bearer, Ophiuchus was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. It was formerly referred to as Serpentarius /sɜːrpənˈtɛəriəs/ and Anguitenens, Ophiuchus is located between Aquila, Serpens and Hercules, northwest of the center of the Milky Way. The southern part lies between Scorpius to the west and Sagittarius to the east, in the northern hemisphere, it is best visible in summer. It is located opposite Orion in the sky, Ophiuchus straddles the equator but lies predominately to its south. However, Rasalhague, a conspicuous star in its north, is circumpolar north of 78° north latitude. The constellation extends southward to −30° declination, segments of the ecliptic that lie within Ophiuchus lie south of −20° declination. A determination of exactly where these stars are visible on Earth would depend on atmospheric refraction, in contrast to Orion, it is in the period November–January when Ophiuchus is in the daytime sky and thus not visible at most latitudes. However, for much of the Arctic Circle in the Northern Hemispheres winter months, stars are then visible at twilight for a few hours around local noon, low in the South. In countries close to the equator Ophiuchus appears overhead in June around midnight, the brightest stars in Ophiuchus include α Ophiuchi, called Rasalhague, at magnitude 2.07, and η Ophiuchi, known as Sabik, at magnitude 2.43. Other bright stars in the constellation include β Ophiuchi, Cebalrai and λ Ophiuchi, RS Ophiuchi is part of a class called recurrent novae, whose brightness increase at irregular intervals by hundreds of times in a period of just a few days. It is thought to be at the brink of becoming a type-1a supernova, barnards Star, one of the nearest stars to the Solar System, lies in Ophiuchus. It is located to the left of β and just north of the V-shaped group of stars in an area that was occupied by the now-obsolete constellation of Taurus Poniatovii. In 2005, astronomers using data from the Green Bank Telescope discovered a superbubble so large that it extends beyond the plane of the galaxy and it is called the Ophiuchus Superbubble. In April 2007, astronomers announced that the Swedish-built Odin satellite had made the first detection of clouds of molecular oxygen in space, the supernova of 1604 was first observed on 9 October 1604, near θ Ophiuchi. Johannes Kepler saw it first on 16 October and studied it so extensively that the supernova was subsequently called Keplers Supernova and he published his findings in a book titled De stella nova in pede Serpentarii. Galileo used its brief appearance to counter the Aristotelian dogma that the heavens are changeless. In 2009 it was announced that GJ1214, a star in Ophiuchus, undergoes repeated, the planets low density suggests that the planet may have a substantial component of low-density gas—possibly hydrogen or steam
Ophiuchus
–
Rho Ophiuchi, a multiple star system in the constellation, can be seen in the right corner.
Ophiuchus
–
List of stars in Ophiuchus
Ophiuchus
–
The constellation Ophiuchus as it can be seen by naked eye.
Ophiuchus
–
Infrared light view of
Rho Ophiuchi molecular cloud complex from
NASA.
9.
Scorpius
–
Scorpius is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is, Scorpius is one of the 48 constellations identified by the Greek astronomer Ptolemy in the second century. It is an ancient constellation that pre-dated the Greeks and it lies between Libra to the west and Sagittarius to the east. It is a constellation located in the southern hemisphere near the center of the Milky Way. Scorpius contains many stars, including Antares, rival of Mars, so named because of its distinct reddish hue, β1 Sco, a triple star, δ Sco, θ Sco, ν Sco, ξ Sco, π Sco, σ Sco. Marking the tip of the scorpions curved tail are λ Sco and υ Sco, given their proximity to one another, λ Sco and υ Sco are sometimes referred to as the Cats Eyes. The constellations bright stars form a pattern like a longshoremans hook, most of them are massive members of the nearest OB association, Scorpius-Centaurus. The star δ Sco, after having been a stable 2.3 magnitude star and it has since become a variable star fluctuating between 2.0 and 1.6. This means that at its brightest it is the second brightest star in Scorpius, U Scorpii is the fastest known nova with a period of about 10 years. ω¹ Scorpii and ω² Scorpii are a double, which can be resolved by the unaided eye. They have contrasting blue and yellow colours, the star once designated γ Sco is today known as σ Lib. The division into Libra was formalised during Roman times, Messier 80 is a globular cluster of magnitude 7.3,33,000 light-years from Earth. It is a compact Shapley class II cluster, the classification indicates that it is highly concentrated, m80 was discovered in 1781 by Charles Messier. It was the site of a discovery in 1860 when Arthur von Auwers discovered the nova T Scorpii. NGC6302, also called the Bug Nebula, is a planetary nebula. NGC6334, also known as the Cats Paw Nebula, is an emission nebula, in Greek mythology, the myths associated with Scorpio almost invariably also contain a reference to Orion. According to one of these myths it is written that Orion boasted to goddess Artemis and her mother, Leto, although Artemis was known to be a hunter herself she offered protection to all creatures. Artemis and her mother Leto sent a scorpion to deal with Orion, the pair battled and the scorpion killed Orion
Scorpius
–
The constellation Scorpius as it can be seen by naked eye (with constellation lines drawn in).
Scorpius
–
List of stars in Scorpius
Scorpius
–
Scorpius as depicted in
Urania's Mirror, a set of constellation cards published in London c.1825.
10.
Supermassive black hole
–
In the case of the Milky Way, the SMBH corresponds with the location of Sagittarius A*. Supermassive black holes have properties that distinguish them from lower-mass classifications, first, the average density of a supermassive black hole can be less than the density of water in the case of some supermassive black holes. This is because the Schwarzschild radius is proportional to mass. In addition, the forces in the vicinity of the event horizon are significantly weaker for massive black holes. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole, donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole. Sagittarius A* was discovered and named on February 13 and 15,1974, by astronomers Bruce Balick and they discovered a radio source that emits synchrotron radiation, it was found to be dense and immobile because of its gravitation. This was, therefore, the first indication that a black hole exists in the center of the Milky Way. The origin of black holes remains an open field of research. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter, there are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or seeds, of supermassive black holes. The most obvious hypothesis is that the seeds are black holes of tens or perhaps hundreds of masses that are left behind by the explosions of massive stars. Yet another model involves a dense stellar cluster undergoing core-collapse as the heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first moments after the Big Bang, formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical, the difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen, normally, the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth. This is a component of the theory of accretion disks. Gas accretion is the most efficient and also the most conspicuous way in black holes grow. The majority of the growth of supermassive black holes is thought to occur through episodes of rapid gas accretion. Observations reveal that quasars were more frequent when the Universe was younger, indicating that supermassive black holes formed
Supermassive black hole
–
Artist concept of a SMBH consuming matter from a nearby star.
Supermassive black hole
–
An artist's conception of a supermassive black hole and accretion disk
Supermassive black hole
–
Artist's impression of the huge outflow ejected from the quasar
SDSS J1106+1939
Supermassive black hole
–
Artist's illustration of galaxy with jets from a supermassive black hole.
11.
Interstellar cloud
–
An interstellar cloud is the generic name given to an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, a cloud is a denser-than-average region of the interstellar medium. Depending on the density, size, and temperature of a given cloud, neutral and ionized clouds are sometimes also called diffuse clouds, while molecular clouds are sometimes also referred to as dense clouds. Analyzing the composition of clouds is achieved by studying electromagnetic radiation that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of radiation which are characteristic of certain molecules spectra. Some interstellar clouds are cold and tend to give out EM radiation of large wavelengths, a map of the abundance of these molecules can be made, enabling an understanding of the varying composition of the clouds. In hot clouds, there are ions of many elements, whose spectra can be seen in visible. Radio telescopes can also scan over the frequencies from one point in the map, peaks of frequencies mean that an abundance of that molecule or atom is present in the cloud. The height of the peak is proportional to the percentage that it makes up. Until recently the rates of reactions in interstellar clouds were expected to be slow, with minimal products being produced due to the low temperature. However, organic molecules were observed in the spectra that scientists would not have expected to find under these conditions, such as formaldehyde, methanol, the reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. These reactions are studied in the CRESU experiment, interstellar clouds also provide a medium to study the presence and proportions of metals in space. These interstellar clouds possess a velocity higher than can be explained by the rotation of the Milky Way, by definition, these clouds must have a vlsr greater than 90 km s−1, where vlsr is the local standard rest velocity. They are detected primarily in the 21 cm line of neutral hydrogen, an example of the latter is the Magellanic Stream. To narrow down the origin of clouds, a better understanding of their distances. High-velocity clouds are identified with an HVC prefix, as with HVC 127-41-330
Interstellar cloud
–
Small part of the
emission nebula NGC 6357. It glows with the characteristic red of an
H II region.
Interstellar cloud
–
Reflection nebula IRAS 10082-5647 taken by
Hubble Space Telescope.
12.
Visible spectrum
–
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm, in terms of frequency, this corresponds to a band in the vicinity of 430–770 THz. The spectrum does not, however, contain all the colors that the human eyes, unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass through the window, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earths atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, the optical window is also referred to as the visible window because it overlaps the human visible response spectrum. The near infrared window lies just out of the vision, as well as the Medium Wavelength IR window. In the 13th century, Roger Bacon theorized that rainbows were produced by a process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light and he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. The result is red light is bent less sharply than violet as it passes through the prism. Newton divided the spectrum into seven named colors, red, orange, yellow, green, blue, indigo, the human eye is relatively insensitive to indigos frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right. However, the evidence indicates that what Newton meant by indigo, comparing Newtons observation of prismatic colors to a color image of the visible light spectrum shows that indigo corresponds to what is today called blue, whereas blue corresponds to cyan. In the 18th century, Goethe wrote about optical spectra in his Theory of Colours, Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow, the spectrum appears only when these edges are close enough to overlap. Young was the first to measure the wavelengths of different colors of light, the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century
Visible spectrum
–
White
light is
dispersed by a
prism into the colors of the visible spectrum.
Visible spectrum
–
Newton's observation of prismatic colors (
David Brewster 1855)
13.
Ultraviolet
–
Ultraviolet is an electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation constitutes about 10% of the light output of the Sun. It is also produced by electric arcs and specialized lights, such as lamps, tanning lamps. Consequently, the effects of UV are greater than simple heating effects. Suntan, freckling and sunburn are familiar effects of over-exposure, along with risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earths atmosphere. More-energetic, shorter-wavelength extreme UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground, Ultraviolet is also responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans. The UV spectrum thus has both beneficial and harmful to human health. Ultraviolet rays are invisible to most humans, the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, near-UV radiation is visible to some insects, mammals, and birds. Small birds have a fourth color receptor for ultraviolet rays, this gives birds true UV vision, reindeer use near-UV radiation to see polar bears, who are poorly visible in regular light because they blend in with the snow. UV also allows mammals to see urine trails, which is helpful for animals to find food in the wild. The males and females of some species look identical to the human eye. Ultraviolet means beyond violet, violet being the color of the highest frequencies of visible light, Ultraviolet has a higher frequency than violet light. He called them oxidizing rays to emphasize chemical reactivity and to them from heat rays. The terms chemical and heat rays were eventually dropped in favour of ultraviolet and infrared radiation, in 1878 the effect of short-wavelength light on sterilizing bacteria was discovered. By 1903 it was known the most effective wavelengths were around 250 nm, in 1960, the effect of ultraviolet radiation on DNA was established. The discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet because it is absorbed by air, was made in 1893 by the German physicist Victor Schumann
Ultraviolet
–
(left) Portable ultraviolet lamp. (right) UV light is also produced by
electric arcs.
Arc welders must wear
eye protection to prevent
welder's flash.
Ultraviolet
Ultraviolet
–
Two black light fluorescent tubes, showing use. The longer tube is a F15T8/BLB 18 inch, 15 watt tube, shown in the bottom image in a standard plug-in fluorescent fixture. The shorter is an F8T5/BLB 12 inch, 8 watt tube, used in a portable battery-powered black light sold as a pet urine detector.
Ultraviolet
14.
X-ray
–
X-radiation is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz, X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. Spelling of X-ray in the English language includes the variants x-ray, xray, X-rays with high photon energies are called hard X-rays, while those with lower energy are called soft X-rays. Due to their ability, hard X-rays are widely used to image the inside of objects, e. g. in medical radiography. The term X-ray is metonymically used to refer to an image produced using this method. Since the wavelengths of hard X-rays are similar to the size of atoms they are useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air, the length of 600 eV X-rays in water is less than 1 micrometer. There is no consensus for a definition distinguishing between X-rays and gamma rays, one common practice is to distinguish between the two types of radiation based on their source, X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. This definition has problems, other processes also can generate these high-energy photons. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m and this criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. Occasionally, one term or the other is used in specific contexts due to precedent, based on measurement technique. Thus, gamma-rays generated for medical and industrial uses, for radiotherapy, in the ranges of 6–20 MeV. X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds and this makes it a type of ionizing radiation, and therefore harmful to living tissue. A very high radiation dose over a period of time causes radiation sickness. In medical imaging this increased risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be utilized in treatment to kill malignant cells using radiation therapy. It is also used for material characterization using X-ray spectroscopy, hard X-rays can traverse relatively thick objects without being much absorbed or scattered. For this reason, X-rays are widely used to image the inside of visually opaque objects, the most often seen applications are in medical radiography and airport security scanners, but similar techniques are also important in industry and research
X-ray
–
Spectrum of the X-rays emitted by an X-ray tube with a
rhodium target, operated at 60
kV. The smooth, continuous curve is due to
bremsstrahlung, and the spikes are
characteristic K lines for rhodium atoms.
X-ray
–
X-rays are part of the
electromagnetic spectrum, with wavelengths shorter than
visible light. Different applications use different parts of the X-ray spectrum.
X-ray
–
A
chest radiograph of a female, demonstrating a
hiatus hernia
X-ray
–
An arm radiograph, demonstrating broken
ulna and
radius with implanted
internal fixation.
15.
Wavelength
–
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
Wavelength
–
Wavelength is decreased in a medium with slower propagation.
Wavelength
–
Wavelength of a
sine wave, λ, can be measured between any two points with the same
phase, such as between crests, or troughs, or corresponding
zero crossings as shown.
Wavelength
–
Various local wavelengths on a crest-to-crest basis in an ocean wave approaching shore
Wavelength
–
A wave on a line of atoms can be interpreted according to a variety of wavelengths.
16.
Gamma ray
–
Gamma ray, denoted by the lower-case Greek letter gamma, is penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy, paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays, Rutherford had previously discovered two other types of radioactive decay, which he named alpha and beta rays. Gamma rays are able to ionize atoms, and are thus biologically hazardous. The decay of a nucleus from a high energy state to a lower energy state. Natural sources of gamma rays on Earth are observed in the decay of radionuclides. There are rare terrestrial natural sources, such as lightning strikes and terrestrial gamma-ray flashes, However, a large fraction of such astronomical gamma rays are screened by Earths atmosphere and can only be detected by spacecraft. Gamma rays typically have frequencies above 10 exahertz, and therefore have energies above 100 keV and wavelengths less than 10 picometers, However, this is not a strict definition, but rather only a rule-of-thumb description for natural processes. Electromagnetic radiation from radioactive decay of nuclei is referred to as gamma rays no matter its energy. This radiation commonly has energy of a few hundred keV, in astronomy, gamma rays are defined by their energy, and no production process needs to be specified. The energies of gamma rays from astronomical sources range to over 10 TeV, a notable example is the extremely powerful bursts of high-energy radiation referred to as long duration gamma-ray bursts, of energies higher than can be produced by radioactive decay. These bursts of gamma rays are thought to be due to the collapse of stars called hypernovae, the first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, a nucleus emits a gamma ray almost immediately upon formation. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, However, Villard did not consider naming them as a different fundamental type. Rutherford also noted that gamma rays were not deflected by a field, another property making them unlike alpha. Gamma rays were first thought to be particles with mass, like alpha, Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, Rutherford and his coworker Edward Andrade measured the wavelengths of gamma rays from radium, and found that they were similar to X-rays, but with shorter wavelengths and higher frequency. This was eventually recognized as giving them more energy per photon
Gamma ray
–
Gamma rays are emitted during
nuclear fission in nuclear explosions.
Gamma ray
–
Illustration of an emission of a gamma ray (γ) from an atomic nucleus
Gamma ray
Gamma ray
–
A
hypernova. Artist's illustration showing the life of a
massive star as
nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a
black hole. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration
gamma-ray burst.
17.
Infrared
–
It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
Infrared
–
This infrared space telescope image has (false color) blue, green and red corresponding to 3.4, 4.6, and 12
µm wavelengths, respectively.
Infrared
–
A
false color image of two people taken in long-wavelength infrared (body-temperature thermal) light.
Infrared
–
Materials with higher
emissivity appear to be hotter. In this thermal image, the ceramic cylinder appears to be hotter than its cubic container (made of silicon carbide), while in fact they have the same temperature.
Infrared
–
Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to the
human eye. Despite a dark back-lit scene, active-infrared night vision delivers identifying details, as seen on the display monitor.
18.
Radio wave
–
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 GHz to as low as 3 kHz, though some definitions describe waves above 1 or 3 GHz as microwaves, at 300 GHz, the corresponding wavelength is 1 mm, and at 3 kHz is 100 km. Like all other electromagnetic waves, they travel at the speed of light, naturally occurring radio waves are generated by lightning, or by astronomical objects. Radio waves are generated by radio transmitters and received by radio receivers, the radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses. Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist James Clerk Maxwell, Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. Radio waves were first used for communication in the mid 1890s by Guglielmo Marconi, different frequencies experience different combinations of these phenomena in the Earths atmosphere, making certain radio bands more useful for specific purposes than others. It does not necessarily require a cleared sight path, at lower frequencies radio waves can pass through buildings, foliage and this is the only method of propagation possible at microwave frequencies and above. On the surface of the Earth, line of propagation is limited by the visual horizon to about 40 miles. This is the used by cell phones, cordless phones, walkie-talkies, wireless networks, FM and television broadcasting. Indirect propagation, Radio waves can reach points beyond the line-of-sight by diffraction, diffraction allows a radio wave to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. Radio waves also reflect from surfaces such as walls, floors, ceilings, vehicles and these effects are used in short range radio communication systems. Ground waves allow mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, the nonzero resistance of the earth absorbs energy from ground waves, so as they propagate the waves lose power and the wavefronts bend over at an angle to the surface. As frequency decreases, the decrease and the achievable range increases. Military very low frequency and extremely low frequency communication systems can communicate over most of the Earth, and with submarines hundreds of feet underwater. Tropospheric propagation, In the VHF and UHF bands, radio waves can travel somewhat beyond the horizon due to refraction in the troposphere. This is due to changes in the index of air with temperature and pressure. At times, radio waves can travel up to 500 -1000 km due to tropospheric ducting and these effects are variable and not as reliable as ionospheric propagation, below. So radio waves directed at an angle into the sky can return to Earth beyond the horizon, by using multiple skips communication at intercontinental distances can be achieved
Radio wave
–
Diagram of the
electric fields (E) and
magnetic fields (H) of radio waves emitted by a
monopole radio transmitting
antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right.
19.
Harlow Shapley
–
Harlow Shapley was an American astronomer. He used RR Lyrae stars to correctly estimate the size of the Milky Way Galaxy, in 1953 he proposed his liquid water belt theory, now known as the concept of a habitable zone. Shapley was born on a farm in Nashville, Missouri, to Willis and Sarah Shapley, after studying at home and covering crime stories as a newspaper reporter, Shapley returned to complete a six-year high school program in only two years, graduating as class valedictorian. In 1907, Shapley went to journalism at the University of Missouri. When he learned that the opening of the School of Journalism had been postponed for a year, rejecting Archaeology, which Harlow later explained he couldnt pronounce, he chose the next subject, Astronomy. He was instrumental in moving away from the idea that Cepheids were spectroscopic binaries. He realized the Milky Way Galaxy was far larger than previously believed, and this discovery by Shapley is a part of the Copernican principle, according to which the Earth is not at the center of our Solar System, our galaxy, or our Universe. Shapley participated in the Great Debate with Heber D. Curtis on the nature of nebulae and galaxies, the debate took place on April 26,1920, in the hall of the United States National Academy of Sciences in Washington D. C. This issue and debate are the start of extragalactic astronomy, while the detailed arguments and data, often with ambiguities, however, Shapleys actual talk and argument given during the Great Debate was completely different from the published paper. Historian Michael Hoskin says His decision was to treat the National Academy of Sciences to an address so elementary that much of it was necessarily uncontroversial. With Shapleys motivation being only to impress a delegation from Harvard who were interviewing him for an offer as the next Director of Harvard College Observatory. With the default by Shapley, Curtis won the debate, the astronomical issues were soon resolved in favor of Curtis position when Edwin Hubble discovered Cepheid variable stars in the Andromeda Galaxy. At the time of the debate, Shapley was working at the Mount Wilson Observatory, after the debate, however, he was hired to replace the recently deceased Edward Charles Pickering as director of the Harvard College Observatory. He is also known to have incorrectly opposed Edwin Hubbles observations that there are additional galaxies in the other than the Milky Way. Shapley fiercely critiqued Hubble and regarded his work as junk science, however, after he received a letter from Hubble showing Hubbles observed light curve of V1, he withdrew his criticism. He reportedly told a colleague, Here is the letter that destroyed my universe and he also encouraged Hubble to write a paper for a joint meeting of the American Astronomical Society and American Association for the Advancement of Science. Hubbles findings went on to reshape fundamentally the scientific view of the universe and he served as director of the HCO from 1921 to 1952. During this time, he hired Cecilia Payne, who, in 1925, became the first person to earn a doctorate at Radcliffe College in the field of astronomy and he wrote many books on astronomy and the sciences
Harlow Shapley
–
Harlow Shapley
Harlow Shapley
–
Shapley (first standing from the right) at a
Science Service board meeting in 1941.
20.
Globular cluster
–
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes, the name of this category of star cluster is derived from the Latin globulus—a small sphere. A globular cluster is known more simply as a globular. Globular clusters are found in the halo of a galaxy and contain considerably more stars and are older than the less dense open clusters. Globular clusters are common, there are about 150 to 158 currently known globular clusters in the Milky Way. These globular clusters orbit the Galaxy at radii of 40 kiloparsecs or more, larger galaxies can have more, Andromeda Galaxy, for instance, may have as many as 500. Some giant elliptical galaxies such as M87, have as many as 13,000 globular clusters, every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters. The Sagittarius Dwarf galaxy and the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way and this demonstrates how many of this galaxys globular clusters might have been acquired in the past. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins, the first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a cluster were not resolved until Charles Messier observed M4 in 1764. The first eight globular clusters discovered are shown in the table, subsequently, Abbé Lacaille would list NGC104, NGC4833, M55, M69, and NGC6397 in his 1751–52 catalogue. The M before a number refers to Charles Messiers catalogue, while NGC is from the New General Catalogue by John Dreyer, when William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters. Herschel discovered another 36 himself and was the first to virtually all of them into stars. He coined the term globular cluster in his Catalogue of a Second Thousand New Nebulae, the number of globular clusters discovered continued to increase, reaching 83 in 1915,93 in 1930 and 97 by 1947. A total of 152 globular clusters have now discovered in the Milky Way galaxy. These additional, undiscovered globular clusters are believed to be hidden behind the gas, beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers. He examined the RR Lyrae variables in the clusters and would use their period–luminosity relationship for distance estimates, later, it was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distance to the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the core
Globular cluster
–
The
Messier 80 globular cluster in the constellation
Scorpius is located about 30,000
light-years from the Sun and contains hundreds of thousands of stars.
Globular cluster
–
NGC 7006 is a highly concentrated, Class I globular cluster.
Globular cluster
–
Globular star cluster
Messier 54.
Globular cluster
–
Djorgovski 1 's stars contain hydrogen and helium, but not much else. In astronomical terms, they are described as "metal-poor".
21.
Mount Wilson Observatory
–
The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 1, 740-metre peak in the San Gabriel Mountains near Pasadena, the observatory was conceived and founded by George Ellery Hale, who had previously built the 1 meter telescope at the Yerkes Observatory, then the worlds largest telescope. The Mount Wilson Solar Observatory was first funded by the Carnegie Institution of Washington in 1904, among the conditions of the lease was that it allow public access. There are three solar telescopes at Mount Wilson Observatory, today, just one of these telescopes, the 60 foot Solar Tower, is still used for solar research. The Snow Solar Telescope was the first telescope installed at the fledgling Mount Wilson Solar Observatory and it was the worlds first permanently mounted solar telescope. Solar telescopes had previously been portable so they could be taken to solar eclipses around the world, the telescope was donated to Yerkes Observatory by Helen Snow of Chicago. George Ellery Hale, then director of Yerkes, had the telescope brought to Mount Wilson to put it into service as a scientific instrument. Stellar research soon followed as the brightest stars could have their spectra recorded with very long exposures on glass plates, today the Snow solar telescope is mostly used by undergraduate students who get hands on training in solar physics and spectroscopy. It was also used publicly for the May 9,2016 transit of Mercury across the face of the sun, the 60-foot Solar Tower soon built on the work started at the Snow telescope. At its completion in 1908, the tower design of the 60 foot focal length solar telescope allowed much higher resolution of the solar image. The higher resolution came from situating the optics higher above the ground, on June 25,1908, Hale would record Zeeman splitting in the spectrum of a sunspot, showing for the first time that magnetic fields existed somewhere besides the earth. A later discovery was of the polarity in sunspots of the new solar cycle of 1912. The success of the 60 foot Tower prompted Hale to pursue yet another, taller tower telescope, in the 1960s, Robert Leighton discovered the sun had a 5-minute oscillation and the field of heliosiesmology was born. The 60 foot Tower is operated by the Department of Physics, the 150-foot focal length solar tower expanded on the solar tower design with its tower-in-a-tower design. An inner tower supports the optics above, while an outer tower and this design allowed complete isolation of the optics from the effect of wind swaying the tower. Two mirrors feed sunlight to a 12-inch lens which focuses light down at the ground floor and it was first completed in 1910, but unsatisfactory optics caused a two-year delay before a suitable doublet lens was installed. Research included solar rotation, sunspot polarities, daily sunspot drawings, the solar telescope would be the worlds largest for 50 years until the McMath-Pierce Solar telescope was completed at Kitt Peak in Arizona in 1962. In 1985, UCLA took over operation of the tower from the Carnegie Observatories after it was decided to stop funding the observatory
Mount Wilson Observatory
–
Looking down on the top of Mount Wilson, including the historic 100"
Hooker telescope (center), the 60" telescope (center left), and the CHARA array
Mount Wilson Observatory
–
At the base of the "150-foot" Solar Tower.
Mount Wilson Observatory
–
Top of the Solar tower containing the mirrors
Mount Wilson Observatory
–
The 60-inch (1.5 m) Hale telescope at Mt. Wilson
22.
Alnasl
–
Gamma² Sagittarii, also named Alnasl, is a 3rd-magnitude star in the zodiac constellation of Sagittarius. The location of this forms the tip of the arrow in the bow of Sagittarius the Centaur. This star is close enough to the Earth that its distance can be determined using parallax and it has an apparent visual magnitude of +2.98, making it the seventh-brightest star in the constellation. It forms part of a star along with a fainter optical companion designated Gamma¹ Sagittarii located about 50 arcminutes north of this star. The latter is a magnitude 4.7 Cepheid variable star also has the variable star designation W Sagittarii. γ² Sagittarii is the stars Bayer designation and it bore the traditional names Alnasl, Nushaba and Awal al Warida. Alnasl is derived from the Arabic النصل al-naşl and Nushaba is derived from the Arabic Zujj al-Nashshaba and this ostrich was thought of as going down to the river to drink, and another ostrich was thought of as coming back up. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue, the WGSN approved the name Alnasl for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names. Gamma² Sagittarii, together with Delta Sagittarii, Epsilon Sagittarii, Zeta Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii, together with Delta Sagittarii and Epsilon Sagittarii, Gamma Sagittarii formed the Akkadian Sin-nun‑tu, or Si-nu-nu‑tum the Swallow. In Chinese, 箕, meaning Winnowing Basket, refers to an asterism consisting of Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii and Eta Sagittarii. Consequently, Gamma Sagittarii itself is known as 箕宿一 A stellar classification of K1 III reveals that this is a giant star and this means it has exhausted the hydrogen in its core and evolved away from the main sequence. The abundance of other than hydrogen and helium in this star. Gamma² Sagittarii has a temperature of 4,760 K. It is this temperature that gives Gamma² Sagittarii the orange hue that is a characteristic of K-type stars
Alnasl
–
Location of γ Sagittarii (circled)
23.
Dover Heights
–
Dover Heights is a coastal, eastern suburb of Sydney, in the state of New South Wales, Australia. Dover Heights is 9 kilometres east of the Sydney central business district, Dover Heights borders Vaucluse to its north, North Bondi to its south and Rose Bay to its west and has the Pacific Ocean to its east. Dover Heights is a residential suburb. Many of the properties have views of Sydney Harbour and the Sydney Harbour Bridge. Its Dudley Page Reserve is a site for tourist buses. Some properties have both harbour and ocean views, the Holman House, Dover Heights, was presented the Wilkinson Award by the Royal Australian Institute of Architects in 2005. The suburb is considered to be the most affluent within the Waverly Local Government Area and this is reflected in property prices - like nearby suburbs Vaucluse and Bellevue Hill, median house prices are above A$2 million. Dover Heights is believed to have named for its cliffs along the Pacific Ocean which resembled those found at Dover, in Kent. The first mention of Dover Heights appeared in records in 1886. The area was first used for market gardens, in 1830 land in the area was owned by Daniel Cooper, a partner in the firm Cooper and Levey, who owned the Waterloo Stores. The retailing emporium was located on the corner of George Street and Market Street in the city, the Dover Heights area was subdivided in 1913. At the 2011 census, there were 3,940 residents in Dover Heights,53. 7% of people were born in Australia with the most common other countries of birth being South Africa 14. 1%, England 3. 6% and Israel 2. 3%. Judaism was the most common response for religion in Dover Heights, the median weekly household income was $2,508 and 34. 4% of households had a weekly income of more than $3,000. Housing costs were high in Dover Heights with the monthly mortgage payment being $4,275. During the Second World War, Rodney Reserve, at the Dover Heights clifftop, was used by the Royal Australian Air Force for coastal defence radar, the site primarily made use of Yagi antennas. Radio astronomy at Dover Heights, Proceedings of the Astronomical Society of Australia,4, CSIRO, Radio astronomy at Dover Heights Orchiston, W. & B. Slee, Ingenuity and Inititative in Australian Radio Astronomy, The Dover Heights ‘Hole-in-the-Ground’ Antenna, stanley, G. J. Recollections of John G. Bolton at Dover-Heights and Caltech, Australian Journal of Physics 47, 507-16. Radhakrishnan, Heights John Gatenby Bolton 1922-1993, Historical Records of Australian Science 10-4, the Book of Sydney Suburbs, Compiled by Frances Pollen, Angus & Robertson Publishers,1990, Published in Australia ISBN 0-207-14495-8 St Therese Catholic Church Dover Heights
Dover Heights
–
Dover Heights with the
Tasman Sea on the left
Dover Heights
–
Houses and apartments line the cliff top in Dover Heights
24.
CSIRO
–
The Commonwealth Scientific and Industrial Research Organisation is the federal government agency for scientific research in Australia. Its chief role is to improve the economic and social performance of industry, CSIRO works with leading organisations around the world, and CSIRO Publishing issues journals with the latest research by leading scientists on a broad range of subjects. From its headquarters in Canberra, CSIRO maintains more than 50 sites across Australia and biological control research stations in France and Mexico, employing about 5000 in total. As of 1 July 2014, CSIRO is governed by a Board, chaired by David Thodey with nine directors inclusive of the CEO, daily management is led by a CEO, presently Dr Larry Marshall, with an executive team of seven. There are ten research flagships and twelve services areas, the CSIRO Flagship initiative was originally designed to integrate, focus and direct national scientific resources. In July 2014, all research Divisions and Flagships were merged into ten research flagships, the Bureau of Meteorology generates the high resolution weather forecasts and CSIRO has created computer models to calculate pollution levels. A precursor to CSIRO, the Advisory Council of Science and Industry, was established in 1916 at the initiative of prime minister Billy Hughes, however, the Advisory Council struggled with insufficient funding during the First World War. In 1920 the Council was renamed the Commonwealth Institute of Science and Industry, and was led by George Handley Knibbs, in 1926 The Science and Industry Research Act replaced the Institute with the Council for Scientific and Industrial Research. The CSIR was structured to represent the structure of Australian government, and had state-level committees. As well as this structure, the CSIR benefited from strong bureaucratic management under George Julius, David Rivett. CSIR research focussed on primary and secondary industries, early in its existence, it established divisions studying animal health and animal nutrition. After the depression, the CSIR extended into secondary industries such as manufacturing, CSIRO today has expanded into a wider range of scientific inquiry. This expansion began with the evolution of CSIR to the CSIRO by the Ben Chifley Labor government in 1949 which amended the Science and Industry Research Act, the amendment enlarged and reconstituted the organisation and its administrative structure. Under Ian Clunies Ross as chairman, CSIRO pursued new areas such as radioastronomy, in October 2005, the journal Nature announced CSIRO scientists had developed near-perfect rubber from resilin, the elastic protein which gives fleas their jumping ability and helps insects fly. On 19 August 2005, CSIRO and the University of Texas at Dallas announced they were able to make transparent carbon nanotube sheets that will bring carbon nanotube products to the masses, CSIRO had a pioneering role in the scientific discovery of the universe through radio eyes. A team led by Paul Wild built and operated the world’s first solar radiospectrograph, for three decades, the Division of Radiophysics had a world-leading role in solar research, attracting prominent solar physicists from around the world. CSIRO owned the first computer in Australia, CSIRAC, built as part of a project began in the Sydney Radiophysics Laboratory in 1947, the CSIR Mk 1 ran its first program in 1949, the fifth electronic computer in the world. It was over 1,000 times faster than the mechanical calculators available at the time and it was decommissioned in 1955 and recommissioned in Melbourne as CSIRAC in 1956 as a general purpose computing machine used by over 700 projects until 1964
CSIRO
–
CSIRO Corporate Headquarters in
Campbell, Australian Capital Territory
CSIRO
–
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
CSIRO
–
CSIRO
Darwin
CSIRO
–
CSIRO
Hobart
25.
Joseph Lade Pawsey
–
Joseph Lade Pawsey was an Australian scientist, radiophysicist and radio astronomer. Pawsey was born in Ararat, Victoria to a family of farmers, at the age of 14 he was awarded a government scholarship to study at Wesley College, Melbourne, followed by a scholarship to study at the University of Melbourne. In 1929, he earned his Bachelor of Science degree from the university, Pawsey was then awarded an Exhibition Research Scholarship to study at Cambridge University, where he worked under the direction of J. A. In 1935, he was awarded a PhD from Cambridge and in September of that year he married Greta Lenore Nicoll, Pawsey then became a research physicist at EMI until 1939. In February 1940, Pawsey returned to Australia to work at the recently formed Division of Radiophysics in CSIR, one group he led developed a microwave set for the Royal Australian Navy while another group under his direction investigated the super-refraction of radio waves in the Earths atmosphere. Pawsey continued as a research physicist at the Division of Radiophysics until 1962, to investigate the latter Pawsey, with Ruby Payne-Scott and Lindsay McCready, used an existing Royal Australian Air Force antenna at Collaroy, a northern Sydney suburb. In addition to confirming that the Sun was a source of radio noise their data showed that the temperature in some regions of the Sun were as high as one million degrees. This temperature was far higher than was possible at the time. Work by the physicist David Forbes Martyn showed that peak in the Suns corona at one million degrees. The introduction of interferometry was probably Pawseys most important contribution to radio astronomy, in early 1946 he turned his attention to sunspots as a source of strong fluctuating radio noise. To overcome the limitations of the antennas, Pawsey used sea interferometry. The observations confirmed beyond doubt that sunspots were the source of the increase in radio noise. This work was confirmed four months later by radio astronomers at Cambridge University led by Martin Ryle, Pawsey did some work on his own, but his main focus was on guiding and administering his research teams, which worked in comparative isolation and sometimes in competition with each other. He was straightforward, honest and humble, and scrupulous in acknowledging his colleagues achievements and his subordinate Paul Wild, who in 1971 became division chief, said, Joe Pawsey was the. Father of radio astronomy in Australia and his influence on the growth of radio astronomy in Australia was great because … he just provided ideal conditions, an ideal environment to allow everyone to use their own initiative. Was a wonderfully inspiring leader, very self-effacing and taking no credit for himself, was often very perceptive, very good. International authorities gave much credit to his leadership, in 1963, in his introduction to the Utrecht Symposium on the Solar Spectrum, Professor M. G. J. Minnaert remarked, The history of solar radio-spectroscopy is mainly the history of Australian work on this subject, at each meeting of the IAU, at each important symposium on radio-astronomy, highly competent specialists such as Wild, Smerd and Christianson, headed by the dynamic personality of
Joseph Lade Pawsey
–
Joe Pawsey as a research physicist in
CSIRO
26.
Sagittarius A
–
Sagittarius A or Sgr A is a complex radio source at the center of the Milky Way. It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the arms of the Milky Way. It consists of three components, the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and these three overlap, Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West. These measurements were made through the use of a wavelength of 7 mm, the result was that Sagittarius A* was determined to be an elliptical Gaussian with a semi-major axis size of 35. 4×12.6 R☉ with an inclination of 95 degrees east of north. The observations for this finding were found from the detection of NIR flares, both flares come from electron excitation, not an enhanced accretion rate onto the black hole, which means that not all high energy events produce variability at radio wavelengths. A study done with the measured parallaxes and motions of 10 massive regions in the arms of Sagittarius in the Milky Way where stars are formed. Data was gathered using the BeSSeL Survey with the VLBA, the results were that the spiral pitch angle of the arms is 7.3 ±1.5 degrees, and the half-width of the arms of the Milky Way were found to be.2 kpc. The nearest arm from the Sun is around 1.4 ±0.2 kpc away and this feature is approximately 25 light-years in width and has the attributes of a supernova remnant from an explosive event that occurred between 35,000 and 100,000 BCE. However, it would take 50 to 100 times more energy than a supernova explosion to create a structure of this size. It is conjectured that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made an approach to the central black hole. Sgr A West has the appearance of a spiral, from the point of view of the Earth. For this reason, it is known as the Minispiral. This appearance and nickname are misleading, though, the structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit, the surface layer of these clouds is ionized. The source of ionisation is the population of stars that also occupy the central parsec. Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source. The Western Arc is interpreted as the inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane and they have been estimated to amount for about 20 solar masses each
Sagittarius A
–
Surface brightness and velocity field of the inner part of Sagittarius A West
Sagittarius A
–
Astronomers have observed stars spinning around the
supermassive black hole in
Sagittarius A*.
Sagittarius A
–
Location
27.
International Astronomical Union
–
The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the recognized authority for assigning designations to celestial bodies. The IAU is a member of the International Council for Science and its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership, the IAU has its head office on the second floor of the Institut dAstrophysique de Paris in the 14th arrondissement of Paris. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams, the Minor Planet Center also operates under the IAU, and is a clearinghouse for all non-planetary or non-moon bodies in the Solar System. The Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers, the IAU was founded on July 28,1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. The 7 initial member states were Belgium, Canada, France, Great Britain, Greece, Japan, the first executive committee consisted of Benjamin Baillaud, Alfred Fowler, and four vice presidents, William Campbell, Frank Dyson, Georges Lecointe, and Annibale Riccò. Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets, the reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, May 2–10,1922. By the end of the first General Assembly, ten nations had joined the Union. Although the Union was officially formed eight months after the end of World War I, the first 50 years of the Unions history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents, twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Unions history in IAU Information Bulletin No.100. Six past IAU Presidents in the period 1976–2003 also contributed their recollections in IAU Information Bulletin No.104, the IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide. 83% of all members are male, while 17% are female, among them the unions current president. Membership also includes 79 national members, professional astronomical communities representing their countrys affiliation with the IAU, the sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union, the right to vote on matters brought before the Assembly varies according to the type of business under discussion. On budget matters, votes are weighted according to the subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid, an absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union, since 1922, the IAU General Assembly meets every three years, with the exception of the period between 1938 and 1948, due to World War II
International Astronomical Union
–
International Astronomical Union
28.
Equatorial coordinate system
–
The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. The origin at the center of the Earth means the coordinates are geocentric, a right-handed convention means that coordinates are positive toward the north and toward the east in the fundamental plane. This description of the orientation of the frame is somewhat simplified. A slow motion of Earths axis, precession, causes a slow, continuous turning of the system westward about the poles of the ecliptic. Superimposed on this is a motion of the ecliptic. In order to fix the primary direction, these motions necessitate the specification of the equinox of a particular date, known as an epoch. The three most commonly used are, Mean equinox of an epoch is a fixed standard direction. Mean equinox of date is the intersection of the ecliptic of date with the mean equator, commonly used in planetary orbit calculation. True equinox of date is the intersection of the ecliptic of date with the true equator and this is the actual intersection of the two planes at any particular moment, with all motions accounted for. A position in the coordinate system is thus typically specified true equinox and equator of date, mean equinox and equator of J2000.0. Note that there is no mean ecliptic, as the ecliptic is not subject to small periodic oscillations, a stars spherical coordinates are often expressed as a pair, right ascension and declination, without a distance coordinate. The direction of distant objects is the same for all observers. Telescopes equipped with equatorial mounts and setting circles employ the equatorial coordinate system to find objects, setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere. The declination symbol δ, measures the distance of an object perpendicular to the celestial equator, positive to the north. For example, the celestial pole has a declination of +90°. The origin for declination is the equator, which is the projection of the Earths equator onto the celestial sphere. Declination is analogous to terrestrial latitude, the right ascension symbol α, measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the object. The vernal equinox point is one of the two where the ecliptic intersects the celestial equator, there are = 15° in one hour of right ascension, 24h of right ascension around the entire celestial equator
Equatorial coordinate system
–
As seen from above the
Earth 's
north pole, a star's
local hour angle (LHA) for an observer near New York (red). Also depicted are the star's
right ascension and Greenwich hour angle (GHA), the
local mean sidereal time (LMST) and
Greenwich mean sidereal time (GMST). The symbol ʏ identifies the
vernal equinox direction.
29.
Right ascension
–
Right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the sphere in the equatorial coordinate system. Right ascension is the equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a direction on an equator. Right ascension is measured continuously in a circle from that equinox towards the east. Any units of measure could have been chosen for right ascension, but it is customarily measured in hours, minutes. Astronomers have chosen this unit to measure right ascension because they measure a stars location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a line onto the celestial sphere. A full circle, measured in units, contains 24 × 60 × 60 = 86 400s, or 24 × 60 = 1 440m. Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees, it is the complement of right ascension with respect to 24h and it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earths axis rotates slowly westward about the poles of the ecliptic and this effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. The right ascension of Polaris is increasing quickly, the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, the easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earths axis
Right ascension
–
Right ascension (blue) and
declination (green) as seen from outside the
celestial sphere.
30.
Declination
–
In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declinations angle is measured north or south of the celestial equator, the root of the word declination means a bending away or a bending down. It comes from the root as the words incline and recline. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of measure can be used for declination, but it is customarily measured in the degrees, minutes. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere, the Earths axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, therefore, equatorial coordinates are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be rotated to match each other. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity. This similarly occurs in the Southern Hemisphere for objects with less than −90° − φ. An extreme example is the star which has a declination near to +90°. Circumpolar stars never dip below the horizon, conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earths surface. Generally, if a star whose declination is δ is circumpolar for some observer, then a star whose declination is −δ never rises above the horizon, as seen by the same observer. Likewise, if a star is circumpolar for an observer at latitude φ, neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, from the poles, declination is uniform around the entire horizon, approximately 0°. Non-circumpolar stars are visible only during certain days or seasons of the year, the Suns declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the summer solstice, leading to the phenomenon of it being above the horizon at midnight
Declination
–
Right ascension (blue) and declination (green) as seen from outside the
celestial sphere.
31.
Solar System
–
The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
Solar System
–
The
Sun and
planets of the Solar System (distances not to scale)
Solar System
–
Solar System
Solar System
–
Andreas Cellarius 's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
Solar System
–
The eight planets of the Solar System (by decreasing size) are
Jupiter,
Saturn,
Uranus,
Neptune,
Earth,
Venus,
Mars and
Mercury.
32.
Cosmic distance ladder
–
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities
Cosmic distance ladder
–
Light green boxes: Technique applicable to
star-forming galaxies.
Cosmic distance ladder
–
Statue of an astronomer and the concept of the cosmic distance ladder by the parallax method, made from the azimuth ring and other parts of the Yale–Columbia Refractor (telescope) (c 1925) wrecked by the
2003 Canberra bushfires which burned out the
Mount Stromlo Observatory; at
Questacon,
Canberra, Australian Capital Territory
Cosmic distance ladder
–
SN 1994D (bright spot on the lower left) in the
NGC 4526 galaxy. Image by
NASA,
ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team
Cosmic distance ladder
–
Galaxy cluster
33.
Variable star
–
For the astronomical object, see Variable star. Variable Star is a 2006 novel written by Spider Robinson based on the seven pages of an eight-page 1955 novel outline by the late Robert A. Heinlein. The book is set in a divergent offshoot of Heinleins Future History and contains references to works by Heinlein. It describes the coming of age of a musician who signs on to the crew of a starship as a way of escaping from a failed romance. Robinson posted a note on his website in 2009 noting that his agent had sold a trilogy of sequels based on the novel, Robinson states in an appendix to the book that he was working from an outline that lacked an ending. He was told by his publisher that they wanted him to write in his own style, not Heinleins, Heinleins original title for Variable Star was The Stars are a Clock. Eighteen-year-old aspiring musician and composer Joel Johnston, a Ganymedean on Earth for his education, both are orphans, and virtually penniless. When Jinny decides their relationship is ready for marriage, she reveals that she is actually Jinnia Conrad, Joel learns that Conrad has already mapped out his future, he is to be groomed for a role in the family business and to produce children to continue the dynasty. Preferring to pursue his own destiny, he flees the Conrad estate with the help of Jinnys cousin, to escape the Conrads and their vast reach, Joel joins the crew of the RSS Charles Sheffield. The ship is headed to a distant star on a 20-year voyage to establish a colony, with experience from his family farm, Joel works as a farmer for the ships crew of 500 and as a part-time musician. He regularly corresponds with Evelyn through the twins on board who maintain contact with Earth via telepathy with their siblings, six relativists are essential to the voyage, controlling the ships quantum ramjet drive with their minds. The drive has to run continuously, at speeds, it is nearly impossible to restart it. Each relativist can only stand the strain reliably for six hours a day, five years into the voyage, one is killed and another mentally incapacitated, leaving only four and no margin for error. The next year, the Sheffield learns through its telepaths that the Sun has gone nova, a wavefront of deadly gamma radiation is expanding at lightspeed, threatening the colonies that are all that is left of humanity. The crew is able to warn one in time, the rest are doomed. The Sun going nova is contrary to all theories, and because over 90% of the suns mass was converted into energy. Unable to bear the catastrophe, one of the relativists commits suicide, despite the other threes efforts, the quantum ramjet drive soon shuts down. The Sheffield will not be able to stop, it will coast by its destination at 97. 6% of the speed of light
Variable star
–
Variable Star
Variable star
34.
RR Lyrae variable
–
RR Lyrae variables are periodic variable stars, commonly found in globular clusters and are used as standard candles to measure galactic distances. This class of star is named after the prototype and brightest example. RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Suns. They are thought to have previously shed mass during the Red-giant branch phase, the period of pulsation and absolute magnitude of RR Lyraes makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group. Beyond the Milky Way they are difficult to due to their low luminosity. They are extensively used in globular cluster studies, and also used to study properties of older stars. In surveys of clusters, these cluster-type variables were being rapidly identified in the mid-1890s. Probably the first star of definitely RR Lyrae type found outside a cluster was U Leporis, the prototype star RR Lyrae was discovered prior to 1899 by Williamina Fleming, and reported by Pickering in 1900 as indistinguishable from cluster-type variables. RR Lyrae variables are metal-poor, Population II stars, RR Lyraes have proven difficult to observe in external galaxies because of their intrinsic faintness. Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & van den Bergh found Lyraes in Andromedas galactic halo and, more recently, several times as many RR Lyraes are known as all Cepheids combined, in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way, though binary star systems are common for typical stars, RR Lyrae are very rarely observed in pairs. RR Lyrae stars pulse in a similar to Cepheid variables. Like all variables on the Cepheid instability strip, pulsations are caused by the κ-mechanism, RR Lyraes are old, relatively low mass, Population II stars, in common with W Virginis and BL Herculis variables, the type II Cepheids. Classical Cepheid variables are higher mass population I stars, RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae star is about +0.75 and their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, include RR Lyrae itself, exhibit the Blazhko effect in which there is a conspicuous phase, unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infrared K band. They are normally analysed using a period-colour-relationship, for using a Wesenheit function. In this way, they can be used as standard candles for distance measurements although there are difficulties with the effects of metallicity, faintness, and blending
RR Lyrae variable
–
The RR Lyrae variable stars fall in a particular area on a
Hertzsprung–Russell diagram of color versus brightness.
35.
Standard candles
–
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities
Standard candles
–
Light green boxes: Technique applicable to
star-forming galaxies.
Standard candles
–
Statue of an astronomer and the concept of the cosmic distance ladder by the parallax method, made from the azimuth ring and other parts of the Yale–Columbia Refractor (telescope) (c 1925) wrecked by the
2003 Canberra bushfires which burned out the
Mount Stromlo Observatory; at
Questacon,
Canberra, Australian Capital Territory
Standard candles
–
SN 1994D (bright spot on the lower left) in the
NGC 4526 galaxy. Image by
NASA,
ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team
Standard candles
–
Galaxy cluster
36.
Red clump
–
The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, the red clump giants are cool horizontal branch stars, stars originally similar to the sun which have undergone a helium flash and are now fusing helium in their cores. Red clump stellar properties vary depending on their origin, most notably on the metallicity of the stars, the absolute visual magnitude of red clump giants near the sun has been measured at an average of +0.81 with metallicities between −0.6 and +0.4 dex. There is a spread in the properties of red clump stars even within a single population of similar stars such as an open cluster. Although red clump stars are hotter than red-giant-branch stars, the two regions overlap and the status of individual stars can only be assigned with a detailed chemical abundance study. Modelling of the branch has shown that stars have a strong tendency to cluster at the cool end of the zero age horizontal branch. This tendency is weaker in low metallicity stars, so the red clump is usually more prominent metal-rich clusters, however, there are other effects, and there are well-populated red clumps in some metal-poor globular clusters. Stars with a mass to the sun evolve towards the tip of the red giant branch with a degenerate helium core. More massive stars leave the red giant branch early and perform a loop, but all stars with a degenerate core reach the tip with very similar core masses, temperatures. After the helium flash they lie along the ZAHB, all with helium cores just under 0.5 M☉, lower envelope masses result in weaker hydrogen shell fusion and give hotter and slightly less luminous stars strung along the horizontal branch. Different initial masses and natural variations in mass loss rates on the red giant branch cause the variations in the masses even though the helium cores are all the same size. Low metallicity stars are more sensitive to the size of the envelope, so with the same envelope masses they are spread further along the horizontal branch. Although red clump stars lie consistently to the hot side of the red giant branch that they evolved from, red clump and this occurs in ω Centauri where metal-poor red-giant-branch stars have the same or hotter temperatures as more metal-rich red clump giants. Other stars, not strictly horizontal branch stars, can lie in the region of the H-R diagram. Stars too massive to develop a degenerate helium core on the red giant branch will ignite helium before the tip of the red giant branch and perform a blue loop. For stars only a more massive than the sun, around 2 M☉. This means that these imposters are much less common in the H-R diagram, stars with 2 -3 M☉ will also pass through the red clump as they evolve along the subgiant branch
Red clump
–
Hertzsprung–Russell diagram showing the evolution of stars of different masses. The red clump is marked RC on the green line showing the evolution of a star of 2 solar masses.
37.
Reddening law
–
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. For stars that lie near the plane of the Milky Way and are within a few parsecs of the Earth. For Earth-bound observers, extinction arises both from the medium and the Earths atmosphere, it may also arise from circumstellar dust around an observed object. The strong atmospheric extinction in some wavelength regions requires the use of space-based observatories, since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium, interstellar reddening is a different phenomenon from redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a spectrum while leaving behind the longer wavelength photons. In any photometric system interstellar reddening can be described by color excess, an objects intrinsic color index is the theoretical color index which it would have if unaffected by extinction. This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets, broadly speaking, interstellar extinction is strongest at short wavelengths, generally observed by using techniques from spectroscopy. Extinction results in a change in the shape of an observed spectrum, superimposed on this general shape are absorption features that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e. g. dust grains. Known absorption features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features. In the solar neighborhood, the rate of extinction in the Johnson-Cousins V-band is usually taken to be 0. 7-1.0 mag/kpc−simply an average due to the clumpiness of interstellar dust. In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band for every kiloparsec it is away from us. The amount of extinction can be higher than this in specific directions. For example, some regions of the Galactic Center have more than 30 magnitudes of extinction in the optical, extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features. R is defined to be A/E, and measures the total, A, to selective, E = A-A, a and A are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A/A at wavelength λ, R is known to be correlated with the average size of the dust grains causing the extinction. For our own galaxy, the Milky Way, the value for R is 3.1
Reddening law
–
Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar. The curves are plotted versus 1/wavelength to emphasize the UV.
38.
Galactic bulge
–
A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
Galactic bulge
–
An example of a spiral galaxy, the
Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
Galactic bulge
–
Barred spiral galaxy UGC 12158.
Galactic bulge
–
NGC 1300 in
infrared light.
Galactic bulge
–
Spiral galaxy NGC 1345
39.
Extinction (astronomy)
–
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. For stars that lie near the plane of the Milky Way and are within a few parsecs of the Earth. For Earth-bound observers, extinction arises both from the medium and the Earths atmosphere, it may also arise from circumstellar dust around an observed object. The strong atmospheric extinction in some wavelength regions requires the use of space-based observatories, since blue light is much more strongly attenuated than red light, extinction causes objects to appear redder than expected, a phenomenon referred to as interstellar reddening. Reddening occurs due to the light scattering off dust and other matter in the interstellar medium, interstellar reddening is a different phenomenon from redshift, which is the proportional frequency shifts of spectra without distortion. Reddening preferentially removes shorter wavelength photons from a spectrum while leaving behind the longer wavelength photons. In any photometric system interstellar reddening can be described by color excess, an objects intrinsic color index is the theoretical color index which it would have if unaffected by extinction. This is similar to the effect seen when dust particles in the atmosphere of Earth contribute to red sunsets, broadly speaking, interstellar extinction is strongest at short wavelengths, generally observed by using techniques from spectroscopy. Extinction results in a change in the shape of an observed spectrum, superimposed on this general shape are absorption features that have a variety of origins and can give clues as to the chemical composition of the interstellar material, e. g. dust grains. Known absorption features include the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features. In the solar neighborhood, the rate of extinction in the Johnson-Cousins V-band is usually taken to be 0. 7-1.0 mag/kpc−simply an average due to the clumpiness of interstellar dust. In general, however, this means that a star will have its brightness reduced by about a factor of 2 in the V-band for every kiloparsec it is away from us. The amount of extinction can be higher than this in specific directions. For example, some regions of the Galactic Center have more than 30 magnitudes of extinction in the optical, extending the extinction law into the mid-infrared wavelength range is difficult due to the lack of suitable targets and various contributions by absorption features. R is defined to be A/E, and measures the total, A, to selective, E = A-A, a and A are the total extinction at the B and V filter bands. Another measure used in the literature is the absolute extinction A/A at wavelength λ, R is known to be correlated with the average size of the dust grains causing the extinction. For our own galaxy, the Milky Way, the value for R is 3.1
Extinction (astronomy)
–
Plot showing the average extinction curves for the MW, LMC2, LMC, and SMC Bar. The curves are plotted versus 1/wavelength to emphasize the UV.
40.
Variable stars
–
A variable star is a star whose brightness as seen from Earth fluctuates. Many, possibly most, stars have at least some variation in luminosity, an ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol. This discovery, combined with supernovae observed in 1572 and 1604, proved that the sky was not eternally invariable as Aristotle. In this way, the discovery of variable stars contributed to the revolution of the sixteenth. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669, chi Cygni was identified in 1686 by G. Kirch, then R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known, John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850 the number of variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography. The latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 suspected variables. The most common kinds of variability involve changes in brightness, but other types of variability also occur, by combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy, measurements of their changes in brightness can be plotted to produce light curves. Peak brightnesses in the curve are known as maxima, while troughs are known as minima. By estimating the magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world, from the light curve the following data are derived, are the brightness variations periodical, semiperiodical, irregular, or unique. What is the period of the brightness fluctuations, what is the shape of the light curve. From the spectrum the following data are derived, what kind of star is it, what is its temperature, is it a single star, or a binary. does the spectrum change with time. In very few cases it is possible to make pictures of a stellar disk and these may show darker spots on its surface. Combining light curves with spectral data often gives a clue as to the changes occur in a variable star. For example, evidence for a star is found in its shifting spectrum because its surface periodically moves toward and away from us
Variable stars
–
Trifid Nebula contains Cepheid variable stars.
Variable stars
–
A photogenic variable star,
Eta Carinae, embedded in the
Carina Nebula.
Variable stars
–
Images showing the expansion of the light echo of a red variable star, the
V838 Monocerotis
Variable stars
–
How eclipsing binaries vary in brightness
41.
Red giant
–
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large, the appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars. The most common red giants are stars on the red-giant branch that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core and they have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue, despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon, the first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch, the stellar limb of a red giant is not sharply-defined, contrary to their depiction in many illustrations. Rather, due to the low mass density of the envelope, such stars lack a well-defined photosphere. The coolest red giants have complex spectra, with lines, emission features. Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M☉ to around 8 M☉, when a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of metals. These elements are all uniformly mixed throughout the star, the star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen and establishes hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium, for the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a lifetime than less massive stars. When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and this brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core. The higher temperatures lead to increasing rates, enough to increase the stars luminosity by a factor of 1. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the stars life, in actuality, though, the color usually is orange. At this time, the star is said to be ascending the red-giant branch of the Hertzsprung–Russell diagram, the outer layers carry the energy evolved from fusion to the surface by way of convection. This causes material exposed to nuclear burning in the interior to be brought to the stars surface for the first time in its history
Red giant
–
Mira, a red giant
Red giant
–
Hertzsprung–Russell diagram
Red giant
–
This image tracks the life of a
Sun -like star, from its
birth on the left side of the frame to its
evolution into a red giant on the right after billions of years.
Red giant
–
Mira A is an old star, already shedding its outer layers into space.
42.
RR Lyr variable
–
RR Lyrae variables are periodic variable stars, commonly found in globular clusters and are used as standard candles to measure galactic distances. This class of star is named after the prototype and brightest example. RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Suns. They are thought to have previously shed mass during the Red-giant branch phase, the period of pulsation and absolute magnitude of RR Lyraes makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group. Beyond the Milky Way they are difficult to due to their low luminosity. They are extensively used in globular cluster studies, and also used to study properties of older stars. In surveys of clusters, these cluster-type variables were being rapidly identified in the mid-1890s. Probably the first star of definitely RR Lyrae type found outside a cluster was U Leporis, the prototype star RR Lyrae was discovered prior to 1899 by Williamina Fleming, and reported by Pickering in 1900 as indistinguishable from cluster-type variables. RR Lyrae variables are metal-poor, Population II stars, RR Lyraes have proven difficult to observe in external galaxies because of their intrinsic faintness. Using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & van den Bergh found Lyraes in Andromedas galactic halo and, more recently, several times as many RR Lyraes are known as all Cepheids combined, in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way, though binary star systems are common for typical stars, RR Lyrae are very rarely observed in pairs. RR Lyrae stars pulse in a similar to Cepheid variables. Like all variables on the Cepheid instability strip, pulsations are caused by the κ-mechanism, RR Lyraes are old, relatively low mass, Population II stars, in common with W Virginis and BL Herculis variables, the type II Cepheids. Classical Cepheid variables are higher mass population I stars, RR Lyrae variables are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae star is about +0.75 and their period is shorter, typically less than one day, sometimes ranging down to seven hours. Some RRab stars, include RR Lyrae itself, exhibit the Blazhko effect in which there is a conspicuous phase, unlike Cepheid variables, RR Lyrae variables do not follow a strict period-luminosity relationship at visual wavelengths, although they do in the infrared K band. They are normally analysed using a period-colour-relationship, for using a Wesenheit function. In this way, they can be used as standard candles for distance measurements although there are difficulties with the effects of metallicity, faintness, and blending
RR Lyr variable
–
The RR Lyrae variable stars fall in a particular area on a
Hertzsprung–Russell diagram of color versus brightness.
43.
Kiloparsec
–
The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond
Kiloparsec
–
The jet erupting from the
active galactic nucleus of
M87 is thought to be 7000150000000000000♠ 1.5 kiloparsecs (7019462629720109201♠ 4890
ly) long. (image from Hubble Space Telescope)
Kiloparsec
–
A parsec is the distance from the
Sun to an
astronomical object that has a
parallax angle of one
arcsecond (the diagram is not to scale).
44.
Star formation
–
Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as stellar nurseries or star-forming regions, fuse to form stars. It is closely related to formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a star, must also account for the statistics of binary stars. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z =6.60, a spiral galaxy like the Milky Way contains stars, stellar remnants, and a diffuse interstellar medium of gas and dust. The interstellar medium consists of 10−4 to 106 particles per cm3 and is composed of roughly 70% hydrogen by mass. This medium has been enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, in the dense nebulae where stars are produced, much of the hydrogen is in the molecular form, so these nebulae are called molecular clouds. These giant molecular clouds have densities of 100 particles per cm3, diameters of 100 light-years, masses of up to 6 million solar masses. About half the mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula,1,300 ly away, however, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex. A more compact site of formation is the opaque clouds of dense gas and dust known as Bok globules. These can form in association with collapsing molecular clouds or possibly independently, the Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars, over half the known Bok globules have been found to contain newly forming stars. An interstellar cloud of gas will remain in equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the theorem, which states that, to maintain equilibrium. If a cloud is massive enough that the gas pressure is insufficient to support it, the mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud and this coincides with the typical mass of an open cluster of stars, which is the end product of a collapsing cloud. In triggered star formation, one of several events occur to compress a molecular cloud
Star formation
–
Star formation
Star formation
–
Hubble telescope image known as
Pillars of Creation, where stars are forming in the
Eagle Nebula.
Star formation
–
Assembly of galaxy in early Universe.
Star formation
–
Stellar cluster and
star-forming region M 17.
45.
Andromeda Galaxy
–
The Andromeda Galaxy, also known as Messier 31, M31, or NGC224, is a spiral galaxy approximately 780 kiloparsecs from Earth. It is the nearest major galaxy to the Milky Way and was referred to as the Great Andromeda Nebula in older texts. It received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda. Andromeda is approximately 220,000 light years across, and it is the largest galaxy of the Local Group, which contains the Milky Way, the Triangulum Galaxy. The mass of the Andromeda Galaxy is estimated to be 1. 5×1012 solar masses, the Milky Way and Andromeda galaxies are expected to collide in 4.5 billion years, eventually merging to form a giant elliptical galaxy or perhaps a large disc galaxy. Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave a description of the Andromeda Galaxy based on telescopic observations. In 1764, Charles Messier catalogued Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye, in 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. In 1850, William Parsons, 3rd Earl of Rosse, saw, in 1864, William Huggins noted that the spectrum of Andromeda differs from a gaseous nebula. The spectra of Andromeda displays a continuum of frequencies, superimposed with dark lines that help identify the chemical composition of an object. Andromedas spectrum is similar to the spectra of individual stars. In 1885, a supernova was seen in Andromeda, the first, at the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly, Nova 1885. In 1887, Isaac Roberts took the first photographs of Andromeda, Roberts actually mistook Andromeda and similar spiral nebulae as solar systems being formed. In 1912, Vesto Slipher used spectroscopy to measure the velocity of Andromeda with respect to our solar system—the largest velocity yet measured. In 1917, Heber Curtis observed a nova within Andromeda, searching the photographic record,11 more novae were discovered. Curtis noticed that these novae were, on average,10 magnitudes fainter than those that occurred elsewhere in the sky, as a result, he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the island universes hypothesis, which held that spiral nebulae were actually independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, in 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars
Andromeda Galaxy
–
The Andromeda Galaxy
Andromeda Galaxy
–
Great Andromeda Nebula by
Isaac Roberts, 1899
Andromeda Galaxy
–
M31 above the
Very Large Telescope.
Andromeda Galaxy
–
The Andromeda Galaxy as seen by
NASA 's
Wide-field Infrared Survey Explorer
46.
Sagittarius A*
–
Sagittarius A* is a bright and very compact astronomical radio source at the center of the Milky Way, near the border of the constellations Sagittarius and Scorpius. Astronomers have been unable to observe Sgr A* in the spectrum because of the effect of 25 magnitudes of extinction by dust. Several teams of researchers have attempted to image Sagittarius A* in the spectrum using very-long-baseline interferometry. The current highest-resolution measurement, made at a wavelength of 1.3 mm, at a distance of 26,000 light-years, this yields a diameter of 44 million kilometers. For comparison, Earth is 150 million kilometers from the Sun, the proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination. There are plans to image Sagittarius A* in much greater detail than before using interferometry combining images from widely spaced observatories at different places on Earth and it is hoped the measurements will test Einsteins theory of relativity more rigorously than has previously been done. Discrepancies may be found between the theory and actual observation or, if there are no discrepancies relativity may be confirmed. Karl Jansky was the first person to determine that a signal was coming from a location at the center of the Milky Way. Sgr A* was discovered on February 13 and 15,1974, by astronomers Bruce Balick, the name Sgr A* was coined by Brown in a 1982 paper because the radio source was exciting, and excited states of atoms are denoted with asterisks. The observations of S2 used near-infra red interferometry because of reduced interstellar extinction in this band, siO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images, the VLBI radio observations of Sagittarius A* could also be aligned centrally with the images so S2 could be seen to orbit Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 2.6 ±0.2 million solar masses, confined in a volume with a radius no more than 17 light-hours. Later observations of the star S14 showed the mass of the object to be about 4.1 million solar masses within a volume with no larger than 6.25 light-hours or about 6.7 billion kilometres. They also determined the distance from Earth to the Galactic Center, in November 2004 a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars and this observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars. After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimate the mass at 4.31 ±0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009, reinhard Genzel, team leader of the research, said the study has delivered what is now considered to be the best empirical evidence that super-massive black holes do really exist. The stellar orbits in the Galactic Center show that the mass concentration of four million solar masses must be a black hole
Sagittarius A*
–
Sgr A* (centre) and two
light echoes from a recent explosion (circled)
Sagittarius A*
–
Detection of an unusually bright
X-ray flare from Sgr A*.
Sagittarius A*
–
Dusty cloud G2 passes the supermassive black hole at the center of the Milky Way.
Sagittarius A*
–
Supernova remnant ejecta producing planet-forming material.
47.
Accretion (astrophysics)
–
In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes, the idea proposed in the 19th century that Earth and the other terrestrial planets formed from meteoric material was developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of planet formation. Since then, the theory has been developed using intensive numerical simulations to study planetesimal accumulation. Stars form by the collapse of interstellar gas. Prior to collapse, this gas is mostly in the form of molecular clouds, as the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, and the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk. A few hundred years after the Big Bang, the Universe cooled to the point where atoms could form. As the Universe continued to expand and cool, the atoms lost enough kinetic energy, as further accretion occurred, galaxies formed. Galaxies grow through mergers and smooth gas accretion, accretion also occurs inside galaxies, forming stars. Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of roughly 300,000 M☉ and 65 light-years in diameter, over millions of years, giant molecular clouds are prone to collapse and fragmentation. These fragments then form small, dense cores, which in turn collapse into stars, the cores range in mass from a fraction to several times that of the Sun and are called protostellar nebulae. They possess diameters of 2, 000–20,000 astronomical units, compare it with the particle number density of the air at the sea level—2. 8×1019/cm3. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years, every nebula begins with a certain amount of angular momentum. This core forms the seed of what will become a star, as the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which eventually forms a disk. Around this time the protostar begins to fuse deuterium, if the protostar is sufficiently massive, hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf and this birth of a new star occurs approximately 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are also called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the star has already accreted much of its mass
Accretion (astrophysics)
–
Atacama Large Millimeter Array image of
HL Tauri, a
protoplanetary disc
Accretion (astrophysics)
48.
Black hole
–
A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—not even particles and electromagnetic radiation such as light—can escape from inside it. The theory of relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon, although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways a black hole acts like a black body. Moreover, quantum theory in curved spacetime predicts that event horizons emit Hawking radiation. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. Black holes were considered a mathematical curiosity, it was during the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality, black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings, by absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies, despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an accretion disk heated by friction. If there are other stars orbiting a black hole, their orbits can be used to determine the black holes mass, such observations can be used to exclude possible alternatives such as neutron stars.3 million solar masses. On 15 June 2016, a detection of a gravitational wave event from colliding black holes was announced. The idea of a body so massive that light could not escape was briefly proposed by astronomical pioneer John Michell in a letter published in 1783-4. Michell correctly noted that such supermassive but non-radiating bodies might be detectable through their effects on nearby visible bodies. In 1915, Albert Einstein developed his theory of general relativity, only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the solution for the point mass. This solution had a peculiar behaviour at what is now called the Schwarzschild radius, the nature of this surface was not quite understood at the time
Black hole
–
Predicted appearance of non-rotating black hole with toroidal ring of ionised matter, such as has been proposed as a model for
Sagittarius A*. The asymmetry is due to the
Doppler effect resulting from the enormous orbital speed needed for centrifugal balance of the very strong gravitational attraction of the hole.
Black hole
–
Simulation of
gravitational lensing by a black hole, which distorts the image of a
galaxy in the background
Black hole
–
A simple illustration of a non-spinning black hole
Black hole
–
A simulated event in the CMS detector, a collision in which a micro black hole may be created.
49.
Accretion disc
–
An accretion disk is a structure formed by diffused material in orbital motion around a massive central body. The central body is typically a star, gravity causes material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, the frequency range of that radiation depends on the central objects mass. Accretion disks of stars and protostars radiate in the infrared. The study of modes in accretion disks is referred to as diskoseismology. Accretion disks are a phenomenon in astrophysics, active galactic nuclei, protoplanetary disks. These disks very often give rise to astrophysical jets coming from the vicinity of the central object, jets are an efficient way for the star-disk system to shed angular momentum without losing too much mass. The most spectacular accretion disks found in nature are those of active galactic nuclei and of quasars, as matter enters the accretion disc, it follows a trajectory called a tendex line, which describes an inward spiral. The loss of angular momentum manifests as a reduction in velocity, at a slower velocity, as the particle falls to this lower orbit, a portion of its gravitational potential energy is converted to increased velocity and the particle gains speed. Thus, the particle has lost energy even though it is now travelling faster than before, however, the large luminosity of quasars is believed to be a result of gas being accreted by supermassive black holes. Elliptical accretion disks formed at tidal disruption of stars can be typical in galactic nuclei, Accretion process can convert about 10 percent to over 40 percent of the mass of an object into energy as compared to around 0.7 percent for nuclear fusion processes. A gas flow then develops from the star to the primary. Angular momentum conservation prevents a straight flow from one star to the other, Accretion disks surrounding T Tauri stars or Herbig stars are called protoplanetary disks because they are thought to be the progenitors of planetary systems. The accreted gas in this case comes from the molecular cloud out of which the star has formed rather than a companion star, in the 1940s, models were first derived from basic physical principles. In order to agree with observations, those models had to invoke a yet unknown mechanism for angular momentum redistribution, if matter is to fall inwards it must lose not only gravitational energy but also lose angular momentum. In other words, angular momentum should be transported outwards for matter to accrete and this prevents the existence of a hydrodynamic mechanism for angular momentum transport. On one hand, it was clear that viscous stresses would eventually cause the matter towards the center to heat up, on the other hand, viscosity itself was not enough to explain the transport of angular momentum to the exterior parts of the disk. Turbulence-enhanced viscosity was the thought to be responsible for such angular-momentum redistribution
Accretion disc
–
Image taken by Hubble space telescope of what may be gas accreting onto a black hole in elliptical galaxy NGC 4261
Accretion disc
–
Artist's conception of a black hole drawing matter from a nearby star, forming an accretion disk.
Accretion disc
–
HH-30, a
Herbig–Haro object surrounded by an accretion disk
Accretion disc
–
Simulation by J.A. Marck of optical appearance of Schwarzschild black hole with thin (Keplerian) disk.
50.
Radio telescope
–
A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky in radio astronomy. Radio telescopes are typically large parabolic antennas similar to those employed in tracking and communicating with satellites and they may be used singly, or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night, Radio waves from space were first detected by engineer Karl Guthe Jansky in 1932 at Bell Telephone Laboratories in Holmdel, New Jersey using an antenna built to study noise in radio receivers. The first purpose-built radio telescope was a 9-meter parabolic dish constructed by radio amateur Grote Reber in his yard in Wheaton. The sky survey he did with it is considered the beginning of the field of radio astronomy. The first radio antenna used to identify an astronomical radio source was one built by Karl Guthe Jansky, Jansky was assigned the job of identifying sources of static that might interfere with radio telephone service. Janskys antenna was an array of dipoles and reflectors designed to receive short wave radio signals at a frequency of 20.5 MHz and it was mounted on a turntable that allowed it to rotate in any direction, earning it the name Janskys merry-go-round. It had a diameter of approximately 100 ft and stood 20 ft tall, by rotating the antenna, the direction of the received interfering radio source could be pinpointed. A small shed to the side of the antenna housed an analog recording system. Jansky finally determined that the faint hiss repeated on a cycle of 23 hours and 56 minutes and this period is the length of an astronomical sidereal day, the time it takes any fixed object located on the celestial sphere to come back to the same location in the sky. An amateur radio operator, Grote Reber, was one of the pioneers of what became known as radio astronomy and he built the first parabolic dish radio telescope, a 9 metres in diameter) in his back yard in Wheaton, Illinois in 1937. The range of frequencies in the spectrum that makes up the radio spectrum is very large. This means that the types of antennas that are used as radio telescopes vary widely in design, size, at wavelengths of 30 meters to 3 meters, they are generally either directional antenna arrays similar to TV antennas or large stationary reflectors with moveable focal points. Since the wavelengths being observed with these types of antennas are so long, at shorter wavelengths parabolic dish antennas predominate. The angular resolution of an antenna is determined by the ratio of the diameter of the dish to the wavelength of the radio waves being observed. This dictates the size a radio telescope needs for a useful resolution. Radio telescopes that operate at wavelengths of 3 meters to 30 cm are usually well over 100 meters in diameter, telescopes working at wavelengths shorter than 30 cm range in size from 3 to 90 meters in diameter. The Wilkinson Microwave Anisotropy Probe mapped the Cosmic microwave background radiation in 5 different frequency bands, centered on 23 GHz,33 GHz,41 GHz,61 GHz, the worlds largest filled-aperture radio telescope is the Five hundred meter Aperture Spherical Telescope completed in 2016 by China
Radio telescope
–
The 64 meter radio telescope at
Parkes Observatory
Radio telescope
–
Full-size replica of the first radio telescope, Jansky's dipole array now at the US
National Radio Astronomy Observatory
Radio telescope
–
A cylindrical paraboloid antenna.
Radio telescope
–
The
Very Large Array, an
interferometric array formed from many smaller telescopes, like many larger radio telescopes.
51.
X-Ray
–
X-radiation is a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz, X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. Spelling of X-ray in the English language includes the variants x-ray, xray, X-rays with high photon energies are called hard X-rays, while those with lower energy are called soft X-rays. Due to their ability, hard X-rays are widely used to image the inside of objects, e. g. in medical radiography. The term X-ray is metonymically used to refer to an image produced using this method. Since the wavelengths of hard X-rays are similar to the size of atoms they are useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air, the length of 600 eV X-rays in water is less than 1 micrometer. There is no consensus for a definition distinguishing between X-rays and gamma rays, one common practice is to distinguish between the two types of radiation based on their source, X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. This definition has problems, other processes also can generate these high-energy photons. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m and this criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. Occasionally, one term or the other is used in specific contexts due to precedent, based on measurement technique. Thus, gamma-rays generated for medical and industrial uses, for radiotherapy, in the ranges of 6–20 MeV. X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds and this makes it a type of ionizing radiation, and therefore harmful to living tissue. A very high radiation dose over a period of time causes radiation sickness. In medical imaging this increased risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be utilized in treatment to kill malignant cells using radiation therapy. It is also used for material characterization using X-ray spectroscopy, hard X-rays can traverse relatively thick objects without being much absorbed or scattered. For this reason, X-rays are widely used to image the inside of visually opaque objects, the most often seen applications are in medical radiography and airport security scanners, but similar techniques are also important in industry and research
X-Ray
–
Spectrum of the X-rays emitted by an X-ray tube with a
rhodium target, operated at 60
kV. The smooth, continuous curve is due to
bremsstrahlung, and the spikes are
characteristic K lines for rhodium atoms.
X-Ray
–
X-rays are part of the
electromagnetic spectrum, with wavelengths shorter than
visible light. Different applications use different parts of the X-ray spectrum.
X-Ray
–
A
chest radiograph of a female, demonstrating a
hiatus hernia
X-Ray
–
An arm radiograph, demonstrating broken
ulna and
radius with implanted
internal fixation.
. doi:10.1134/S1063772913020078. Russian original Малкин, З. М. (2013). "Об определении расстояния от Солнца до центра Галактики". Astronomicheskii Zhurnal (in Russian). 90 (2): 152–157. doi:10.7868/S0004629913020072.
