1.
Luminosity
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In astronomy, luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time. It is related to the brightness, which is the luminosity of an object in a spectral region. In SI units luminosity is measured in joules per second or watts, values for luminosity are often given in the terms of the luminosity of the Sun, which has a total power output of 7026384600000000000♠3. 846×1026 W. The symbol for solar luminosity is L⊙. Luminosity can also be given in terms of magnitude, the absolute bolometric magnitude of an object is a logarithmic measure of its total energy emission. In astronomy, luminosity is the amount of energy a body radiates per unit of time. It is most frequently measured in two forms, visual and bolometric, although luminosities at other wavelengths are increasingly being used as instruments become available to measure them, a bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. When not qualified, the term luminosity means bolometric luminosity, which is measured either in the SI units, watts, a star also radiates neutrinos, which carry off some energy, contributing to the stars total luminosity. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths, a stars luminosity can be determined from two stellar characteristics, size and effective temperature. The former is represented in terms of solar radii, R⊙, while the latter is represented in kelvins. To determine a stars radius, two metrics are needed, the stars angular diameter and its distance from Earth, often calculated using parallax. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty, an alternate way to measure stellar luminosity is to measure the stars apparent brightness and distance. Because luminosity is proportional to temperature to the power, the large variation in stellar temperatures produces an even vaster variation in stellar luminosity. Because the luminosity depends on a power of the stellar mass. The most luminous stars are young stars, no more than a few million years for the most extreme. In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the sequence with blue Class 0 stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are found above and to the right of the main sequence, blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars. A star like Deneb, for example, has a luminosity around 200,000 L⊙, a type of A2
2.
Active galactic nucleus
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Such excess non-stellar emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy, the radiation from an AGN is believed to be a result of accretion of matter by a supermassive black hole at the center of its host galaxy. Numerous subclasses of AGN have been defined based on their observed characteristics, early photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, in 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of study included NGC1068, NGC4151, NGC3516. Active galaxies such as these are known as Seyfert galaxies in honor of Seyferts pioneering work, the development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87, the 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, a major breakthrough was the measurement of the redshift of the quasar 3C273 by Maarten Schmidt, published in 1963. Shortly afterward, optical spectra were used to measured the redshifts of a number of quasars including 3C48. The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars, Accretion of gas onto a supermassive black hole was suggested as the source of quasars power in papers by Edwin Salpeter and Yakov ZelDovich in 1964. Today, AGN are a topic of astrophysical research, both observational and theoretical. For a long time it has argued that an AGN must be powered by accretion of mass onto massive black holes. AGN are both compact and persistently extremely luminous, thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to a black hole forms an accretion disc, dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The radiation from the accretion disc excites cold atomic material close to the black hole, a large fraction of the AGNs radiation may be obscured by interstellar gas and dust close to the accretion disc, but this will be re-radiated at some other waveband, most likely the infrared. Some accretion discs produce jets of twin, highly collimated, the direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low, there exists a class of radiatively inefficient solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow, radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc
3.
Galaxy
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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
4.
Supermassive black hole
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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
5.
Solar mass
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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
6.
Accretion disk
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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
7.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
8.
Electromagnetic radiation
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In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, light, ultraviolet, X-, classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to other and perpendicular to the direction of energy and wave propagation. The wavefront of electromagnetic waves emitted from a point source is a sphere, the position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves are produced whenever charged particles are accelerated, and these waves can interact with other charged particles. EM waves carry energy, momentum and angular momentum away from their source particle, quanta of EM waves are called photons, whose rest mass is zero, but whose energy, or equivalent total mass, is not zero so they are still affected by gravity. Thus, EMR is sometimes referred to as the far field, in this language, the near field refers to EM fields near the charges and current that directly produced them, specifically, electromagnetic induction and electrostatic induction phenomena. In the quantum theory of electromagnetism, EMR consists of photons, quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation. The energy of a photon is quantized and is greater for photons of higher frequency. This relationship is given by Plancks equation E = hν, where E is the energy per photon, ν is the frequency of the photon, a single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light. The effects of EMR upon chemical compounds and biological organisms depend both upon the power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules, the effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons. In contrast, high ultraviolet, X-rays and gamma rays are called ionizing radiation since individual photons of high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the speed of light. Maxwell’s equations were confirmed by Heinrich Hertz through experiments with radio waves, according to Maxwells equations, a spatially varying electric field is always associated with a magnetic field that changes over time. Likewise, a varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the field are always accompanied by a wave in the magnetic field in one direction
9.
Electromagnetic spectrum
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The electromagnetic spectrum is the collective term for all known frequencies and their linked wavelengths of the known photons. The electromagnetic spectrum of an object has a different meaning, and is instead the characteristic distribution of radiation emitted or absorbed by that particular object. Visible light lies toward the end, with wavelengths from 400 to 700 nanometres. The limit for long wavelengths is the size of the universe itself, until the middle of the 20th century it was believed by most physicists that this spectrum was infinite and continuous. Nearly all types of radiation can be used for spectroscopy, to study. Other technological uses are described under electromagnetic radiation, for most of history, visible light was the only known part of the electromagnetic spectrum. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, the study of light continued, and during the 16th and 17th centuries conflicting theories regarded light as either a wave or a particle. The first discovery of electromagnetic radiation other than visible light came in 1800 and he was studying the temperature of different colors by moving a thermometer through light split by a prism. He noticed that the highest temperature was beyond red and he theorized that this temperature change was due to calorific rays that were a type of light ray that could not be seen. The next year, Johann Ritter, working at the end of the spectrum. These behaved similarly to visible light rays, but were beyond them in the spectrum. They were later renamed ultraviolet radiation, during the 1860s James Maxwell developed four partial differential equations for the electromagnetic field. Two of these equations predicted the possibility of, and behavior of, analyzing the speed of these theoretical waves, Maxwell realized that they must travel at a speed that was about the known speed of light. This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave, maxwells equations predicted an infinite number of frequencies of electromagnetic waves, all traveling at the speed of light. This was the first indication of the existence of the electromagnetic spectrum. Maxwells predicted waves included waves at very low compared to infrared. Hertz found the waves and was able to infer that they traveled at the speed of light, Hertz also demonstrated that the new radiation could be both reflected and refracted by various dielectric media, in the same manner as light. For example, Hertz was able to focus the waves using a lens made of tree resin, in a later experiment, Hertz similarly produced and measured the properties of microwaves
10.
Radio astronomy
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Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an object was in 1932. Subsequent observations have identified a number of different sources of radio emission and these include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources, in the 1860s, James Clerk Maxwells equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. These attempts were unable to detect any emission due to limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions, using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun exactly and he concluded that since the Sun were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy. Jansky announced his discovery in 1933 and he wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of astronomy have been recognized by the naming of the fundamental unit of flux density. Grote Reber was inspired by Janskys work, and built a radio telescope 9m in diameter in his backyard in 1937. He began by repeating Janskys observations, and then conducted the first sky survey in the radio frequencies, on February 27,1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year George Clark Southworth, at Bell Labs like Jansky, both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark, at Cambridge University, where ionospheric research had taken place during World War II, J. A. This early research soon branched out into the observation of celestial radio sources. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis, the radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s
11.
Infrared astronomy
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Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers, Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves. Infrared astronomy began in the 1830s, a few decades after the discovery of infrared light by William Herschel in 1800. Early progress was limited, and it was not until the early 20th century that conclusive detections of astronomical objects other than the Sun, both fields also use solid state detectors, though the specific type of solid state detectors used are different. Infrared light is absorbed at many wavelengths by water vapor in the Earths atmosphere, so most infrared telescopes are at elevations in dry places. There are also infrared observatories in space, including the Spitzer Space Telescope, the discovery of infrared radiation is attributed to William Herschel, who performed an experiment where he placed a thermometer in sunlight of different colors after it passed through a prism. He noticed that the temperature increase induced by sunlight was highest outside the visible spectrum and he dubbed this radiation calorific rays, and went on to show that it could be reflected, transmitted, and absorbed just like visible light. Efforts were made starting in the 1830s and continuing through the 19th century to detect infrared radiation from astronomical sources. Radiation from the Moon was first detected in 1873 by William Parsons, ernest Fox Nichols used a modified Crookes radiometer in an attempt to detect infrared radiation from Arcturus and Vega, but Nichols deemed the results inconclusive. The field was mostly neglected by traditional astronomers though until the 1960s, Infrared radiation with wavelengths just longer than visible light, known as near-infrared, behaves in a very similar way to visible light, and can be detected using similar solid state devices. For this reason, the infrared region of the spectrum is commonly incorporated as part of the optical spectrum. Many optical telescopes, such as those at Keck Observatory, operate effectively in the infrared as well as at visible wavelengths. The far-infrared extends to submillimeter wavelengths, which are observed by such as the James Clerk Maxwell Telescope at Mauna Kea Observatory. Like all other forms of radiation, infrared is utilized by astronomers to study the universe. Indeed, infrared measurements taken by the 2MASS and WISE astronomical surveys have been effective at unveiling previously undiscovered star clusters. Examples of such embedded star clusters are FSR1424, FSR1432, Camargo 394, Camargo 399, Majaess 30, and Majaess 99. Infrared telescopes, which includes most major optical telescopes as well as a few dedicated infrared telescopes, need to be chilled with liquid nitrogen, the reason for this is that objects with temperatures of a few hundred Kelvin emit most of their thermal energy at infrared wavelengths. If infrared detectors were not kept cooled, the radiation from the detector itself would contribute noise that would dwarf the radiation from any celestial source and this is particularly important in the mid-infrared and far-infrared regions of the spectrum
12.
Visible-light astronomy
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Visible-light astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. An example of spectroscopy is the study of lines to understand of what kind of matter light is going through. Visible astronomy also includes looking up at night and he later made improved versions with up to about 30x magnification. With a Galilean telescope the observer could see magnified, upright images on the earth—it was what is known as a terrestrial telescope or a spyglass. He could also use it to observe the sky, for a time he was one of those who could construct telescopes good enough for that purpose. On 25 August 1609, he demonstrated one of his telescopes, with a magnification of about 8 or 9. His telescopes were also a sideline for Galileo selling them to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March 1610 in a treatise entitled Sidereus Nuncius. The visibility of objects in the night sky is affected by light pollution. The presence of the Moon in the sky has historically hindered astronomical observation by increasing the amount of ambient lighting. With the advent of light sources, however, light pollution has been a growing problem for viewing the night sky. List of largest optical reflecting telescopes
13.
Ultraviolet astronomy
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An ultraviolet light is not visible to the human eye. Light at these wavelengths is absorbed by the Earths atmosphere, so observations at these wavelengths must be performed from the atmosphere or from space. Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of the medium. UV observations can provide essential information about the evolution of galaxies. The ultraviolet Universe looks quite different from the stars and galaxies seen in visible light. Most stars are relatively cool objects emitting much of their electromagnetic radiation in the visible or near-infrared part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early, in the Earths sky seen in ultraviolet light, most stars would fade in prominence. Some very young stars and some very old stars and galaxies, growing hotter and producing higher-energy radiation near their birth or death. Clouds of gas and dust would block the vision in many directions along the Milky Way, charles Stuart Bowyer is generally given credit for starting this field
14.
X-ray astronomy
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X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earths atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy is the science related to a type of space telescope that can see farther than standard light-absorption telescopes, such as the Mauna Kea Observatories. X-ray emission is expected from astronomical objects that contain extremely hot gasses at temperatures from about a million kelvin to hundreds of millions of kelvin. Although X-rays have been observed emanating from the Sun since the 1940s and this source is called Scorpius X-1, the first X-ray source found in the constellation Scorpius. The X-ray emission of Scorpius X-1 is 10,000 times greater than its visual emission, in addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. Based on discoveries in this new field of X-ray astronomy, starting with Scorpius X-1 and it is now known that such X-ray sources as Sco X-1 are compact stars, such as neutron stars or black holes. Material falling into a hole may emit X-rays, but the black hole itself does not. The energy source for the X-ray emission is gravity, infalling gas and dust is heated by the strong gravitational fields of these and other celestial objects. Many thousands of X-ray sources are known, in addition, the space between galaxies in galaxy clusters is filled with a very hot, but very dilute gas at a temperature between 10 and 100 megakelvins. The total amount of hot gas is five to ten times the mass in the visible galaxies. The first sounding rocket flights for X-ray research were accomplished at the White Sands Missile Range in New Mexico with a V-2 rocket on January 28,1949. A detector was placed in the nose section and the rocket was launched in a suborbital flight to an altitude just above the atmosphere. X-rays from the Sun were detected by the U. S. Naval Research Laboratory Blossom experiment on board, an Aerobee 150 rocket was launched on June 12,1962 and it detected the first X-rays from other celestial sources. The largest drawback to rocket flights is their short duration. A rocket launched from the United States will not be able to see sources in the southern sky, in astronomy, the interstellar medium is the gas and cosmic dust that pervade interstellar space, the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the intergalactic medium. The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, the energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field
15.
Gamma-ray astronomy
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Gamma-ray astronomy is the astronomical observation of gamma rays, the most energetic form of electromagnetic radiation, with photon energies above 100 keV. Radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy, the highest photon energies measured to date are in the TeV range, the record being held by the Crab Pulsar in 2004, yielding photons with as much as 80 TeV. Observation of gamma rays first became possible in the 1960s, very energetic gamma rays, with photon energies over ~30 GeV, can also be detected by ground based experiments. The extremely low photon fluxes at such high energies require detector effective areas that are large for current space-based instruments. The Imaging Atmospheric Cherenkov Telescope technique currently achieves the highest sensitivity, Gamma radiation in the TeV range emanating from the Crab Nebula was first detected in 1989 by the Whipple Observatory at Mt. Hopkins, in Arizona in the USA. Modern Cherenkov telescope experiments like H. E. S. S, VERITAS, MAGIC, and CANGAROO III can detect the Crab Nebula in a few minutes. The most energetic photons observed from an extragalactic object originate from the blazar and these measurements were done by the High-Energy-Gamma-Ray Astronomy air Cherenkov telescopes. Gamma-ray astronomy observations are limited by non-gamma-ray backgrounds at lower energies. Larger area detectors and better background suppression are essential for progress in the field, a discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV, the index of refraction starts to increase again, long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing them. Work by Eugene Feenberg and Henry Primakoff in 1948, Sachio Hayakawa and these processes included cosmic ray interactions with interstellar gas, supernova explosions, and interactions of energetic electrons with magnetic fields. However, it was not until the 1960s that our ability to detect these emissions came to pass. The first gamma-ray telescope carried into orbit, on the Explorer 11 satellite in 1961 and they appeared to come from all directions in the Universe, implying some sort of uniform gamma-ray background. Such a background would be expected from the interaction of cosmic rays with interstellar gas, the first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. These first gamma-ray line observations were from OSO-3, OSO-7, and the Solar Maximum Mission, the solar observations inspired theoretical work by Reuven Ramaty and others. Significant gamma-ray emission from our galaxy was first detected in 1967 by the detector aboard the OSO-3 satellite and it detected 621 events attributable to cosmic gamma rays. However, the field of gamma-ray astronomy took great leaps forward with the SAS-2 and these two satellites provided an exciting view into the high-energy universe. They confirmed the findings of the gamma-ray background, produced the first detailed map of the sky at gamma-ray wavelengths
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Milky Way
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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
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Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The HST was built by the United States space agency NASA, the Space Telescope Science Institute selects Hubbles targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923, Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, the fifth mission was canceled on safety grounds following the Columbia disaster. However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal
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Interacting galaxy
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Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of an interaction is a satellite galaxys disturbing the primary galaxys spiral arms. An example of an interaction is a galactic collision, which may lead to a galaxy merger. A giant galaxy interacting with its satellites is common and that can possibly trigger a small amount of star formation. Colliding galaxies are common during galaxy evolution, the extremely tenuous distribution of matter in galaxies means these are not collisions in the traditional sense of the word, but rather gravitational interactions. Colliding may lead to merging if two collide and do not have enough momentum to continue traveling after the collision. In that case, they fall back into other and eventually merge into one galaxy after many passes through each other. If one of the galaxies is much larger than the other. The larger galaxy will look much the same, while the smaller galaxy will be stripped apart, when galaxies pass through each other, unlike during mergers, they largely retain their material and shape after the pass. Dynamical friction slows the relative motion galaxy pairs, which may merge at some point. The most common result of the merger between two or more galaxies is an irregular galaxy, but elliptical galaxies may also result. It has been suggested that galactic cannibalism is currently occurring between the Milky Way and the Large and Small Magellanic Clouds, streams of gravitationally-attracted hydrogen arcing from these dwarf galaxies to the Milky Way is taken as evidence for the theory. The result would be the conversion of low-luminosity spiral galaxies into dwarf spheroidals, evidence for the hypothesis has been found by studying early-type dwarf galaxies of the Virgo Cluster and finding structures, such as disks and spiral arms. That suggests they are former disk systems, transformed by the mentioned interactions, astronomers have estimated the Milky Way galaxy, will collide with the Andromeda galaxy in about 4.5 billion years. It is thought that the two galaxies will eventually merge to become an elliptical galaxy or perhaps a large disk galaxy. NGC7318 Galaxy Collisions Galactic cannibalism Galactic Collision Simulation GALMER, Galaxy Merger Simulations
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Galaxy merger
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Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction, the exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the rate is fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time, during the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, the gravitational potential begins changing so quickly that star orbits are greatly affected and this process is called violent relaxation. Thus if two galaxies collide, they begin with their stars in an orderly rotation in the plane of the disk. During the merger, the motion is transformed into random energy. The resultant galaxy is dominated by stars that orbit the galaxy in a complex and this is what we see in elliptical galaxies, stars on random unordered orbits. Mergers are also locations of extreme amounts of star formation, the star formation rate during a major merger can reach thousands of solar masses worth of new stars each year, depending on the gas content of each galaxy and its redshift. Typical merger SFRs are less than 100 new solar masses per year and this is large compared to our Galaxy, which makes only a few new stars each year. Though stars almost never get enough to actually collide in galaxy mergers. These collisions then induce condensations of these clouds into new stars and we can see this phenomenon in merging galaxies in the nearby universe. Yet, this process was pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1-10 billion years ago. Also, away from the center of the gas clouds will run into each other producing shocks which stimulate the formation of new stars in gas clouds. The result of all violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a merger, and then a few billion years pass. This is what we see in elliptical galaxies, very little molecular gas. Galaxy mergers can be simulated in computers, to more about galaxy formation
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Astrophysical jet
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An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the approaches the speed of light. One possible explanation is that magnetic fields aim two diametrically opposing beams away from the central source by angles only several degrees wide. According to another hypothesis, the jets are due to an effect in general relativity known as frame-dragging, most of the largest and most active jets are created by supermassive black holes in the centre of active galaxies such as quasars and radio galaxies or within galaxy clusters. Such jets can exceed millions of parsecs in length, other astronomical objects that contain jets include cataclysmic variable stars, X-ray binaries and Gamma ray bursters. Others are associated with star forming regions including T Tauri stars and Herbig–Haro objects, bipolar outflows or jets may also be associated with protostars, or with evolved post-AGB stars, planetary nebulae and bipolar nebulae. Relativistic jets are beams of ionised matter accelerated close to the speed of light, most have been observationally associated with central black holes of some active galaxies, radio galaxies or quasars, and also by galactic stellar black holes, neutron stars or pulsars. Beam lengths may extend several thousand, hundreds of thousands or millions of parsecs. Jet velocities when approaching the speed of light show significant effects of the Special Theory of Relativity, for example, massive central black holes in galaxies have the most powerful jets, but their structure and behaviours are similar to those of smaller galactic neutron stars and black holes. These SMBH systems are often called microquasars and show a range of velocities. SS433 jet, for example, has a velocity of 0. 23c, relativistic jet formation may also explain observed gamma-ray bursts. Notably, even weaker and less relativistic jets may be associated with many binary systems, because of the enormous amount of energy needed to launch a relativistic jet, some jets are thought to be powered by spinning black holes. There are two theories for how energy is transferred from a black hole to a jet. This theory explains the extraction of energy from magnetic fields around an accretion disk, relativistic material is then feasibly launched by the tightening of the field lines. Jets may also be observed from spinning neutron stars, an example is pulsar IGR J11014-6103, which has the largest jet so far observed in the Milky Way Galaxy whose velocity is estimated at 80% the speed of light. X-ray observations have been obtained but there is no detected radio signature nor accretion disk, initially, this pulsar was presumed to be rapidly spinning but later measurements indicate the spin rate is only 15.9 Hz. Hubble Video Shows Shock Collision inside Black Hole Jet
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Extinction (astronomy)
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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
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Cosmic dust
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Cosmic dust, or Extraterrestrial dust, is dust which exists in outer space, as well as all over planet Earth. Most cosmic dust particles are between a few molecules to 0.1 µm in size, a smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left the stars. It is called stardust and is included in a section below. The dust density falling to Earth is approximately 10−6/m3 with each grain having a mass between 10−16kg and 10−4 kg, cosmic dust can be further distinguished by its astronomical location, intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust. In the Solar System, interplanetary dust causes the zodiacal light, sources of Solar System dust include comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. The terminology has no application for describing materials found on the planet Earth except for dust that has demonstrably fallen to Earth. By one estimate, as much as 40,000 tons of cosmic dust reaches the Earths surface every year, in October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. In August 2014, scientists announced the collection of interstellar dust particles from the Stardust spacecraft since returning to Earth in 2006. In March 2017, scientists reported that extraterrestrial dust particles have been identified all over planet Earth. According to one of the researchers, “Once I knew what to look for, I found them everywhere. ”Cosmic dust was once solely an annoyance to astronomers, when infrared astronomy began, those dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of the Solar System, for example, cosmic dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In the Solar System, dust plays a role in the zodiacal light, Saturns B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune. These disparate research areas can be linked by the following theme, the astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the Universes complicated recycling steps. Parameters such as the initial motion, material properties, intervening plasma. Slightly changing any of these parameters can give significantly different dust dynamical behavior, therefore, one can learn about where that object came from, and what is the intervening medium. Cosmic dust can be detected by indirect methods utilizing the properties of cosmic dust. Cosmic dust can also be detected using a variety of collection methods. Estimates of the influx of extraterrestrial material entering the Earths atmosphere range between 5 and 300 tonnes
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Redshift
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In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens, a redshift occurs whenever a light source moves away from an observer. Finally, gravitational redshift is an effect observed in electromagnetic radiation moving out of gravitational fields. However, redshift is a common term and sometimes blueshift is referred to as negative redshift. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns, Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z, a special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in contexts, such as black holes and Big Bang cosmology. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws, the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842, the hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler correctly predicted that the phenomenon should apply to all waves, before this was verified, however, it was found that stellar colors were primarily due to a stars temperature, not motion. Only later was Doppler vindicated by verified redshift observations, the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is called the Doppler–Fizeau effect. In 1868, British astronomer William Huggins was the first to determine the velocity of a moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, in 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The word does not appear unhyphenated until about 1934 by Willem de Sitter, perhaps indicating that up to point its German equivalent. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies, Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years later, he wrote a review in the journal Popular Astronomy, Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable positive velocities
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Universe
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The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, the size of the entire Universe is unknown. The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of gravitation, Sir Isaac Newton built upon Copernicuss work as well as observations by Tycho Brahe. Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy and it is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning, the majority of mass in the Universe appears to exist in an unknown form called dark matter. The Big Bang theory is the prevailing cosmological description of the development of the Universe, under this theory, space and time emerged together 13. 799±0.021 billion years ago with a fixed amount of energy and matter that has become less dense as the Universe has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form, giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist, the Universe can be defined as everything that exists, everything that has existed, and everything that will exist. According to our current understanding, the Universe consists of spacetime, forms of energy, the Universe encompasses all of life, all of history, and some philosophers and scientists suggest that it even encompasses ideas such as mathematics and logic. The word universe derives from the Old French word univers, which in turn derives from the Latin word universum, the Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used. Another synonym was ὁ κόσμος ho kósmos, synonyms are also found in Latin authors and survive in modern languages, e. g. the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world, the prevailing model for the evolution of the Universe is the Big Bang theory. The Big Bang model states that the earliest state of the Universe was extremely hot and dense, the model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The Big Bang model accounts for such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms. The initial hot, dense state is called the Planck epoch, after the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang, the observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength
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Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Astrophysics
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Astrophysics is the branch of astronomy that employs the principles of physics and chemistry to ascertain the nature of the heavenly bodies, rather than their positions or motions in space. Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. In practice, modern astronomical research often involves an amount of work in the realms of theoretical and observational physics. Although astronomy is as ancient as recorded history itself, it was separated from the study of terrestrial physics. Their challenge was that the tools had not yet been invented with which to prove these assertions, for much of the nineteenth century, astronomical research was focused on the routine work of measuring the positions and computing the motions of astronomical objects. Kirchhoff deduced that the lines in the solar spectrum are caused by absorption by chemical elements in the Solar atmosphere. In this way it was proved that the elements found in the Sun. Among those who extended the study of solar and stellar spectra was Norman Lockyer and he thus claimed the line represented a new element, which was called helium, after the Greek Helios, the Sun personified. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types, most significantly, she discovered that hydrogen and helium were the principal components of stars. This discovery was so unexpected that her dissertation readers convinced her to modify the conclusion before publication, however, later research confirmed her discovery. By the end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray and it is the practice of observing celestial objects by using telescopes and other astronomical apparatus. The majority of observations are made using the electromagnetic spectrum. Radio astronomy studies radiation with a greater than a few millimeters. The study of these waves requires very large radio telescopes, infrared astronomy studies radiation with a wavelength that is too long to be visible to the naked eye but is shorter than radio waves. Infrared observations are made with telescopes similar to the familiar optical telescopes. Objects colder than stars are studied at infrared frequencies. Optical astronomy is the oldest kind of astronomy, telescopes paired with a charge-coupled device or spectroscopes are the most common instruments used. The Earths atmosphere interferes somewhat with optical observations, so adaptive optics, in this wavelength range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies and nebulae
27.
Hong-Yee Chiu
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Hong-Yee Chiu is an American astrophysicist and successful publisher of EHGBooks micro-publishing company, born in Shanghai in October 1932. Chiu graduated from National Taiwan University and later received his PhD in Astrophysics from Cornell University, after graduation, he was a member at the Institute for Advanced Study in Princeton, New Jersey. He served as a Space Astrophysics Scientist for NASA for over 35 years, in 1969, Chiu was the first Chinese-American scientist to receive the NASA Exceptional Scientific Achievement Medal. He was also the first scientist to use the term quasar. Later on, Chiu led the EHanism Group to develop the first Amazon Micropublishing Chinese Solution in 2012, Chiu is the host of the 2013 Taiwan Reunion Program for NTU Early Graduates in commemoration of the 85th anniversary of National Taiwan University. His father, Han-Ping Chiu, was a famous economist and lawyer in Shanghai during the Republican Era in China and his late brother Hungdah Chiu was a notable scholar of international law. Chiu, China Needs East and West World Journal News Report, Chiu, Lecture Report World Journal News Report, November 18,2012. Chiu, Lecture Report World Journal News Report, November 8,2012, CTITV interview CTITV, November 8,2012. Chiu interview, Human Rights Award for Dr. Li-Zhi Fang Yuan-Jian Magazine, NTU Early Graduate official website Taiwan Fellowship Publishing App Hong-Yee Chius publications on Amazon
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Sloan Digital Sky Survey
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The project was named after the Alfred P. Sloan Foundation, which contributed significant funding. Data collection began in 2000, and the imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects. Data release 8, released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky. Data release 9, released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey spectrograph, over 500,000 of the new spectra are of objects in the Universe 7 billion years ago. DR10 also includes over 670,000 new BOSS spectra of galaxies, the publicly available images from the survey were made between 1998 and 2009. SDSS uses a dedicated 2. 5-m wide-angle optical telescope, from 2000-2009 it observed in both imaging and spectroscopic modes, the imaging camera was retired in late 2009, since when the telescope has observed entirely in spectroscopic mode. Images were taken using a system of five filters. For imaging observations, the SDSS telescope used the drift scanning technique and this method allows consistent astrometry over the widest possible field, and minimises overheads from reading out the detectors. The disadvantage is minor distortion effects, the telescopes imaging camera is made up of thirty CCD chips each with a resolution of 2048×2048 pixels, totaling approximately 120 Megapixels. The chips are arranged in five rows of six chips, the filters are placed on the camera in the order r, i, u, z, g. To reduce noise the camera is cooled to 190 kelvin by liquid nitrogen, using these photometric data, stars, galaxies, and quasars are also selected for spectroscopy. The spectrograph operates by feeding an optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned specifically for a target, so every field in which spectra are to be acquired requires a unique plate. The original spectrograph attached to the telescope was capable of recording 640 spectra simultaneously, over the course of each night, between six and nine plates are typically used for recording spectra. In spectroscopic mode, the tracks the sky in the standard way. Every night the telescope produces about 200 GB of data and it also obtained repeated imaging of a 300 square degree stripe in the southern Galactic cap. The survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars. The information on the position and distance of the objects has allowed the structure of the Universe, with its voids and filaments
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3C 273
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3C273 is a quasar located in the constellation Virgo. It was the first quasar ever to be identified and it is the optically brightest quasar in the sky, and one of the closest with a redshift, z, of 0.158. A luminosity distance of DL =749 megaparsecs may be calculated from z. It is also one of the most luminous quasars known, with a magnitude of −26.7. Since the suns absolute magnitude is 4.83, it means that the quasar is over 4 trillion times more luminous than the Sun at visible wavelengths. The mass of its black hole has been measured to be 886 ±187 million solar masses through broad emission-line reverberation mapping. The quasar has a large-scale visible jet, which measures ~200 kly long, having an apparent size of 23″, in 1995 optical imaging of the jet using the Hubble Space Telescope revealed a structured morphology evidenced by repeated bright knots interlaced by areas of weak emission. The name signifies that it was the 273rd object of the Third Cambridge Catalog of Radio Sources, published in 1959. In 1963, Maarten Schmidt and Bev Oke published a pair of papers in Nature reporting that 3C273 has a redshift of 0.158. Prior to the discovery of 3C273, several other sources had been associated with optical counterparts. Also, many active galaxies had been misidentified as variable stars, including the famous BL Lac, W Com, however, it wasnt understood what these objects were, since their spectra were unlike those of any known stars. Its spectrum did not resemble that of any stars with typical stellar elements. 3C273 was the first object to be identified as what we now know quasars to be — extremely luminous objects at cosmological distances, 3C273 is a radio-loud quasar, and was also one of the first extragalactic X-ray sources discovered in 1970. However, even to this day, the process gives rise to the X-ray emissions is controversial. The luminosity is variable at nearly every wavelength from radio waves to Gamma rays on timescales of a few days to decades, such jets are believed to be created by the interaction of the central black hole and the accretion disk. VLBI radio observations of 3C273 have revealed proper motion of some of the radio emitting regions, 3C273 lies at the center of a giant elliptical galaxy with an apparent magnitude of 16 and an apparent size of 30 arc seconds. 3C273 is visible in May in both the northern and southern hemispheres, situated in the Virgo constellation, It is bright enough to be observed with larger amateur telescopes. Given its distance from Earth and visual magnitude, 3C273 is the most distant celestial object average amateur astronomers are likely to see through their telescopes
30.
Coronagraph
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A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the stars bright glare – can be resolved. The coronagraph was introduced in 1931 by the French astronomer Bernard Lyot, since then, coronagraphs operating within Earths atmosphere suffer from scattered light in the sky itself, due primarily to Rayleigh scattering of sunlight in the upper atmosphere. At view angles close to the Sun, the sky is much brighter than the background corona even at high altitude sites on clear, dry days. Coronagraph instruments are examples of stray light rejection and precise photometry because the total brightness from the solar corona is less than one millionth the brightness of the Sun. The apparent surface brightness is even fainter because, in addition to delivering less total light, during a solar eclipse, the Moon acts as an occulting disk and any camera in the eclipse path may be operated as a coronagraph until the eclipse is over. More common is an arrangement where the sky is imaged onto a focal plane containing an opaque spot. Either way, the instrument design must take into account scattering, as an example, imaging instruments on the Hubble Space Telescope offer coronagraphic capability. A band-limited coronagraph uses a kind of mask called a band-limited mask. This mask is designed to light and also manage potangina effects caused by removal of the light. The band-limited coronagraph has served as the design for the canceled Terrestrial Planet Finder coronagraph. Band-limited masks will also be available on the James Webb Space Telescope, see also, A phase-mask coronagraph uses a transparent mask to shift the phase of the stellar light in order to create a self-destructive interference, rather than a simple opaque disc to block it. See also, An optical vortex coronagraph uses a phase-mask in which the phase-shift varies azimuthally around the center, several varieties of optical vortex coronagraphs exist, the scalar optical vortex coronagraph based on a phase ramp directly etched in a dielectric material, like fused silica. The vector vortex coronagraph employs a mask that rotates the angle of polarization of photons, a mask of this kind can be synthesized by various technologies, ranging from liquid crystal polymer, and micro-structured surfaces. Such a vector vortex coronagraph made out of liquid crystal polymers is currently in use at the 200-inch Hale telescope at the Palomar Observatory and it has recently been operated with adaptive optics to image extrasolar planets. This works with other than the sun because they are so far away their light is, for this purpose. The coronagraph using interference masks out the light along the axis of the telescope. Coronagraphs in outer space are more effective than the same instruments would be if located on the ground. This is because the absence of atmospheric scattering eliminates the largest source of glare present in a terrestrial coronagraph
31.
Heber Doust Curtis
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Heber Doust Curtis was an American astronomer. He participated in 11 expeditions for the study of solar eclipses and he was born on June 27,1872, the elder son of Orson Blair Curtis and Sarah Eliza Doust. He studied at the University of Michigan and at the University of Virginia, from 1902 to 1920 Curtis worked at Lick Observatory, continuing the survey of nebulae initiated by Keeler. In 1912 he was elected president of the Astronomical Society of the Pacific, in 1918 he observed Messier 87 and was the first to notice the polar jet which he described as a curious straight ray. apparently connected with the nucleus by a thin line of matter. In 1920 he was appointed director of the Allegheny Observatory, in the same year he participated in the Great Debate with Harlow Shapley on the nature of nebulae and galaxies, and the size of the universe. Curtis advocated the view that other galaxies apart from the Milky Way existed. This device is packed in crates and resided at UCO Lick Observatory as of Aug 2011 and his article describing the device appears in the Publications of the Allegheny Observatory, vol. He contributed to develop the McMath–Hulbert private observatory at Lake Angelus and he died on January 9,1942. The Great Debate National Academy of Sciences Biographical Memoir Portraits of Heber D. Curtis from the Lick Observatory Records Digital Archive, UC Santa Cruz Librarys Digital Collection
32.
Nebula
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A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, the Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Most nebulae are of vast size, even millions of years in diameter. The Orion Nebula, the brightest nebula in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Many nebulae are visible due to their fluorescence caused by the hot stars, while others are so diffuse they can only be detected with long exposures. Some nebulae are variably illuminated by T Tauri variable stars, Nebulae are often star-forming regions, such as in the Pillars of Creation in the Eagle Nebula. In these regions the formations of gas, dust, and other materials together to form denser regions, which attract further matter. The remaining material is believed to form planets and other planetary system objects. Around 150 AD, Claudius Ptolemaeus recorded, in books VII–VIII of his Almagest and he also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star. The first true nebula, as distinct from a cluster, was mentioned by the Persian astronomer Abd al-Rahman al-Sufi. He noted a little cloud where the Andromeda Galaxy is located and he also cataloged the Omicron Velorum star cluster as a nebulous star and other nebulous objects, such as Brocchis Cluster. The supernova that created the Crab Nebula, the SN1054, was observed by Arabic, in 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, in 1715, Edmund Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 in 1746, from 1751 to 1753, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 nebulae by 1781, his interest was detecting comets, the number of nebulae was then greatly expanded by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786, a second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars, in 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster
33.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
34.
Spectral line
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Spectral lines are often used to identify atoms and molecules from their characteristic spectral lines. Spectral lines are the result of interaction between a system and a single photon. When a photon has about the amount of energy to allow a change in the energy state of the system. A spectral line may be observed either as a line or an absorption line. Which type of line is observed depends on the type of material, an absorption line is produced when photons from a hot, broad spectrum source pass through a cold material. The intensity of light, over a frequency range, is reduced due to absorption by the material. By contrast, a bright, emission line is produced when photons from a hot material are detected in the presence of a spectrum from a cold source. The intensity of light, over a frequency range, is increased due to emission by the material. Spectral lines are highly atom-specific, and can be used to identify the composition of any medium capable of letting light pass through it. Several elements were discovered by means, such as helium, thallium. Mechanisms other than atom-photon interaction can produce spectral lines, depending on the exact physical interaction, the frequency of the involved photons will vary widely, and lines can be observed across the electromagnetic spectrum, from radio waves to gamma rays. In other cases the lines are designated according to the level of ionization by adding a Roman numeral to the designation of the chemical element, so that Ca+ also has the designation Ca II. Neutral atoms are denoted with the roman number I, singly ionized atoms with II, more detailed designations usually include the line wavelength and may include a multiplet number or band designation. Many spectral lines of hydrogen also have designations within their respective series. A spectral line extends over a range of frequencies, not a single frequency, in addition, its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift and these reasons may be divided into two general categories – broadening due to local conditions and broadening due to extended conditions. Broadening due to conditions is due to effects which hold in a small region around the emitting element. Broadening due to extended conditions may result from changes to the distribution of the radiation as it traverses its path to the observer
35.
Chemical element
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A chemical element or element is a species of atoms having the same number of protons in their atomic nuclei. There are 118 elements that have identified, of which the first 94 occur naturally on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have exclusively radioactive isotopes, iron is the most abundant element making up Earth, while oxygen is the most common element in the Earths crust. Chemical elements constitute all of the matter of the universe. The two lightest elements, hydrogen and helium, were formed in the Big Bang and are the most common elements in the universe. The next three elements were formed mostly by cosmic ray spallation, and are rarer than those that follow. Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis, the high abundance of oxygen, silicon, and iron on Earth reflects their common production in such stars. The term element is used for atoms with a number of protons as well as for a pure chemical substance consisting of a single element. A single element can form multiple substances differing in their structure, when different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds. Only a minority of elements are found uncombined as relatively pure minerals, among the more common of such native elements are copper, silver, gold, carbon, and sulfur. All but a few of the most inert elements, such as gases and noble metals, are usually found on Earth in chemically combined form. While about 32 of the elements occur on Earth in native uncombined forms. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, the history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, copper and gold. Later civilizations extracted elemental copper, tin, lead and iron from their ores by smelting, using charcoal, alchemists and chemists subsequently identified many more, almost all of the naturally occurring elements were known by 1900. Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are produced in nucleogenic reactions, or in cosmogenic processes. Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope, Isotopes considered stable are those for which no radioactive decay has yet been observed. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected, the very heaviest elements undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized
36.
Solar System
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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
37.
3C 48
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3C48 is a quasar discovered in 1960, it was the first source conclusively identified as such. 3C48 was the first source in the Third Cambridge Catalogue of Radio Sources for which an optical identification was found by Allan Sandage and Thomas A. Matthews in 1960 through interferometry. Jesse L. Greenstein and Thomas Matthews found that it had a redshift of 0.367 and it was not until 1982 that the surrounding faint galactic nebulosity was confirmed to have the same redshift as 3C48, cementing its identification as an object in a distant galaxy. This was also the first solid identification of a quasar with a galaxy at the same redshift. 3C48 is one of four primary calibrators used by the Very Large Array, visibilities of all other sources are calibrated using observed visibilities of one of these four calibrators
38.
Lovell Telescope
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The Lovell Telescope /ˈlʌvəl/ is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire in the north-west of England. It was originally known as the 250 ft telescope or the Radio Telescope at Jodrell Bank and it was renamed to the Lovell Telescope in 1987 after Sir Bernard Lovell, and became a Grade I listed building in 1988. The telescope forms part of the MERLIN and European VLBI Network arrays of radio telescopes, both Bernard Lovell and Charles Husband were knighted for their roles in creating the telescope. In September 2006, the won the BBCs online competition to find the UKs greatest Unsung Landmark. 2007 marked the 50th anniversary of the telescope and it can also be seen from the Terminal 1 restaurant area and departure lounges of Manchester Airport. Bernard Lovell built the Transit Telescope at Jodrell Bank in the late 1940s and this turned out to be Charles Husband, whom Lovell first met on 8 September 1949. Two bearing assemblies from 15-inch gun turrets were bought cheaply in 1950, these came from the World War I battleships HMS Revenge and Royal Sovereign, the bearings became the two main altitude rotator bearings of the telescope, with the appropriate parts of the telescope being designed around them. Husband presented the first drawings of the giant, fully steerable radio telescope in 1950. After refinements, these plans were detailed in a Blue Book, which was presented to the DSIR on 20 March 1951, construction began on 3 September 1952. The foundations for the telescope were completed on 21 May 1953 after being sunk 90 ft into the ground and it then took until Mid-March 1954 to get the double railway lines completed due to their required accuracy. The central pivot was delivered to the site on 11 May 1954, although the funding was not ultimately made available from the Air Ministry, the planning process had already progressed too far and so this improvement was made anyway. The telescope was constructed so that the bowl could be completely inverted, originally, it was intended to use a movable tower at the base of the telescope to change the receivers at the focus. However, the tower was never built, due jointly to funding constraints. Instead, receivers were mounted on 50-foot long steel tubes, which were inserted by a winch into the top of the aerial tower while the bowl was inverted. The cables from the receivers then ran down the inside of this tube, the telescope moved for the first time on 3 February 1957, by an inch. It was first moved azimuthally under power on 12 June 1957, the bowl was tilted under power for the first time on 20 June 1957. By the end of July the dish surface was completed, and first light was on 2 August 1957, the telescope was first controlled from the control room on 9 October 1957, by a purpose-built analogue computer. There were large cost overruns with the construction, mainly due to the steeply rising cost of steel at the time the telescope was constructed
39.
Allan Sandage
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Allan Rex Sandage was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena and he determined the first reasonably accurate values for the Hubble constant and the age of the universe. He also discovered the first quasar, Sandage was one of the most influential astronomers of the 20th century. He was born in Iowa City, Iowa, United States and he graduated from the University of Illinois in 1948. In 1953 he received a Ph. D. from the California Institute of Technology, during this time Sandage was a graduate student assistant to cosmologist Edwin Hubble. He continued Hubbles research program after Hubble died in 1953, Hubble had posited the earlier value, he had considered only the weaker Population II Cepheid variables as standard candles. This resulted in another 1.5 factor increase in the age of the universe. Sandage began working at the Palomar Observatory, in 1958 he published the first good estimate for the Hubble constant, revising Hubbles value of 250 down to 75 km/s/Mpc, which is close to todays accepted value. Later he became the advocate of an even lower value, around 50. Sandage performed photometric studies of clusters, and calculated their age to be at least 25 billion years. This led him to speculate that the universe did not merely expand, the current cosmological estimates of the age of the universe, in contrast, are typically of the order of 14 billion years. He later defended the paper in 1990 and this paper influenced observational cosmology for at least three decades as it carefully specified the types of observational tests that could be performed with a large telescope. He also published two atlases of galaxies, in 1961 and 1981, based on the Hubble classification scheme, in 1962 Sandage studied the possibility of directly measuring the temporal variation of the redshift of extra-galactic sources. This analysis became known as the Sandage–Loeb test, Sandage discovered jets erupting from the core of the so-called Cigar Galaxy. These must have been caused by explosions in the core. Sandage was a researcher, during his career he published more than 500 papers. Until his death he continued to be a researcher at the Carnegie Observatories. In 1959, Sandage married Mary Connelley, also an astronomer, in 1983 Sandage announced he had become a Christian and began to write essays on the subject of religion and science
. Bibcode:2015Natur.518..512W. doi:10.1038/nature14241. Retrieved 5 March 2017.
