1.
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)
2.
Dorado
–
Dorado is a constellation in the southern sky. It was named in the late 16th century and is now one of the 88 modern constellations and its name refers to the dolphinfish, which is known as dorado in Portuguese, although it has also been depicted as a swordfish. Dorado contains most of the Large Magellanic Cloud, the remainder being in the constellation Mensa, the South Ecliptic pole also lies within this constellation. Its first depiction in an atlas was in Johann Bayers Uranometria of 1603 where it was also named Dorado. Dorado has been represented historically as a dolphinfish and a swordfish and it has also been represented as a goldfish. The constellation was known in the 17th and 18th century as Xiphias. The name Dorado ultimately become dominant and was adopted by the IAU, alpha Doradus is a blue-white star of magnitude 3.3,176 light-years from Earth. It is the brightest star in Dorado, Beta Doradus is a notably bright Cepheid variable star. It is a supergiant star that has a minimum magnitude of 4.1. One thousand and forty light-years from Earth, Beta Doradus has a period of 9 days and 20 hours, R Doradus is one of the many variable stars in Dorado. S Dor,9.721 hypergiant in the Large Magellanic Cloud, is the prototype of S Doradus variable stars, the variable star R Doradus 5.73 has the largest known apparent size of any star other than the Sun. Gamma Doradus is the prototype of the Gamma Doradus variable stars, supernova 1987A was the closest supernova to occur since the invention of the telescope. SNR 0509-67.5 is the remnant of an unusually energetic Type 1a supernova from about 400 years ago, HE 0437-5439 is a hypervelocity star escaping from the Milky Way/Magellanic Cloud system. Dorado is also the location of the South Ecliptic pole, which lies near the fishs head, the pole was called Polus Doradinalis by Willem Jansson Blaeu. Because Dorado contains part of the Large Magellanic Cloud, it is rich in deep sky objects, the Large Magellanic Cloud,1,000 light-years in diameter, is a satellite galaxy of the Milky Way Galaxy, located at a distance of 179,000 light-years. It has been deformed by its interactions with the larger Milky Way. In 1987, it became host to SN 1987A, the first supernova of 1987 and this 25, 000-light-year-wide galaxy contains over 10,000 million stars. All coordinates given are for Epoch J2000.0, NGC1566 is a face-on spiral galaxy
Dorado
–
The constellation Dorado as it can be seen by the naked eye.
Dorado
–
List of stars in Dorado
Dorado
–
NGC 1566 is an intermediate spiral galaxy.
Dorado
–
LEDA 89996 is a classic example of a spiral galaxy.
3.
Mensa (constellation)
–
Mensa is a small constellation in the Southern Celestial Hemisphere between Scorpius and Centaurus, one of twelve drawn up in the 18th century by French astronomer Nicolas Louis de Lacaille. Its name is Latin for table, though it originally depicted Table Mountain and was known as Mons Mensae. One of the 88 modern constellations, it covers a keystone-shaped wedge of sky stretching from approximately 4h to 7. 5h of right ascension, other than the south polar constellation of Octans, it is the most southerly of constellations. As a result, it is essentially unobservable from the Northern Hemisphere, one of the faintest constellations in the night sky, Mensa boasts no bright stars. Its brightest star, Alpha Mensae is barely visible in suburban skies, two star systems in Mensa have been found to have planets, and part of the Large Magellanic Cloud lies within the constellations borders. Initially known as Mons Mensae, Mensa was created by Nicolas Louis de Lacaille out of dim Southern Hemisphere stars in honor of Table Mountain, a South African mountain overlooking Cape Town. He recalled that the Magellanic clouds were sometimes known as Cape clouds, hence he made a table in the sky under the clouds. Lacaille had observed and catalogued 10,000 southern stars during a stay at the Cape of Good Hope. He devised 14 new constellations in uncharted regions of the Southern Celestial Hemisphere not visible from Europe, Mensa was the only constellation that did not honor an instrument that symbolised the Age of Enlightenment. John Herschel proposed shrinking the name to one word in 1844, although the stars of Mensa do not feature in any ancient mythology, the mountain it is named after has a rich mythology. Called Tafelberg in Dutch and German, the mesa has two neighboring mountains called Devils Peak and Lions Head, Table Mountain features in the mythology of the Cape of Good Hope, notorious for its storms—the explorer Bartolomeu Dias saw the mesa as a mythical anvil for storms. Another myth relating to its dangers comes from Sinbad the Sailor, Mensa is bordered by Dorado to the north, Hydrus to the northwest and west, Octans to the south, Chamaeleon to the east and Volans to the northeast. Covering 153.5 square degrees and 0. 372% of the night sky, the three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is Men. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of eight segments, the whole constellation is visible to observers south of latitude 5°N. Lacaille labelled eleven stars with Bayer designations Alpha through to Lambda, Gould later added Kappa, Mu, Nu, Xi and Pi Mensae. Stars as dim as these were not generally given designations, however, overall, there are 22 stars within the constellations borders brighter than or equal to apparent magnitude 6.5. Mensa contains no stars, with Alpha Mensae its brightest star with a barely visible apparent magnitude of 5.09. Alpha Mensae is a solar-type star 33.26 ±0.05 light-years from Earth, an infrared excess has been detected around this star, most likely indicating the presence of a circumstellar disk at a radius of over 147 AU
Mensa (constellation)
–
The constellation Mensa as it can be seen by the naked eye.
Mensa (constellation)
–
List of stars in Mensa
4.
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.
5.
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.
6.
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
7.
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).
8.
Apparent magnitude
–
The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
Apparent magnitude
–
Asteroid
65 Cybele and two stars, with their magnitudes labeled
Apparent magnitude
–
30 Doradus image taken by
ESO 's
VISTA. This
nebula has an apparent magnitude of 8.
9.
Galaxy morphological classification
–
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented, Hubble’s scheme divides galaxies into three broad classes based on their visual appearance, Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a disk, with stars forming a spiral structure, and a central concentration of stars known as the bulge. They are given the symbol S, roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB and these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, the de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. In particular, he argued that rings and lenses are important structural components of spiral galaxies, the de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. For example, a barred spiral galaxy with loosely wound arms. Visually, the de Vaucouleurs system can be represented as a version of Hubble’s tuning fork, with stage on the x-axis, family on the y-axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme, values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three stages, compact ellipticals, normal ellipticals and late types, lenticulars are similarly subdivided into early, intermediate and late types. Irregular galaxies can be of type magellanic irregulars or compact, the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan, together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy, the shape, real and apparent, thus, for example, the Andromeda Galaxy is classified as kS5. H
Galaxy morphological classification
–
Tuning-fork style diagram of the Hubble sequence
Galaxy morphological classification
–
The Hubble sequence throughout the universe's history.
Galaxy morphological classification
Galaxy morphological classification
–
NGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.
10.
Mass
–
In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States
Mass
–
Depiction of early
balance scales in the
Papyrus of Hunefer (dated to the
19th dynasty, ca. 1285 BC). The scene shows
Anubis weighing the heart of Hunefer.
Mass
–
The kilogram is one of the seven
SI base units and one of three which is defined ad hoc (i.e. without reference to another base unit).
Mass
–
Galileo Galilei (1636)
Mass
–
Distance traveled by a freely falling ball is proportional to the square of the elapsed time
11.
Solar mass
–
The solar mass is a standard unit of mass in astronomy, equal to approximately 1.99 ×1030 kilograms. It is used to indicate the masses of stars, as well as clusters, nebulae. It is equal to the mass of the Sun, about two kilograms, M☉ = ×1030 kg The above mass is about 332946 times the mass of Earth. Because Earth follows an orbit around the Sun, its solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. The value he obtained differs by only 1% from the modern value, the diurnal parallax of the Sun was accurately measured during the transits of Venus in 1761 and 1769, yielding a value of 9″. From the value of the parallax, one can determine the distance to the Sun from the geometry of Earth. The first person to estimate the mass of the Sun was Isaac Newton, in his work Principia, he estimated that the ratio of the mass of Earth to the Sun was about 1/28700. Later he determined that his value was based upon a faulty value for the solar parallax and he corrected his estimated ratio to 1/169282 in the third edition of the Principia. The current value for the parallax is smaller still, yielding an estimated mass ratio of 1/332946. As a unit of measurement, the solar mass came into use before the AU, the mass of the Sun has been decreasing since the time it formed. This occurs through two processes in nearly equal amounts, first, in the Suns core, hydrogen is converted into helium through nuclear fusion, in particular the p–p chain, and this reaction converts some mass into energy in the form of gamma ray photons. Most of this energy eventually radiates away from the Sun, second, high-energy protons and electrons in the atmosphere of the Sun are ejected directly into outer space as a solar wind. The original mass of the Sun at the time it reached the main sequence remains uncertain, the early Sun had much higher mass-loss rates than at present, and it may have lost anywhere from 1–7% of its natal mass over the course of its main-sequence lifetime. The Sun gains a small amount of mass through the impact of asteroids. However, as the Sun already contains 99. 86% of the Solar Systems total mass, M☉ G / c2 ≈1.48 km M☉ G / c3 ≈4.93 μs I. -J. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars
Solar mass
–
Internal structure
Solar mass
–
Size and mass of very large stars: Most massive example, the blue
Pistol Star (150 M ☉). Others are
Rho Cassiopeiae (40 M ☉),
Betelgeuse (20 M ☉), and
VY Canis Majoris (17 M ☉). The
Sun (1 M ☉) which is not visible in this thumbnail is included to illustrate the scale of example stars. Earth's orbit (grey), Jupiter's orbit (red), and Neptune's orbit (blue) are also given.
12.
Kiloparsecs
–
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
Kiloparsecs
–
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)
Kiloparsecs
–
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).
13.
Angular diameter
–
The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences it is called the angle and in optics it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side, Angular radius equals half the angular diameter. When D ≫ d, we have δ ≈ d / D, for practical use, the distinction is only significant for spherical objects that are relatively close, since the small-angle approximation holds for x ≪1, arcsin x ≈ arctan x ≈ x. Estimates of angular diameter may be obtained by holding the hand at right angles to an extended arm. In astronomy the sizes of objects in the sky are given in terms of their angular diameter as seen from Earth. Since these angular diameters are typically small, it is common to present them in arcseconds, an arcsecond is 1/3600th of one degree, and a radian is 180/ π degrees, so one radian equals 3600*180/ π arcseconds, which is about 206265 arcseconds. Therefore, the diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by. These objects have a diameter of one arcsecond, an object of diameter 725. The angular diameter of the Sun, from a distance of one light-year, is 0. 03″, the angular diameter 0. 03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth. Thus the angular diameter of the Sun is about 250,000 times that of Sirius, the angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A. The angular diameter of the Sun is about the same as that of the Moon, even though Pluto is physically larger than Ceres, when viewed from Earth Ceres has a much larger apparent size. While angular sizes measured in degrees are useful for larger patches of sky, we need much finer units when talking about the size of galaxies. The Moons motion across the sky can be measured in size, approximately 15 degrees every hour. A one-mile-long line painted on the face of the Moon would appear to us to be about one arc-second in length, in astronomy, it is typically difficult to directly measure the distance to an object. But the object may have a physical size and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the Angular diameter distance to distant objects as d ≡2 D tan . In non-Euclidean space, such as our universe, the angular diameter distance is only one of several definitions of distance
Angular diameter
–
Diagram for the formula of the angular diameter
Angular diameter
–
Angular diameter: the angle subtended by an object
14.
Degree (angle)
–
A degree, usually denoted by °, is a measurement of a plane angle, defined so that a full rotation is 360 degrees. It is not an SI unit, as the SI unit of measure is the radian. Because a full rotation equals 2π radians, one degree is equivalent to π/180 radians, the original motivation for choosing the degree as a unit of rotations and angles is unknown. One theory states that it is related to the fact that 360 is approximately the number of days in a year. Ancient astronomers noticed that the sun, which follows through the path over the course of the year. Some ancient calendars, such as the Persian calendar, used 360 days for a year, the use of a calendar with 360 days may be related to the use of sexagesimal numbers. The earliest trigonometry, used by the Babylonian astronomers and their Greek successors, was based on chords of a circle, a chord of length equal to the radius made a natural base quantity. One sixtieth of this, using their standard sexagesimal divisions, was a degree, Aristarchus of Samos and Hipparchus seem to have been among the first Greek scientists to exploit Babylonian astronomical knowledge and techniques systematically. Timocharis, Aristarchus, Aristillus, Archimedes, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minutes, eratosthenes used a simpler sexagesimal system dividing a circle into 60 parts. Furthermore, it is divisible by every number from 1 to 10 except 7 and this property has many useful applications, such as dividing the world into 24 time zones, each of which is nominally 15° of longitude, to correlate with the established 24-hour day convention. Finally, it may be the case more than one of these factors has come into play. For many practical purposes, a degree is a small enough angle that whole degrees provide sufficient precision. When this is not the case, as in astronomy or for geographic coordinates, degree measurements may be written using decimal degrees, with the symbol behind the decimals. Alternatively, the sexagesimal unit subdivisions can be used. One degree is divided into 60 minutes, and one minute into 60 seconds, use of degrees-minutes-seconds is also called DMS notation. These subdivisions, also called the arcminute and arcsecond, are represented by a single and double prime. For example,40. 1875° = 40° 11′ 15″, or, using quotation mark characters, additional precision can be provided using decimals for the arcseconds component. The older system of thirds, fourths, etc. which continues the sexagesimal unit subdivision, was used by al-Kashi and other ancient astronomers, but is rarely used today
Degree (angle)
–
One degree (shown in red) and eighty nine (shown in blue)
15.
Principal Galaxies Catalogue
–
The Catalogue of Principal Galaxies is an astronomical catalog published in 1989 that lists B1950 and J2000 equatorial coordinates and cross-identifications for 73,197 galaxies. It is based on the Lyon-Meudon Extragalactic Database, which was started in 1983. 40,932 coordinates have standard deviations smaller than 10″, a total of 131,601 names from the 38 most common sources are listed. The Lyon-Meudon Extragalactic Database was eventually expanded into HyperLEDA, a database of a few million galaxies, Galaxies in the original PGC catalogue are numbered with their original PGC number in HyperLEDA. Numbers have also assigned for the other galaxies, although for those galaxies not in the original PGC catalogue. PGC6240 is a lenticular galaxy in the constellation Hydrus. It is located about 106 million parsecs away from Earth, PGC39058 is a dwarf galaxy which is located approximately 14 million light years away in the constellation of Draco. It is relatively nearby, however it is obscured by a star which is in front of the galaxy. Astronomical catalogue PGC info at ESOs archive of astronomical catalogues PGC readme at Centre de Données astronomiques de Strasbourg
Principal Galaxies Catalogue
–
Hubble image of the
elliptical galaxy PGC 6240.
16.
Nubecula Major
–
The Magellanic Clouds are two irregular dwarf galaxies visible from the southern hemisphere, they are members of the Local Group and are orbiting the Milky Way galaxy. Because they both show signs of a bar structure, they are reclassified as Magellanic spiral galaxies. Collectively they were known to Māori of New Zealand as Nga Patari-Kaihau or as Te Reporepo and were used as predictors of winds. The Magellanic Clouds have been known since the first millennium in Western Asia, the first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi. In Europe, the Clouds were first observed by Italian explorers Peter Martyr dAnghiera, subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. However, naming the clouds after Magellan did not become widespread until much later, in Bayers Uranometria they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage. In Sri Lanka, from ancient times, these clouds have been referred to as the Maha Mera Paruwathaya meaning the great mountain, roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, the LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC, for comparison, the Milky Way is about 100,000 ly across. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other and their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk. Aside from their different structure and lower mass, they differ from our galaxy in two major ways, first, they are gas-rich, a higher fraction of their mass is hydrogen and helium compared to the Milky Way. They are also more metal-poor than the Milky Way, the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. Both are noted for their nebulae and young populations, but as in our own galaxy their stars range from the very young to the very old. The Large Magellanic Cloud was host galaxy to a supernova, the brightest observed in four centuries. Measurements with the Hubble Space Telescope, announced in 2006, suggest the Magellanic Clouds may be moving too fast to be long term companions of the Milky Way. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and they have dubbed this smaller remnant the Mini Magellanic Cloud
Nubecula Major
–
The Large and Small Magellanic Clouds
Nubecula Major
–
ALMA antennae bathed in red light. In the background there is the southern Milky Way on the left and the Magellanic Clouds at the top.
Nubecula Major
–
The
Large Magellanic Cloud (LMC)
Nubecula Major
–
Part of the
Small Magellanic Cloud (SMC)
17.
Galaxy
–
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380, See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, all of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major
Galaxy
–
NGC 4414, a typical spiral galaxy in the
constellation Coma Berenices, is about 55,000
light-years in diameter and approximately 60 million light-years away from Earth
Galaxy
–
A
fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark-sky location in Chile
Galaxy
–
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the
Andromeda Galaxy
Galaxy
–
NGC 3923 Elliptical Shell Galaxy-Hubble Space Telescope photograph
18.
List of galaxies
–
The following is a list of notable galaxies. There are about 51 galaxies in the Local Group, on the order of 100,000 in our Local Supercluster, the discovery of the nature of galaxies as distinct from other nebulae was made in the 1920s. In the 1980s, the Lyons Groups of Galaxies listed 485 galaxy groups with 3,933 member galaxies, there is no universal naming convention for galaxies, as they are mostly catalogued before it is established whether the object is or isnt a galaxy. This is a list of galaxies that are visible to the naked-eye, for at the very least, keen-eyed observers in a very dark-sky environment that is high in altitude, during clear and stable weather. Sagittarius Dwarf Spheroidal Galaxy is not listed, because it is not discernible as being a separate galaxy in the sky and this is a list of galaxies that became prototypes for a class of galaxies. MACS0647-JD, discovered in 2012, with z=10.7, does not appear on this list because it has not been confirmed with a spectroscopic redshift, uDFy-38135539, discovered in 2009, with z=8.6, does not appear on this list because its claimed redshift is disputed. Follow-up observations have failed to replicate the cited redshift measurement, a1689-zD1, discovered in 2008, with z=7.6, does not appear on this list because it has not been confirmed with a spectroscopic redshift. Abell 68 c1 and Abell 2219 c1, discovered in 2007, with z=9, iOK4 and IOK5, discovered in 2007, with z=7, do not appear on this list because they have not been confirmed with a spectroscopic redshift. Abell 1835 IR1916, discovered in 2004, with z=10.0, some follow-up observations have failed to find the object at all. STIS 123627+621755, discovered in 1999, with z=6.68, bR1202-0725 LAE, discovered in 1998 at z=5.64 does not appear on the list because it was not definitively pinned. BR1202-0725 refers to a quasar that the Lyman alpha emitting galaxy is near, the quasar itself lies at z=4.6947 BaasdR2237-0607 LA1 and BR2237-0607 LA2 were found at z=4.55 while investigating around the quasar BR2237-0607 in 1996. Neither of these appear on the list because they were not definitively pinned down at the time, the quasar itself lies at z=4.558 Two absorption dropouts in the spectrum of quasar BR 1202-07 were found, one in early 1996, another later in 1996. Neither of these appear on the list because they were not definitively pinned down at the time. The early one was at z=4.38, the one at z=4.687, the quasar itself lies at z=4.695 In 1986. However, its redshift was only determined in 1991, at z=2.237, by which time, it would no longer be the most distant galaxy known. An absorption drop was discovered in 1985 in the spectrum of quasar PKS 1614+051 at z=3.21 This does not appear on the list because it was not definitively fixed down. At the time, it was claimed to be the first non-QSO galaxy found beyond redshift 3. The quasar itself is at z=3.197 In 1975, 3C123 was incorrectly determined to lie at z=0.637 From 1964 to 1997 and that list is available at list of quasars
List of galaxies
–
Size (left) and distance (right) of a few well-known galaxies put to scale.
19.
Satellite galaxy
–
A satellite galaxy is a galaxy that orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a number of objects that are not connected to each other, it has a center of mass. This is similar to how an object has a center of mass which is the weighted average of the positions of all its component atoms. Even within a galaxy, the stars in turn orbit the galactic center. In a pair of orbiting galaxies, if one is larger than the other, then the larger is the primary. If two orbiting galaxies are about the size, then they are said to form a binary system. Galaxies which encounter one another from certain directions may interact, collide, merge, rip each other apart, in these situations, it can be difficult to tell where one galaxy ends and where another begins. Collisions between galaxies do not necessarily involve collisions between objects from one galaxy and objects from the other, since galaxies are composed of empty space. Rather, gravitational forces from nearby clouds of matter are capable of distorting one another, the largest satellite galaxy of the Milky Way is the Large Magellanic Cloud. Dwarf galaxy Galactic tide Interacting galaxy Spiral galaxy
Satellite galaxy
–
The
Andromeda Galaxy with its two closest satellite galaxies
M32, and
M110.
Satellite galaxy
–
The
Large Magellanic Cloud, the largest satellite galaxy, and fourth largest in the
Local Group.
Satellite galaxy
–
Satellite galaxies of the
Milky Way, our own galaxy.
20.
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.
21.
Sagittarius Dwarf Elliptical Galaxy
–
The Sagittarius Dwarf Spheroidal Galaxy, also known as the Sagittarius Dwarf Elliptical Galaxy, is an elliptical loop-shaped satellite galaxy of the Milky Way. It consists of four clusters, the main cluster having been discovered in 1994. In its looping, spiraling path, it has passed through the plane of the Milky Way several times in the past, Sgr dSph should not be confused with the Sagittarius Dwarf Irregular Galaxy, or the Sag DIG, a small galaxy 3.4 million light-years distant. Officially discovered in 1994, by Rodrigo Ibata, Mike Irwin, Sgr dSph appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor, as compared to the Milky Way. No neutral hydrogen gas related to Sgr dSph has been found, Sgr dSph has four known globular clusters with one, M54, apparently residing at its core. It is also linked to the young globular Terzan 7 as well as to Terzan 8. Additionally, Palomar 12 is now thought to also be associated with Sgr dSph as well as Whiting 1. Sgr dSph has multiple stellar populations, ranging in age from the oldest globular clusters to trace populations as young as several hundred million years and it also exhibits an age-metallicity relationship, in that its old populations are metal poor while its youngest populations have super-solar abundances. At first, many thought that Sgr dSph had already reached an advanced state of destruction. However, Sgr dSph still has coherence as a dispersed elongated ellipse, although it may have begun as a spherical object before falling towards the Milky Way, Sgr dSph is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be out in a long stellar stream along its path. However, some contend that Sgr dSph has been in orbit around the Milky Way for some billions of years. Its ability to retain some coherence despite such strains would indicate a high concentration of dark matter within that galaxy. In 1999, Johnston et al. concluded that Sgr dSph has orbited the Milky Way for at least one Gya and its orbit is found to have galactocentric distances that oscillate between ~13 and ~41 kpc with a period of 550 to 750 million years. The last perigalacticon was approximately fifty years ago. Also in 1999, Jiang & Binney found that it may have started its infall into the Milky Way at a point more than 200 kpc away if its mass was as large as ~1011M☉. The models of both its orbit and the Milky Ways potential field could be improved by proper motion observations of Sgr dSphs stellar debris and this issue is under intense investigation, with computational support by the MilkyWay@Home project. A simulation published in 2011 suggested that the Milky Way may have obtained its spiral structure as a result of repeated collisions with Sgr dSph, ids - Bibliography - Image - B&W Image
Sagittarius Dwarf Elliptical Galaxy
–
Palomar 12, believed to have been captured from the Sgr dSph about 1.7
Gya
Sagittarius Dwarf Elliptical Galaxy
–
Messier 54, believed to be at the core of Sgr dSph. Greyscale image created from the
HST's Advanced Camera for Surveys
Sagittarius Dwarf Elliptical Galaxy
–
Location
22.
Canis Major Dwarf Galaxy
–
The Canis Major Dwarf Galaxy or Canis Major Overdensity is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major. The supposed small galaxy contains a high percentage of red giants. It has an elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy. This structure is located closer to the Sun than the center of our galaxy, astronomers believe that the dwarf galaxy is in the process of being pulled apart by the gravitational field of the more massive Milky Way galaxy. The main body of the galaxy is extremely degraded, the stream of stars was first discovered in the early 21st century by astronomers conducting the Sloan Digital Sky Survey. NGC1261 is another cluster, but its velocity is different enough from that of the others to make its relation to the system unclear. The Canis Major Dwarf Galaxy may also have associated open clusters, including Dolidze 25 and H18 and it is thought that the open clusters may have formed due to the dwarf galaxys gravity perturbing material in the galactic disk and stimulating star formation. Martin et al. believe that the preponderance of evidence points to the accretion of a satellite galaxy of the Milky Way which was orbiting roughly in the plane of the galactic disk. Several studies cast doubts on the nature of this overdensity. Investigation of the area in 2009 yielded only ten RR Lyrae variable stars which is consistent with the Milky Ways halo, messier 79 Galaxy formation and evolution SEDS page on the Canis Major dwarf galaxy Canis Major Overdensity at the SIMBAD Astronomical Database
Canis Major Dwarf Galaxy
–
Location
23.
Galactic Center
–
The Galactic Center is the rotational center of the Milky Way. The estimates for its range from 7.6 to 8.7 kiloparsecs from Earth in the direction of the constellations Sagittarius, Ophiuchus. There is strong evidence consistent with the existence of a black hole at the Galactic Center of the Milky Way. Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, the available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths. In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime conditions in nearby Los Angeles to conduct a search for the center with the 100 inch Hooker Telescope. This gap has been known as Baades Window ever since, by 1954 they had built an 80 feet fixed dish antenna and used it to make a detailed study of an extended, extremely powerful belt of radio emission that was detected in Sagittarius. In 1958 the International Astronomical Union decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of latitude and longitude. In the equatorial system the location is, RA 17h 45m 40. 04s. The exact distance between the Solar System and the Galactic Center is not certain, although estimates since 2000 have remained within the range 7. 2–8.8 kpc. The nature of the Milky Ways bar, which extends across the Galactic Center, is also debated, with estimates for its half-length. Certain authors advocate that the Milky Way features two bars, one nestled within the other. The bar is delineated by red-clump stars, however, RR Lyr variables do not trace a prominent Galactic bar. The bar may be surrounded by a called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way, accretion of gas onto the black hole, probably involving a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments, a study in 2008 which linked radio telescopes in Hawaii, Arizona and California measured the diameter of Sagittarius A* to be 44 million kilometers. For comparison, the radius of Earths orbit around the Sun is about 150 million kilometers, thus the diameter of the radio source is slightly less than the distance from Mercury to the Sun.3 million solar masses. On 5 January 2015, NASA reported observing an X-ray flare 400 times brighter than usual, the central cubic parsec around Sagittarius A* contains around 10 million stars. Although most of them are old red giant stars, the Galactic Center is also rich in massive stars, more than 100 OB and Wolf–Rayet stars have been identified there so far
Galactic Center
–
The Galactic Center, as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image.
Galactic Center
–
There is a supermassive black hole in the bright white area to the right of the center of image. This composite photograph covers about half of a degree.
Galactic Center
–
Artist impression of a supermassive black hole at the centre of a galaxy.
Galactic Center
–
Galactic Center of the
Milky Way and a meteor.
24.
Local Group
–
The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies and its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group covers a diameter of 10 Mly and has a binary distribution, the group itself is a part of the larger Virgo Supercluster, which in turn may be a part of the Laniakea Supercluster. The three largest members of the group are the Andromeda Galaxy, the Milky Way and the Triangulum Galaxy, the larger two of these spiral galaxies each have their own system of satellite galaxies. The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy, pisces Dwarf is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either. The membership of NGC3109, with its companions Sextans A, the term The Local Group was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. He also identified IC10 as a possible Local Group member, by 2003, the number of known Local Group members had increased from his initial 12 to 36. Smiths Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, updated Information on the Local Group. The Publications of the Astronomical Society of the Pacific
Local Group
–
Distribution of the
iron content (in logarithmic scale) in four dwarf neighbouring galaxies of the Milky Way
Local Group
–
The Local Group of galaxies. The
Milky Way and
Andromeda are the most massive galaxies by far. (Click to enlarge)
Local Group
–
A diagram of our location in the
observable universe. (Click here for smaller image.)
Local Group
–
Location
25.
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
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The Andromeda Galaxy
Andromeda Galaxy
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Great Andromeda Nebula by
Isaac Roberts, 1899
Andromeda Galaxy
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M31 above the
Very Large Telescope.
Andromeda Galaxy
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The Andromeda Galaxy as seen by
NASA 's
Wide-field Infrared Survey Explorer
26.
Triangulum Galaxy
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The Triangulum Galaxy is a spiral galaxy approximately 3 million light-years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC598, and is informally referred to as the Pinwheel Galaxy. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Milky Way and it is one of the most distant permanent objects that can be viewed with the naked eye. It also has an H-II nucleus, the galaxy gets its name from the constellation Triangulum, which is where it can be spotted. The Triangulum Galaxy is sometimes referred to as the Pinwheel Galaxy by some amateur astronomy references. Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen with the naked eye and it is one of the most distant permanent objects that can be viewed without the aid of a telescope. Being a diffuse object, its visibility is affected by small amounts of light pollution. It ranges from easily visible by direct vision in dark skies to a difficult averted vision object in rural or suburban skies, for this reason, Triangulum is one of the critical sky marks of the Bortle Dark-Sky Scale. The Triangulum Galaxy was probably discovered by the Italian astronomer Giovanni Battista Hodierna before 1654, in his work De systemate orbis cometici, deque admirandis coeli caracteribus, he listed it as a cloud-like nebulosity or obscuration and gave the cryptic description, near the Triangle hinc inde. This is in reference to the constellation of Triangulum as a pair of triangles, the magnitude of the object matches M33, so it is most likely a reference to the Triangulum galaxy. The galaxy was discovered by Charles Messier on the night of August 25–26,1764. It was published in his Catalog of Nebulae and Star Clusters as object number 33, when William Herschel compiled his extensive catalogue of nebulae, he was careful not to include most of the objects identified by Messier. However, M33 was an exception and he catalogued this object on September 11,1784 as H V-17, Herschel also catalogued the Triangulum Galaxys brightest and largest H II region as H III.150 separately from the galaxy itself, the nebula eventually obtained NGC number 604. As seen from Earth, NGC604 is located northeast of the central core. It is one of the largest H II regions known, with a diameter of nearly 1500 light-years, Herschel also noted 3 other smaller H II regions. It was among the first spiral nebulae identified as such by Lord Rosse in 1850, in 1922–23, John Charles Duncan and Max Wolf discovered variable stars in the nebulae. Edwin Hubble showed in 1926 that 35 of these stars were classical Cepheids, the results were consistent with the concept of spiral nebulae being independent galactic systems of gas and dust, rather than just nebulae in the Milky Way. With a diameter of about 60,000 light-years, the Triangulum galaxy is the third largest member of the Local Group of galaxies and it may be a gravitationally bound companion of the Andromeda Galaxy
Triangulum Galaxy
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Triangulum Galaxy – Messier 33
Triangulum Galaxy
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NGC 604 in the Triangulum Galaxy
Triangulum Galaxy
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Triangulum Galaxy taken by VLT Survey Telescope.
Triangulum Galaxy
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Infrared image of M33 taken with the
Spitzer Space Telescope
27.
Magellanic spiral
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Magellanic spiral galaxies are dwarf galaxies which are classified as the type Sm. They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud and they can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies. SAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy, sABm are a type of intermediate spiral galaxy. Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure, Sm galaxies are typically disrupted and asymmetric. DSm galaxies are spiral galaxies or dwarf irregular galaxies, depending on categorization scheme. The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular, Large Magellanic Cloud Small Magellanic Cloud NGC1311 NGC4618 NGC4236 NGC55 NGC4214 NGC3109 NGC4625 NGC5713 NGC5204 NGC2552 Galaxy morphological classification
Magellanic spiral
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Large Magellanic Cloud
28.
Dwarf spiral galaxy
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A dwarf spiral galaxy is the dwarf version of a spiral galaxy. Dwarf galaxies are characterized as having low luminosities, small diameters, low surface brightnesses, the galaxies may be considered a subclass of low-surface-brightness galaxies. Dwarf spiral galaxies, particularly the dwarf counterparts of Sa-Sc type spiral galaxies, are quite rare, in contrast, dwarf elliptical galaxies, dwarf irregular galaxies, and the dwarf versions of Magellanic type galaxies are very common. It is suggested that dwarf spiral galaxies can transform into dwarf elliptical galaxies, NGC5474 NGC5204 NGC247 NGC6503 NGC3928 NGC625 NGC1051 NGC1311 NGC2188 IC4710 Sculptor Dwarf Galaxy Most identified dwarf spiral galaxies are located outside clusters. Strong gravitational interactions between galaxies and interactions between galaxies and intracluster gas are expected to destroy the disks of most dwarf spiral galaxies, nonetheless, dwarf galaxies with spiral-like structure have been identified within the Virgo Cluster and Coma Cluster
Dwarf spiral galaxy
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NGC 5474 an example of a dwarf spiral galaxy
29.
Small Magellanic Cloud
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The Small Magellanic Cloud, or Nebucula Minor, is a dwarf galaxy near the Milky Way. It is classified as an irregular galaxy. It has a diameter of about 7,000 light-years, contains several hundred million stars, the SMC contains a central bar structure and it is speculated that it was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. At a distance of about 200,000 light-years, it is one of the Milky Ways nearest neighbors and it is also one of the most distant objects that can be seen with the naked eye. With a mean declination of approximately −73 degrees, it can only be viewed from the Southern Hemisphere, since it has a very low surface brightness, it is best viewed from a dark site away from city lights. It forms a pair with the Large Magellanic Cloud, which lies a further 20 degrees to the east, in the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south sea islanders and indigenous Australians. Persian astronomer Al Sufi labelled the larger of the two clouds as Al Bakr, the White Ox, european sailors may have first noticed the clouds during the Middle Ages when they were used for navigation. Portuguese and Dutch sailors called them the Cape Clouds, a name that was retained for several centuries, during the circumnavigation of the Earth by Ferdinand Magellan in 1519–22, they were described by Antonio Pigafetta as dim clusters of stars. In Johann Bayers celestial atlas Uranometria, published in 1603, he named the smaller cloud, in Latin, Nubecula means a little cloud. Between 1834 and 1838, John Frederick William Herschel made observations of the skies with his 14-inch reflector from the Royal Observatory at the Cape of Good Hope. While observing the Nubecula Minor, he described it as a mass of light with an oval shape. Within the area of this cloud he catalogued a concentration of 37 nebulae, in 1891, Harvard College Observatory opened an observing station at Arequipa in Peru. Between 1893 and 1906, under the direction of Solon Bailey, henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study the variations in relative luminosity of stars in the SMC. This important period-luminosity relation allowed the distance to any other variable to be estimated in terms of the distance to the SMC. Hence, once the distance to the SMC was known with greater accuracy, using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt and this later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique. Announced in 2006, measurements with the Hubble Space Telescope suggest the Large, there is a bridge of gas connecting the Small Magellanic Cloud with the Large Magellanic Cloud, which is evidence of tidal interaction between the galaxies
Small Magellanic Cloud
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Small Magellanic Cloud. Source: Digitized Sky Survey 2
Small Magellanic Cloud
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Panoramic Large and Small Magellanic Clouds as seen from
ESO 's
VLT observation site. The galaxies are on the left side of the image.
Small Magellanic Cloud
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Location
30.
Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
Moon
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Full moon as seen from Earth's
northern hemisphere
Moon
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The Moon, tinted reddish, during a
lunar eclipse
Moon
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Near side of the Moon
Moon
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Far side of the Moon
. doi:10.1038/nature11878.
