Telescope, device used to form magnified images of distant objects. The telescope is undoubtedly the most important investigative tool in astronomy. It provides a means of collecting and analyzing radiation from celestial objects, even those in the far reaches of the universe.
Galileo's telescopes
Galileo's telescopes
Two of Galileo's first telescopes; in the Museo Galileo, Florence.
Galileo revolutionized astronomy when he applied the telescope to the study of extraterrestrial bodies in the early 17th century. Until then, magnification instruments had never been used for this purpose. Since Galileo’s pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring radiation in every region of the electromagnetic spectrum. Observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems. These developments have contributed dramatically to advances in scientific knowledge about the solar system, the Milky Way Galaxy, and the universe as a whole.
This article describes the operating principles and historical development of optical telescopes. For explanation of instruments that operate in other portions of the electromagnetic spectrum, see radio telescope; X-ray telescope; and gamma-ray telescope.
Refracting telescopes
refractor at Lick Observatory
refractor at Lick Observatory
The historical 91-cm (36-inch) refractor at the Lick Observatory on Mount Hamilton, near San Jose, California, U.S.
focal length of a lens
focal length of a lens
refracting telescope
refracting telescope
Commonly known as refractors, telescopes of this kind are typically used to examine the Moon, other objects of the solar system such as Jupiter and Mars, and binary stars. The name refractor is derived from the term refraction, which is the bending of light when it passes from one medium to another of different density—e.g., from air to glass. The glass is referred to as a lens and may have one or more components. The physical shape of the components may be convex, concave, or plane-parallel. This diagram illustrates the principle of refraction and the term focal length. The focus is the point, or plane, at which light rays from infinity converge after passing through a lens and traveling a distance of one focal length. In a refractor the first lens through which light from a celestial object passes is called the objective lens. It should be noted that the light will be inverted at the focal plane. A second lens, referred to as the eyepiece lens, is placed behind the focal plane and enables the observer to view the enlarged, or magnified, image. Thus, the simplest form of refractor consists of an objective and an eyepiece, as illustrated in the diagram.
View of the Andromeda Galaxy (Messier 31, M31).
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The diameter of the objective is referred to as the aperture; it typically ranges from a few centimetres for small spotting telescopes up to one metre for the largest refractor in existence. The objective, as well as the eyepiece, may have several components. Small spotting telescopes may contain an extra lens behind the eyepiece to erect the image so that it does not appear upside-down. When an object is viewed with a refractor, the image may not appear sharply defined, or it may even have a predominant colour in it. Such distortions, or aberrations, are sometimes introduced when the lens is polished into its design shape. The major kind of distortion in a refractor is chromatic aberration, which is the failure of the differently coloured light rays to come to a common focus. Chromatic aberration can be minimized by adding components to the objective. In lens-design technology, the coefficients of expansion of different kinds of glass are carefully matched to minimize the aberrations that result from temperature changes of the telescope at night.
Eyepieces, which are used with both refractors and reflectors (see below Reflecting telescopes), have a wide variety of applications and provide observers with the ability to select the magnification of their instruments. The magnification, sometimes referred to as magnifying power, is determined by dividing the focal length of the objective by the focal length of the eyepiece. For example, if the objective has a focal length of 254 cm (100 inches) and the eyepiece has a focal length of 2.54 cm (1 inch), then the magnification will be 100. Large magnifications are very useful for observing the Moon and the planets. However, since stars appear as point sources owing to their great distances, magnification provides no additional advantage when viewing them. Another important factor that one must take into consideration when attempting to view at high magnification is the stability of the telescope mounting. Any vibration in the mounting will also be magnified and may severely reduce the quality of the observed image. Thus, great care is usually taken to provide a stable platform for the telescope. This problem should not be associated with that of atmospheric seeing, which may introduce a disturbance to the image because of fluctuating air currents in the path of the light from a celestial or terrestrial object. Generally, most of the seeing disturbance arises in the first 30 metres (100 feet) of air above the telescope. Large telescopes are frequently installed on mountain peaks in order to get above the seeing disturbances.
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Light gathering and resolution
The most important of all the powers of an optical telescope is its light-gathering power. This capacity is strictly a function of the diameter of the clear objective—that is, the aperture—of the telescope. Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared; for example, a 25-cm (10-inch) objective will collect four times the light of a 12.5-cm (5-inch) objective ([25 × 25] ÷ [12.5 × 12.5] = 4). The advantage of collecting more light with a larger-aperture telescope is that one can observe fainter stars, nebulae, and very distant galaxies.
Resolving power is another important feature of a telescope. This is the ability of the instrument to distinguish clearly between two points whose angular separation is less than the smallest angle that the observer’s eye can resolve. The resolving power of a telescope can be calculated by the following formula: resolving power = 11.25 seconds of arc/d, where d is the diameter of the objective expressed in centimetres. Thus, a 25-cm-diameter objective has a theoretical resolution of 0.45 second of arc and a 250-cm (100-inch) telescope has one of 0.045 second of arc. An important application of resolving power is in the observation of visual binary stars. There, one star is routinely observed as it revolves around a second star. Many observatories conduct extensive visual binary observing programs and publish catalogs of their observational results. One of the major contributors in this field is the United States Naval Observatory in Washington, D.C.
Most refractors currently in use at observatories have equatorial mountings. The mounting describes the orientation of the physical bearings and structure that permits a telescope to be pointed at a celestial object for viewing. In the equatorial mounting, the polar axis of the telescope is constructed parallel to Earth’s axis. The polar axis supports the declination axis of the instrument. Declination is measured on the celestial sky north or south from the celestial equator. The declination axis makes it possible for the telescope to be pointed at various declination angles as the instrument is rotated about the polar axis with respect to right ascension. Right ascension is measured along the celestial equator from the vernal equinox (i.e., the position on the celestial sphere where the Sun crosses the celestial equator from south to north on the first day of spring). Declination and right ascension are the two coordinates that define a celestial object on the celestial sphere. Declination is analogous to latitude, and right ascension is analogous to longitude. Graduated dials are mounted on the axis to permit the observer to point the telescope precisely. To track an object, the telescope’s polar axis is driven smoothly by an electric motor at a sidereal rate—namely, at a rate equal to the rate of rotation of Earth with respect to the stars. Thus, one can track or observe with a telescope for long periods of time if the sidereal rate of the motor is very accurate. High-accuracy motor-driven systems have become readily available with the rapid advancement of quartz-clock technology. Most major observatories now rely on either quartz or atomic clocks to provide accurate sidereal time for observations as well as to drive telescopes at an extremely uniform rate.
A notable example of a refracting telescope is the 66-cm (26-inch) refractor of the U.S. Naval Observatory. This instrument was used by the astronomer Asaph Hall to discover the two moons of Mars, Phobos and Deimos, in 1877. Today, the telescope is used primarily for observing binary stars. The 91-cm (36-inch) refractor at Lick Observatory on Mount Hamilton, California, U.S., is the largest refracting system currently in operation. (The 1-metre [40-inch] instrument at Yerkes Observatory in Williams Bay, Wisconsin, U.S., has been inactive since 2018 [see table].)
Some important ground-based optical telescopes
name aperture (metres) type observatory location date observations began
Gran Telescopio Canarias 10.4 reflector Roque de los Muchachos Observatory La Palma, Canary Islands, Spain 2007
Keck I, Keck II 10, 10 reflector Keck Observatory Mauna Kea, Hawaii 1993, 1996
Southern African Large Telescope 11.1 × 9.8 reflector Sutherland, South Africa 2005
Hobby-Eberly Telescope 11.1 × 9.8 reflector McDonald Observatory Fort Davis, Texas 1999
Large Binocular Telescope 2 mirrors, each 8.4 reflector Mount Graham, Arizona 2008
Subaru 8.3 reflector Mauna Kea, Hawaii 1999
Antu, Kueyen, Melipal, Yepun 8.2, 8.2, 8.2, 8.2 reflector Very Large Telescope Cerro Paranal, Chile 1998, 1999, 2000, 2000
Frederick C. Gillett Gemini North Telescope 8.1 reflector International Gemini Observatory Mauna Kea, Hawaii 2000
Gemini South Telescope 8.1 reflector International Gemini Observatory Cerro Pachon, Chile 2000
MMT 6.5 reflector MMT Observatory Mount Hopkins, Arizona 2000
Walter Baade, Landon Clay 6.5, 6.5 reflector Magellan Telescopes Cerro Las Campanas, Chile 2000, 2002
Bolshoi Teleskop 6 reflector Special Astrophysical Observatory Zelenchukskaya, Russia 1976
Hale Telescope 5 reflector Palomar Observatory Mount Palomar, California 1948
William Herschel Telescope 4.2 reflector Roque de los Muchachos Observatory La Palma, Canary Islands, Spain 1987
Victor M. Blanco Telescope 4 reflector Cerro Tololo Inter-American Observatory Cerro Tololo, Chile 1974
Anglo-Australian Telescope 3.9 reflector Siding Spring Observatory Siding Spring Mountain, New South Wales, Austl. 1974
Nicholas U. Mayall Telescope 3.8 reflector Kitt Peak National Observatory Kitt Peak, Arizona 1970
Canada-France-Hawaii Telescope 3.6 reflector Mauna Kea, Hawaii 1979
3.6 reflector La Silla Observatory La Silla, Chile 1977
Hooker Telescope 2.5 reflector Mount Wilson Observatory Mount Wilson, California 1918
Samuel Oschin Telescope 1.2 reflector Palomar Observatory Mount Palomar, California 1948
1 refractor Yerkes Observatory Williams Bay, Wisconsin 1897
Lick Refractor 0.9 refractor Lick Observatory Mount Hamilton, California 1888
Another type of refracting telescope is the astrograph, which usually has an objective diameter of approximately 20 cm (8 inches). The astrograph has a photographic plateholder mounted in the focal plane of the objective so that photographs of the celestial sphere can be taken. The photographs are usually taken on glass plates. The principal application of the astrograph is to determine the positions of a large number of faint stars. These positions are then published in catalogs such as the AGK3 and serve as reference points for deep-space photography.
Reflecting telescopes
telescope at Birr Castle
telescope at Birr Castle
The 72-inch reflecting telescope at Birr Castle, County Offaly, Leinster, Ireland, was the largest in the world at the time of its construction in the 1840s.
concave mirror
concave mirror
Reflectors are used not only to examine the visible region of the electromagnetic spectrum but also to explore both the shorter- and longer-wavelength regions adjacent to it (i.e., the ultraviolet and the infrared). The name of this type of instrument is derived from the fact that the primary mirror reflects the light back to a focus instead of refracting it. The primary mirror usually has a concave spherical or parabolic shape, and, as it reflects the light, it inverts the image at the focal plane. The diagram illustrates the principle of a concave reflecting mirror. The formulas for resolving power, magnifying power, and light-gathering power, as discussed for refractors, apply to reflectors as well.
The primary mirror is located at the lower end of the telescope tube in a reflector and has its front surface coated with an extremely thin film of metal, such as aluminum. The back of the mirror is usually made of glass, although other materials have been used from time to time. Pyrex was the principal glass of choice for many of the older large telescopes, but new technology has led to the development and widespread use of a number of glasses with very low coefficients of expansion. A low coefficient of expansion means that the shape of the mirror will not change significantly as the temperature of the telescope changes during the night. Since the back of the mirror serves only to provide the desired form and physical support, it does not have to meet the high optical quality standards required for a lens.
Newton's reflecting telescope
Newton's reflecting telescope
Isaac Newton's reflecting telescope, 1668.
Reflecting telescopes have a number of other advantages over refractors. They are not subject to chromatic aberration because reflected light does not disperse according to wavelength. Also, the telescope tube of a reflector is shorter than that of a refractor of the same diameter, which reduces the cost of the tube. Consequently, the dome for housing a reflector is smaller and more economical to construct. So far only the primary mirror for the reflector has been discussed. One might wonder about the location of the eyepiece. The primary mirror reflects the light of the celestial object to the prime focus near the upper end of the tube. Obviously, if an observer put his eye there to observe with a modest-sized reflector, he would block out the light from the primary mirror with his head. Isaac Newton placed a small plane mirror at an angle of 45° inside the prime focus and thereby brought the focus to the side of the telescope tube. The amount of light lost by this procedure is very small when compared to the total light-gathering power of the primary mirror. The Newtonian reflector is popular among amateur telescope makers.
Cassegrain reflector
Cassegrain reflector
Laurent Cassegrain of France, a contemporary of Newton, invented another type of reflector. Called the Cassegrain telescope, this instrument employs a small convex mirror to reflect the light back through a small hole in the primary mirror to a focus located behind the primary. The diagram illustrates a typical Cassegrain reflector. Some large telescopes of this kind do not have a hole in the primary mirror but use a small plane mirror in front of the primary to reflect the light outside the main tube and provide another place for observation. The Cassegrain design usually permits short tubes relative to their mirror diameter.
View of the Andromeda Galaxy (Messier 31, M31).
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One more variety of reflector was invented by another of Newton’s contemporaries, the Scottish astronomer James Gregory. Gregory placed a concave secondary mirror outside the prime focus to reflect the light back through a hole in the primary mirror. Notable is the fact that the Gregorian design was adopted for the Earth-orbiting space observatory, the Solar Maximum Mission (SMM), launched in 1980.
Most large reflecting telescopes that are currently in use have a cage at their prime focus that permits the observer to sit inside the telescope tube while operating the instrument. The 5-metre (200-inch) reflector at Palomar Observatory, near San Diego, Calif., is so equipped. While most reflectors have equatorial mounts similar to refractors, the world’s largest reflector, the 10.4-metre (34.1-foot) instrument at the Gran Telescopio Canarias at La Palma, Canary Islands, Spain, has an altitude-azimuth mounting. The significance of the latter design is that the telescope must be moved both in altitude and in azimuth as it tracks a celestial object. Equatorial mountings, by contrast, require motion in only one coordinate while tracking, since the declination coordinate is constant. Reflectors, like refractors, usually have small guide telescopes mounted parallel to their main optical axis to facilitate locating the desired object. These guide telescopes have low magnification and a wide field of view, the latter being a desirable attribute for finding stars or other remote cosmic objects.
The parabolic shape of a primary mirror has a basic failing in that it produces a narrow field of view. This can be a problem when one wishes to observe extended celestial objects. To overcome this difficulty, most large reflectors now have a modified Cassegrain design. The central area of the primary mirror has its shape deepened from that of a paraboloid, and the secondary mirror is configured to compensate for the altered primary. The result is the Ritchey-Chrétien design, which has a curved rather than a flat focus. Obviously, the photographic medium must be curved to collect high-quality images across the curved focal plane. The 1-metre telescope of the U.S. Naval Observatory in Flagstaff, Arizona, was one of the early examples of this design.
The Schmidt telescope
Schmidt telescope
Schmidt telescope
The Ritchey-Chrétien design has a good field of view of about 1°. For some astronomical applications, however, photographing larger areas of the sky is mandatory. In 1930 Bernhard Schmidt, an optician at the Hamburg Observatory in Bergedorf, Germany, designed a catadioptric telescope that satisfied the requirement of photographing larger celestial areas. A catadioptric telescope design incorporates the best features of both the refractor and the reflector—i.e., it has both reflective and refractive optics. The Schmidt telescope has a spherically shaped primary mirror. Since parallel light rays that are reflected by the centre of a spherical mirror are focused farther away than those reflected from the outer regions, Schmidt introduced a thin lens (called the correcting plate) at the radius of curvature of the primary mirror. Since this correcting plate is very thin, it introduces little chromatic aberration. The resulting focal plane has a field of view several degrees in diameter. The diagram illustrates a typical Schmidt design.
The National Geographic Society–Palomar Observatory Sky Survey made use of a 1.2-metre (47-inch) Schmidt telescope to photograph the northern sky in the red and blue regions of the visible spectrum. The survey produced 900 pairs of photographic plates (about 7° by 7° each) taken from 1949 to 1956. Schmidt telescopes of the European Southern Observatory in Chile and of the Siding Spring Observatory in Australia have photographed the remaining part of the sky that cannot be observed from Palomar Observatory. (The survey undertaken at the latter included photographs in the infrared as well as in the red and blue spectral regions.)
Multimirror telescopes
The main reason astronomers build larger telescopes is to increase light-gathering power so that they can see deeper into the universe. Unfortunately, the cost of constructing larger single-mirror telescopes increases rapidly—approximately with the cube of the diameter of the aperture. Thus, in order to achieve the goal of increasing light-gathering power while keeping costs down, it has become necessary to explore new, more economical and nontraditional telescope designs.
Mauna Kea Observatory: Keck telescopes
Mauna Kea Observatory: Keck telescopes
The Keck telescopes at Mauna Kea Observatory, Hawaii.
The two 10-metre (33-foot) Keck Observatory multimirror telescopes represent such an effort. The first was installed on Mauna Kea on the island of Hawaii in 1992, and a second telescope was completed in 1996. Each of the Keck telescopes comprises 36 contiguous adjustable mirror segments, all under computer control. Even-larger multimirror instruments are currently being planned by American and European astronomers.
Mars
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Mars: Early telescopic observations
Special types of optical telescopes
Solar telescopes
solar corona
solar corona
The Sun's corona as seen by the Large Angle Spectrometric Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO).
Either a refractor or a reflector may be used for visual observations of solar features, such as sunspots or solar prominences. Special solar telescopes have been constructed, however, for investigations of the Sun that require the use of such ancillary instruments as spectroheliographs and coronagraphs. These telescopes are mounted in towers and have very long focus objectives. Typical examples of tower solar telescopes are found at the Mount Wilson Observatory in California and the McMath-Hulbert Observatory in Michigan. The long focus objective produces a very good scale factor, which in turn makes it possible to look at individual wavelengths of the solar electromagnetic spectrum in great detail. A tower telescope has an equatorially mounted plane mirror at its summit to direct the sunlight into the telescope objective. This plane mirror is called a coelostat. Bernard Lyot constructed another type of solar telescope in 1930 at Pic du Midi Observatory in France. This instrument was specifically designed for photographing the Sun’s corona (the outer layer), which up to that time had been successfully photographed only during solar eclipses. The coronagraph, as this special telescope is called, must be located at a high altitude to be effective. The high altitude is required to reduce the scattered sunlight, which would reduce the quality of the photograph. The High Altitude Observatory in Colorado has such a coronagraph. The principle has been extended to build instruments that can search for extrasolar planets by blocking out the light of their parent stars. Coronagraphs are also used on board satellites, such as the Solar and Heliospheric Observatory, that study the Sun.
Earth-orbiting space telescopes
Hubble Space Telescope
Hubble Space Telescope
Cutaway of the Hubble Space Telescope, revealing the Optical Telescope Assembly (OTA), the heart of this orbiting observational system.
While astronomers continue to seek new technological breakthroughs with which to build larger ground-based telescopes, it is readily apparent that the only solution to some scientific problems is to make observations from above Earth’s atmosphere. A series of Orbiting Astronomical Observatories (OAOs) was launched by the National Aeronautics and Space Administration (NASA). The OAO launched in 1972—later named Copernicus—had an 81-cm (32-inch) telescope on board. The most sophisticated observational system placed in Earth orbit so far is the Hubble Space Telescope (HST; see photograph). Launched in 1990, the HST is essentially a telescope with a 2.4-metre (94-inch) primary mirror. It has been designed to enable astronomers to see into a volume of space 300 to 400 times larger than that permitted by other systems. At the same time, the HST is not impeded by any of the problems caused by the atmosphere. It is equipped with five principal scientific instruments: (1) a wide-field and planetary camera, (2) a faint-object spectrograph, (3) a high-resolution spectrograph, (4) a high-speed photometer, and (5) a faint-object camera. The HST was launched into orbit from the U.S. space shuttle at an altitude of more than 570 km (350 miles) above Earth. Shortly after its deployment in Earth orbit, HST project scientists found that a manufacturing error affecting the shape of the telescope’s primary mirror severely impaired the instrument’s focusing capability. The flawed mirror caused spherical aberration, which limited the ability of the HST to distinguish between cosmic objects that lie close together and to image distant galaxies and quasars. Project scientists devised measures that enabled them to compensate for the defective mirror and correct the imaging problem.
Astronomical transit instruments
These small but extremely important telescopes have played a vital role in mapping the celestial sphere. Astronomical transit instruments are usually refractors with apertures of 15 to 20 cm (6 to 8 inches). (Ole Rømer, a Danish astronomer, is credited with having invented this type of telescope system.) The main optical axis of the instrument is aligned on a north-south line such that its motion is restricted to the plane of the meridian of the observer. The observer’s meridian is a great circle on the celestial sphere that passes through the north and south points of the horizon as well as through the zenith of the observer. Restricting the telescope to motion only in the meridian provides an added degree of stability, but it requires the observer to wait for the celestial object to rotate across his meridian. The latter process is referred to as transiting the meridian, from which the name of the telescope is derived. There are various types of transit instruments—for example, the transit circle telescope, the vertical circle telescope, and the horizontal meridian circle telescope. The transit circle determines the right ascension of celestial objects, while the vertical circle measures only their declinations. Transit circles and horizontal meridian circles measure both right ascension and declination at the same time. The final output data of all transit instruments are included in star or planetary catalogs. A notable example of this class of telescopes is the transit circle of the National Astronomical Observatory in Tokyo.
Astrolabes
Another special type of telescopic instrument is the modern version of the astrolabe. Known as a prismatic astrolabe, it too is used for making precise determinations of the positions of stars and planets. It may sometimes be used inversely to determine the latitude and longitude of the observer, assuming the star positions are accurately known. The aperture of a prismatic astrolabe is small, usually only 8 to 10 cm (3 to 4 inches). A small pool of mercury and a refracting prism make up the other principal parts of the instrument. An image reflected off the mercury is observed along with a direct image to give the necessary position data. The most notable example of this type of instrument is the French-constructed Danjon astrolabe. During the 1970s, however, the Chinese introduced various innovations that resulted in a more accurate and automatic kind of astrolabe, which is now in use at the National Astronomical Observatories of China’s headquarters in Beijing.
B.L. Klock
The development of the telescope and auxiliary instrumentation
Evolution of the optical telescope
Galileo is credited with having developed telescopes for astronomical observation in 1609. While the largest of his instruments was only about 120 cm (47 inches) long and had an objective diameter of 5 cm (2 inches), it was equipped with an eyepiece that provided an upright (i.e., erect) image. Galileo used his modest instrument to explore such celestial phenomena as the valleys and mountains of the Moon, the phases of Venus, and the four largest Jovian satellites, which had never been systematically observed before.
The reflecting telescope was developed in 1668 by Newton, though John Gregory had independently conceived of an alternative reflector design in 1663. Cassegrain introduced another variation of the reflector in 1672. Near the end of the century, others attempted to construct refractors as long as 61 metres, but these instruments were too awkward to be effective.
The most significant contribution to the development of the telescope in the 18th century was that of Sir William Herschel. Herschel, whose interest in telescopes was kindled by a modest 5-cm Gregorian, persuaded the king of England to finance the construction of a reflector with a 12-metre (39-foot) focal length and a 120-cm mirror. Herschel is credited with having used this instrument to lay the observational groundwork for the concept of extragalactic “nebulas”—i.e., galaxies outside the Milky Way system.
Learn about Lord Rosse's Birr Leviathan telescope and its various important discoveries, the most important being the detailed observations of many nebulae at Birr Castle, Ireland
Learn about Lord Rosse's Birr Leviathan telescope and its various important discoveries, the most important being the detailed observations of many nebulae at Birr Castle, Ireland
Discover the historical significance of Lord Rosse's "Leviathan" telescope at Birr Castle, County Offaly, Ireland.
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Reflectors continued to evolve during the 19th century with the work of William Parsons, 3rd earl of Rosse, and William Lassell. In 1845 Lord Rosse constructed in Ireland a reflector with a 185-cm (73-inch) mirror and a focal length of about 16 metres (52 feet). For 75 years this telescope ranked as the largest in the world and was used to explore thousands of nebulae and star clusters. Lassell built several reflectors, the largest of which was on Malta; this instrument had a 124-cm (49-inch) primary mirror and a focal length of more than 10 metres (33 feet). His telescope had greater reflecting power than Rosse’s, and it enabled him to catalog 600 new nebulae as well as to discover several satellites of the outer planets—Triton (Neptune’s largest moon), Hyperion (Saturn’s 8th moon), and Ariel and Umbriel (two of Uranus’s moons).
Refractor telescopes, too, underwent development during the 18th and 19th centuries. The last significant one to be built was the 1-metre (40-inch) refractor at Yerkes Observatory. Installed in 1897, it was the largest refracting system in the world. Its objective was designed and constructed by the optician Alvan Clark, while the mount was built by the firm of Warner & Swasey.
The reflecting telescope predominated in the 20th century. The rapid proliferation of increasingly larger instruments of this type began with the installation of the 2.5-metre (60-inch) reflector at the Mount Wilson Observatory near Pasadena, Calif., U.S. The technology for mirrors underwent a major advance when the Corning Glass Works (in Steuben county, N.Y., U.S.) developed Pyrex. This borosilicate glass, which undergoes substantially less expansion than ordinary glass does, was used in the 5-metre (200-inch) Hale Telescope built in 1948 at the Palomar Observatory. Pyrex also was utilized in the main mirror of the 6-metre (236-inch) reflector of the Special Astrophysical Observatory in Zelenchukskaya, Russia. Since then, much better materials for mirrors have become available. Cer-Vit, for example, was used for the 4.2-metre (165-inch) William Herschel Telescope of the Roque de los Muchachos Observatory in the Canary Islands, and Zerodur was used for the 10.4-metre (410-inch) reflector at the Gran Telescopio Canarias in the Canary Islands.
B.L. Klock
Advances in auxiliary instrumentation
Almost as important as the telescope itself are the auxiliary instruments that the astronomer uses to exploit the light received at the focal plane. Examples of such instruments are the camera, spectrograph, photomultiplier tube, charge-coupled device (CCD), and charge injection device (CID). Each of these instrument types is discussed below.
Cameras
American John Draper photographed the Moon as early as 1840 by applying the daguerreotype process. The French physicists A.-H.-L. Fizeau and J.-B.-L. Foucault succeeded in making a photographic image of the Sun in 1845. Five years later astronomers at Harvard Observatory took the first photographs of the stars.
The use of photographic equipment in conjunction with telescopes benefited astronomers greatly, giving them two distinct advantages: first, photographic images provided a permanent record of celestial phenomena, and, second, photographic plates integrated the light from celestial sources over long periods of time and thereby permitted astronomers to see much-fainter objects than they would be able to observe visually. Typically, the camera’s photographic plate (or film) was mounted in the focal plane of the telescope. The plate or film consisted of glass or of a plastic material that was covered with a thin layer of a silver compound. The light striking the photographic medium caused the silver compound to undergo a chemical change. When processed, a negative image resulted; i.e., the brightest spots (the Moon and the stars, for example) appeared as the darkest areas on the plate or the film. In the 1980s the CCD supplanted photography in the production of astronomical images.
Spectrographs
spectrograph
spectrograph
A spectrograph and its components.
Newton noted the interesting way in which a piece of glass can break up light into different bands of colour, but it was not until 1814 that the German physicist Joseph von Fraunhofer discovered the lines of the solar spectrum and laid the basis for spectroscopy. The spectrograph consists of a slit, a collimator, a prism for dispersing the light, and a focusing lens. The collimator is an optical device that produces parallel rays from a focal plane source—i.e., it gives the appearance that the source is located at an infinite distance. The spectrograph enables astronomers to analyze the chemical composition of planetary atmospheres, stars, nebulae, and other celestial objects. A bright line in the spectrum indicates the presence of a glowing gas radiating at a wavelength characteristic of the chemical element in the gas. A dark line in the spectrum usually means that a cooler gas has intervened and absorbed the lines of the element characteristic of the intervening material. The lines may be displaced to either the red end or the blue end of the spectrum. This effect was first noted in 1842 by the Austrian physicist Christian Johann Doppler. When a light source is approaching, the lines are shifted toward the blue end of the spectrum, and when the source is receding, the lines are shifted toward its red end. This effect, known as the Doppler effect, permits astronomers to study the relative motions of celestial objects with respect to Earth’s motion.
The slit of the spectrograph is placed at the focal plane of the telescope. The resulting spectrum may be recorded photographically or with some kind of electronic detector, such as a photomultiplier tube, CCD, or CID. If no recording device is used, then the optical device is technically referred to as a spectroscope.
Photomultiplier tubes
The photomultiplier tube is an enhanced version of the photocell, which was first used by astronomers to record data electronically. The photocell contains a photosensitive surface that generates an electric current when struck by light from a celestial source. The photosensitive surface is positioned just behind the focus. A diaphragm of very small aperture is usually placed in the focal plane to eliminate as much of the background light of the sky as possible. A small lens is used to focus the focal plane image on the photosensitive surface, which, in the case of a photomultiplier tube, is referred to as the photocathode. In the photomultiplier tube a series of special sensitive plates are arranged geometrically to amplify or multiply the electron stream. Frequently, magnifications of a million are achieved by this process.
The photomultiplier tube has a distinct advantage over the photographic plate. With the photographic plate the relationship between the brightness of the celestial source and its registration on the plate is not linear. In the case of the photomultiplier tube, however, the release of electrons in the tube is directly proportional to the intensity of light from the celestial source. This linear relationship is very useful for working over a wide range of brightness. A disadvantage of the photomultiplier tube is that only one object can be recorded at a time. The output from such a device is sent to a recorder or digital storage device to produce a permanent record.
Charge-coupled devices
The charge-coupled device (CCD) uses a light-sensitive material on a silicon chip to electronically detect photons in a way similar to the photomultiplier tube. The principal difference is that the chip also contains integrated microcircuitry required to transfer the detected signal along a row of discrete picture elements (or pixels) and thereby scan a celestial object or objects very rapidly. When individual pixels are arranged simply in a single row, the detector is referred to as a linear array. When the pixels are arranged in rows and columns, the assemblage is called a two-dimensional array.
Pixels can be assembled in various sizes and shapes. The Hubble Space Telescope has a CCD detector with a 1,600 × 1,600 pixel array. Actually, there are four 800 × 800 pixel arrays mosaicked together. The sensitivity of a CCD is 100 times greater than a photographic plate and so has the ability to quickly scan objects such as planets, nebulae, and star clusters and record the desired data. Another feature of the CCD is that the detector material may be altered to provide more sensitivity at different wavelengths. Thus, some detectors are more sensitive in the blue region of the spectrum than in the red region.
Today, most large observatories use CCDs to record data electronically. Another similar device, the charge injection device, is sometimes employed. The basic difference between the CID and the CCD is in the way the electric charge is transferred before it is recorded; however, the two devices may be used interchangeably as far as astronomical work is concerned.
Impact of technological developments
Computers
Besides the telescope itself, the electronic computer has become the astronomer’s most important tool. Indeed, the computer has revolutionized the use of the telescope to the point where the collection of observational data is now completely automated. The astronomer need only identify the object to be observed, and the rest is carried out by the computer and auxiliary electronic equipment.
A telescope can be set to observe automatically by means of electronic sensors appropriately placed on the telescope axis. Precise quartz or atomic clocks send signals to the computer, which in turn activates the telescope sensors to collect data at the proper time. The computer not only makes possible more efficient use of telescope time but also permits a more detailed analysis of the data collected than could have been done manually. Data analysis that would have taken a lifetime or longer to complete with a mechanical calculator can now be done within hours or even minutes with a high-speed computer.
Improved means of recording and storing computer data also have contributed to astronomical research. Optical disc data-storage technology, such as the CD-ROM (compact disc read-only memory) or the DVD-ROM (digital video disc read-only memory), has provided astronomers with the ability to store and retrieve vast amounts of telescopic and other astronomical data.
Rockets and spacecraft
The quest for new knowledge about the universe has led astronomers to study electromagnetic radiation other than just visible light. Such forms of radiation, however, are blocked for the most part by Earth’s atmosphere, and so their detection and analysis can only be achieved from above this gaseous envelope.
During the late 1940s, single-stage sounding rockets were sent up to 160 km (100 miles) or more to explore the upper layers of the atmosphere. From 1957, more sophisticated multistage rockets were launched as part of the International Geophysical Year. These rockets carried artificial satellites equipped with a variety of scientific instruments. Beginning in 1959, the Soviet Union and the United States, engaged in a “space race,” intensified their efforts and launched a series of robotic probes to explore the Moon. Lunar exploration culminated with the first crewed landing on the Moon, by the U.S. Apollo 11 astronauts on July 20, 1969. Numerous other U.S. and Soviet spacecraft were sent to further study the lunar environment until the mid-1970s. Lunar exploration revived in the early years of the 21st century with the United States, China, Japan, and India all sending robotic probes to the Moon.
Mars Exploration Rover
Mars Exploration Rover
Artist's conception of Mars Exploration Rover.
Starting in the early 1960s, both the United States and the Soviet Union launched a multitude of robotic deep-space probes to learn more about the other planets and satellites of the solar system. Carrying television cameras, detectors, and an assortment of other instruments, these probes sent back impressive amounts of scientific data and close-up pictures. Among the most successful missions were those involving the U.S. Messenger flybys of Mercury (2008–15), the Soviet Venera probes to Venus (1967–83), the U.S. Mars Exploration Rover landings on Mars (2004–18), and the U.S. Voyager 2 flybys of Jupiter, Saturn, Uranus, and Neptune (1979–89). When the Voyager 2 probe flew past Neptune and its moons in August 1989, every known major planet had been explored by spacecraft. Many long-held views, particularly those about the outer planets, were altered by the findings of the Voyager probe. These findings included the discovery of several rings and six additional satellites around Neptune, all of which are undetectable to ground-based telescopes.
Specially instrumented spacecraft have enabled astronomers to investigate other celestial phenomena as well. The Orbiting Solar Observatories and Solar Maximum Mission (Earth-orbiting U.S. satellites equipped with ultraviolet detector systems) have provided a means for studying solar activity. Another example is the Giotto probe of the European Space Agency, which enabled astronomers to obtain detailed photographs of the nucleus of Halley’s Comet during its 1986 passage.
Astronomy
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From Wikipedia, the free encyclopedia
This article is about the scientific study of celestial objects. For other uses, see Astronomy (disambiguation).
Not to be confused with astrology, a pseudoscience.
The Paranal Observatory of European Southern Observatory shooting a laser guide star to the Galactic Center
Astronomy is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole.
Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Egyptians, Babylonians, Greeks, Indians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars.
Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.
Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets.
Etymology
Astronomical Observatory, New South Wales, Australia 1873
19th-century Quito Astronomical Observatory is located 12 minutes south of the Equator in Quito, Ecuador.[1]
Astronomy (from the Greek ἀστρονομία from ἄστρον astron, "star" and -νομία -nomia from νόμος nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.[2] Although the two fields share a common origin, they are now entirely distinct.[3]
Use of terms "astronomy" and "astrophysics"
"Astronomy" and "astrophysics" are synonyms.[4][5][6] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties,"[7] while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[8] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[9] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[4] Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,[5] and many professional astronomers have physics rather than astronomy degrees.[6] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy & Astrophysics.
History
Main article: History of astronomy
For a chronological guide, see Timeline of astronomy.
Further information: Archaeoastronomy and List of astronomers
A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.
Ancient times
In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that may have had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year.[10]
Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Egypt, Mesopotamia, Greece, Persia, India, China, and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy.[11]
The Suryaprajnaptisūtra, a 6th-century BC astronomy text of Jains at The Schoyen Collection, London. Above: its manuscript from c. 1500 AD.[12]
A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations.[13] The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.[14]
Greek equatorial sundial, Alexandria on the Oxus, present-day Afghanistan 3rd–2nd century BC.
Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena.[15] In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model.[16] In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe.[17] Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.[18] The Antikythera mechanism (c. 150–80 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[19]
Middle Ages
Medieval Europe housed a number of important astronomers. Richard of Wallingford (1292–1336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astronomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes and could predict eclipses. Nicole Oresme (1320–1382) and Jean Buridan (1300–1361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia) which was able to show planets were capable of motion without the intervention of angels.[20] Georg von Peuerbach (1423–1461) and Regiomontanus (1436–1476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.
Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.[21][22][23] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars.[24] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Iranian scholar Al-Biruni observed that, contrary to Ptolemy, the Sun's apogee (highest point in the heavens) was mobile, not fixed.[25] Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Abd al-Rahman al-Sufi, Biruni, Abū Ishāq Ibrāhīm al-Zarqālī, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.[26][27]
It is also believed that the ruins at Great Zimbabwe and Timbuktu[28] may have housed astronomical observatories.[29] In Post-classical West Africa, Astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens as well as precise diagrams of orbits of the other planets based on complex mathematical calculations. Songhai historian Mahmud Kati documented a meteor shower in August 1583.[30][31] Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during the pre-colonial Middle Ages, but modern discoveries show otherwise.[32][33][34][35]
For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.[36]
Scientific revolution
Galileo's sketches and observations of the Moon revealed that the surface was mountainous.
An astronomical chart from an early scientific manuscript, c. 1000.
During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.[37] It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope.[38]
Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars,[39] More extensive star catalogues were produced by Nicolas Louis de Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[40]
During the 18–19th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[41]
Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814–15, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.[26]
The existence of the Earth's galaxy, the Milky Way, as its own group of stars was only proved in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe.[42] Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory was formulated, heavily evidenced by cosmic microwave background radiation, Hubble's law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.[43] In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.[44][45]
Observational astronomy
Main article: Observational astronomy
The main source of information about celestial bodies and other objects is visible light, or more generally electromagnetic radiation.[46] Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.
Radio astronomy
The Very Large Array in New Mexico, an example of a radio telescope
Main article: Radio astronomy
Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range.[47] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[47]
Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields.[47] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.[9][47]
A wide variety of other objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[9][47]
Infrared astronomy
ALMA Observatory is one of the highest observatory sites on Earth. Atacama, Chile.[48]
Main article: Infrared astronomy
Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galactic protostars and their host star clusters.[49][50] With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.[51] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.[52]
Optical astronomy
The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.
Main article: Optical astronomy
Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[53] Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),[53] that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.
Ultraviolet astronomy
Main article: Ultraviolet astronomy
Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).[47] Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[47] However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.[47]
X-ray astronomy
Main article: X-ray astronomy
X-ray jet made from a supermassive black hole found by NASA's Chandra X-ray Observatory, made visible by light from the early Universe
X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 107 (10 million) kelvins, and thermal emission from thick gases above 107 Kelvin.[47] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[47]
Gamma-ray astronomy
Main article: Gamma ray astronomy
Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[47] The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.[54]
Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[47]
Fields not based on the electromagnetic spectrum
In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.
In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A.[47] Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories.[55] Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.[47]
Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole.[56] A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.[57][58]
The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.[59][60]
Astrometry and celestial mechanics
Main articles: Astrometry and Celestial mechanics
Star cluster Pismis 24 with a nebula
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.[61]: 39
Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects.[62]
The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy.[63]
During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.[64]
Theoretical astronomy
Nucleosynthesis
Stellar nucleosynthesis
Big Bang nucleosynthesis
Supernova nucleosynthesis
Cosmic ray spallation
Related topics
Astrophysics
Nuclear fusion
R-process
S-process
Nuclear fission
vte
Main article: Theoretical astronomy
Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.[65][66]
Theorists in astronomy endeavor to create theoretical models that are based on existing observations and known physics, and to predict observational consequences of those models. The observation of phenomena predicted by a model allows astronomers to select between several alternative or conflicting models. Theorists also modify existing models to take into account new observations. In some cases, a large amount of observational data that is inconsistent with a model may lead to abandoning it largely or completely, as for geocentric theory, the existence of luminiferous aether, and the steady-state model of cosmic evolution.
Phenomena modeled by theoretical astronomers include:
stellar dynamics and evolution
galaxy formation
large-scale distribution of matter in the Universe
the origin of cosmic rays
general relativity and physical cosmology, including string cosmology and astroparticle physics.
Modern theoretical astronomy reflects dramatic advances in observation since the 1990s, including studies of the cosmic microwave background, distant supernovae and galaxy redshifts, which have led to the development of a standard model of cosmology. This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.[67]
Specific subfields
Astrophysics
Main article: Astrophysics
Astrophysics applies physics and chemistry to understand the measurements made by astronomy. Representation of the Observable Universe that includes images from Hubble and other telescopes.
Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space".[68][69] Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background.[70][71] Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.
In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe.[70] Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.
Astrochemistry
Main article: Astrochemistry
Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form. Studies in this field contribute to the understanding of the formation of the Solar System, Earth's origin and geology, abiogenesis, and the origin of climate and oceans.[72]
Astrobiology
Main article: Astrobiology
Astrobiology is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and how humans can detect it if it does.[73] The term exobiology is similar.[74]
Astrobiology makes use of molecular biology, biophysics, biochemistry, chemistry, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth.[75] The origin and early evolution of life is an inseparable part of the discipline of astrobiology.[76] Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.
This interdisciplinary field encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space.[77][78][79]
Physical cosmology
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Main article: Physical cosmology
Cosmology (from the Greek κόσμος (kosmos) "world, universe" and λόγος (logos) "word, study" or literally "logic") could be considered the study of the Universe as a whole.
Hubble Extreme Deep Field
Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the Big Bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years[80] to its present condition.[81] The concept of the Big Bang can be traced back to the discovery of the microwave background radiation in 1965.[81]
In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early Universe.[81] (See also nucleocosmochronology.)
When the first neutral atoms formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.[82]
A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.[83]
Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.[84]
Fundamental to the structure of the Universe is the existence of dark matter and dark energy. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.[85]
Extragalactic astronomy
This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lens effect of the cluster of yellow galaxies near the middle of the photograph. The lens is produced by the cluster's gravitational field that bends light to magnify and distort the image of a more distant object.
Main article: Extragalactic astronomy
The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.[61]
Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[86]
As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and older stars.[61]: 877–878 Elliptical galaxies may have been formed by other galaxies merging.[61]: 939
A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.[61]: 875
Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical.[61]: 879 About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.[87]
An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a supermassive black hole that is emitting radiation from in-falling material.[61]: 907 A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, quasars, and blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[88]
The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between.[89]
Galactic astronomy
Observed structure of the Milky Way's spiral arms
Main article: Galactic astronomy
The Solar System orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.[61]: 837–842, 944
In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.[90]
Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[91]
As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.[92]
Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[93]
Stellar astronomy
Mz 3, often referred to as the Ant planetary nebula. Ejecting gas from the dying central star shows symmetrical patterns unlike the chaotic patterns of ordinary explosions.
Main article: Star
See also: Solar astronomy
The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.[94] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[91]
Almost all elements heavier than hydrogen and helium were created inside the cores of stars.[94]
The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core is in turn consumed. Very massive stars can also undergo a series of evolutionary phases, as they fuse increasingly heavier elements.[95]
The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae;[96] while smaller stars blow off their outer layers and leave behind the inert core in the form of a white dwarf. The ejection of the outer layers forms a planetary nebula.[97] The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[98] Closely orbiting binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova.[99] Planetary nebulae and supernovae distribute the "metals" produced in the star by fusion to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.[100]
Solar astronomy
An ultraviolet image of the Sun's active photosphere as viewed by the TRACE space telescope. NASA photo
Solar observatory Lomnický štít (Slovakia) built in 1962
Main article: Sun
See also: Solar telescope
At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 billion years (Gyr) old. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year oscillation in sunspot number. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.[101]
The Sun has steadily increased in luminosity by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[102] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[103]
At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. Above that is the convection zone where the gas material transports energy primarily through physical displacement of the gas known as convection. It is believed that the movement of mass within the convection zone creates the magnetic activity that generates sunspots.[101] The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, and finally by the super-heated corona.[61]: 498–502
A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth's magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines then descend into the atmosphere.[104]
Planetary science
The black spot at the top is a dust devil climbing a crater wall on Mars. This moving, swirling column of Martian atmosphere (comparable to a terrestrial tornado) created the long, dark streak.
Main articles: Planetary science and Planetary geology
Planetary science is the study of the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The Solar System has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of the Sun's planetary system, although many new discoveries are still being made.[105]
The Solar System is divided into the inner Solar System (subdivided into the inner planets and the asteroid belt), the outer Solar System (subdivided into the outer planets and centaurs), comets, the trans-Neptunian region (subdivided into the Kuiper belt, and the scattered disc) and the farthest regions (e.g., boundaries of the heliosphere, and the Oort Cloud, which may extend as far as a light-year). The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer giant planets are the gas giants (Jupiter and Saturn) and the ice giants (Uranus and Neptune).[106]
The planets were formed 4.6 billion years ago in the protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided and one such collision may have formed the Moon.[107]
Once a planet reaches sufficient mass, the materials of different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer crust. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[108]
A planet or moon's interior heat is produced from the collisions that created the body, by the decay of radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating caused by interactions with other bodies. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[109]
Interdisciplinary studies
Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence. Astrobiology is the study of the advent and evolution of biological systems in the Universe, with particular emphasis on the possibility of non-terrestrial life. Astrostatistics is the application of statistics to astrophysics to the analysis of a vast amount of observational astrophysical data.[110]
The study of chemicals found in space, including their formation, interaction and destruction, is called astrochemistry. These substances are usually found in molecular clouds, although they may also appear in low-temperature stars, brown dwarfs and planets. Cosmochemistry is the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios. Both of these fields represent an overlap of the disciplines of astronomy and chemistry. As "forensic astronomy", finally, methods from astronomy have been used to solve problems of art history[111][112] and occasionally of law.[113]
Amateur astronomy
Amateur astronomers can build their own equipment, and hold star parties and gatherings, such as Stellafane.
Main article: Amateur astronomy
Astronomy is one of the sciences to which amateurs can contribute the most.[114]
Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with consumer-level equipment or equipment that they build themselves. Common targets of amateur astronomers include the Sun, the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs are located throughout the world and many have programs to help their members set up and complete observational programs including those to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues of points of interest in the night sky. One branch of amateur astronomy, astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events that interest them.[115][116]
Most amateurs work at visible wavelengths, but many experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky, who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).[117][118]
Amateur astronomers continue to make scientific contributions to the field of astronomy and it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.[119][120][121]
Unsolved problems in astronomy
Main article: List of unsolved problems in astronomy
In the 21st century there remain important unanswered questions in astronomy. Some are cosmic in scope: for example, what are dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.[122] What will be the ultimate fate of the universe?[123] Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model?[124] Others pertain to more specific classes of phenomena. For example, is the Solar System normal or atypical?[125] What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions?[126] Likewise, questions remain about the formation of the first galaxies,[127] the origin of supermassive black holes,[128] the source of ultra-high-energy cosmic rays,[129] and more.
Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.[130][131]
See also
Cosmogony – Branch of science or a theory concerning the origin of the universe
Outline of astronomy
Outline of space science – Overview of and topical guide to space science
Space exploration – Exploration of space, planets, and moons
Lists
Glossary of astronomy – List of definitions of terms and concepts commonly used in the study of astronomy
List of astronomical instruments
List of astronomical observatories
List of astronomy acronyms
List of software for astronomy research and education
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