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{{About|the scientific study of celestial objects}}{{distinguish|text=astrology, the pseudoscience}}{{pp-move-indef}}{{Good article}}File:Crab Nebula.jpg|thumb|upright=1.3|A giant Hubble mosaic of the Crab Nebula, a supernova remnantsupernova remnantFile:The Galactic Centre and Bulge above the ESO 3.6-metre telescope.jpg|290px|thumb|The Milky Way as viewed from La Silla ObservatoryLa Silla ObservatoryAstronomy (from ) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars blazars, pulsars, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, which is the study of the Universe as a whole.BOOK, Unsöld, Albrecht, Baschek, Bodo, Classical Astronomy and the Solar System – Introduction, 2001, 1, Astronomy is one of the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, Maya, and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is now often considered to be synonymous with astrophysics.BOOK, Unsöld, Albrecht, Baschek, Bodo, Classical Astronomy and the Solar System, 2001, 6–9, Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, which 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. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results.Astronomy is one of the few sciences in which amateurs still play an active role, especially in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets.

Etymology

File:SLNSW 479519 16 Observatory SH 198.jpg|thumb|19th-century Sydney Observatory, Australia (1873)weblink{{dead link|date=August 2017 |bot=InternetArchiveBot |fix-attempted=yes }} Official Web Site of the Sydney ObservatorySydney ObservatoryFile:Quito Observatory.JPG|thumb|19th-century Quito Astronomical Observatory is located 12 minutes south of the Equator in Quito, Ecuadorweblink Official Web Site of one of the oldest Observatories in South America, the Quito Astronomical ObservatoryQuito Astronomical ObservatoryAstronomy (from the Greek (wikt:ἀστρονομία#Ancient Greek|ἀστρονομία) from (wikt:ἄστρον#Ancient Greek|ἄστρον) astron, "star" and -νομία (wikt:-nomy|-nomia) from (wikt:νόμος#Ancient Greek|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.Losev A., (2012), 'Astronomy' or 'astrology': a brief history of an apparent confusion, Journal of Astronomical History and Heritage, Vol. 15, No. 1, pp. 42–46 . Although the two fields share a common origin, they are now entirely distinct.BOOK, Albrecht, Unsöld, Baschek, Bodo, Translated by Brewer, W.D., The New Cosmos: An Introduction to Astronomy and Astrophysics, 2001, Berlin, New York, Springer, 978-3-540-67877-9,

Use of terms "astronomy" and "astrophysics"

Generally, both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject.WEB,weblink Curious About Astronomy: What is the difference between astronomy and astrophysics?, Scharringhausen, B., 17 November 2016,weblink" title="web.archive.org/web/20070609102139weblink">weblink 9 June 2007, yes, WEB,weblink Archive of Astronomy Questions and Answers: What is the difference between astronomy and astrophysics?, Odenwald, Sten, astronomycafe.net, The Astronomy Cafe, 20 June 2007,weblink" title="web.archive.org/web/20070708092148weblink">weblink 8 July 2007, no, WEB
, Penn State Erie-School of Science-Astronomy and Astrophysics
,weblink
, 20 June 2007
, yes
,weblink" title="web.archive.org/web/20071101100832weblink">weblink
, 1 November 2007
, dmy
, 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,"WEB, Merriam-Webster Online, Results for "astronomy",weblink
archiveurl=weblink" title="web.archive.org/web/20070617131203weblink">weblink deadurl= no, while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena."WEB, Merriam-Webster Online, Results for "astrophysics",weblink 20 June 2007, 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.F.H. >LAST=SHU, The Physical Universe, University Science Books, 1983, Mill Valley, California, 978-0-935702-05-7, However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics. 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, and many professional astronomers have physics rather than astronomy degrees. Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy and Astrophysics.

History

{{Further|Archaeoastronomy|List of astronomers}}File:Planisphæri cœleste.jpg|thumb|A celestial map from the 17th century, by the Dutch cartographer Frederik de WitFrederik 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 possibly 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.Forbes, 1909Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Mesopotamia, Greece, Persia, India, China, Egypt, 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.BOOK, DeWitt, Richard, Worldviews: An Introduction to the History and Philosophy of Science, 2010, Wiley, Chichester, England, 978-1-4051-9563-8, 113, The Ptolemaic System, 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.JOURNAL, Scientific Astronomy in Antiquity, Aaboe, A., Philosophical Transactions of the Royal Society, 276, 1257, 1974, 21–42, 74272, 10.1098/rsta.1974.0007, 1974RSPTA.276...21A, The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.WEB,weblink Eclipses and the Saros, NASA, 28 October 2007,weblink" title="web.archive.org/web/20071030225501weblink">weblink 30 October 2007, yes, dmy, File:AiKhanoumSunDial.jpg|thumb|Greek equatorial sundial, Alexandria on the OxusAlexandria on the OxusFollowing 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.BOOK, Krafft, Fritz, 2009, Astronomy, Cancik, Hubert, Schneider, Helmuth, Brill's New Pauly, harv, Brill's New Pauly, 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.JOURNAL, Aristarchus's On the Sizes and Distances of the Sun and the Moon: Greek and Arabic Texts, Archive for History of Exact Sciences, May 2007, J.L., Berrgren, Nathan Sidoli, 61, 3, 213–54, 10.1007/s00407-006-0118-4, 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.WEB,weblink Hipparchus of Rhodes, School of Mathematics and Statistics, University of St Andrews, Scotland, 28 October 2007,weblink" title="web.archive.org/web/20071023062202weblink">weblink 23 October 2007, no, Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.Thurston, H., Early Astronomy. Springer, 1996. {{ISBN|0-387-94822-8}} p. 2 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.JOURNAL, Marchant, Jo, In search of lost time, Nature, 444, 7119, 534–38, 2006, 17136067, 10.1038/444534a, 2006Natur.444..534M,

Middle Ages

During the Middle Ages, astronomy was mostly stagnant in medieval Europe, at least until the 13th century. Nevertheless medieval Europe housed a number of astronomers who made a progress. 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 astornomical 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.Hannam, James. God's philosophers: how the medieval world laid the foundations of modern science. Icon Books Ltd, 2009, 180 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.JOURNAL, Kennedy, Edward S., 1962, Review: The Observatory in Islam and Its Place in the General History of the Observatory by Aydin Sayili, Isis (journal), Isis, 53, 2, 237–39, 10.1086/349558, JOURNAL, Micheau, Francoise, Rashed, Roshdi, Morelon, Régis, The Scientific Institutions in the Medieval Near East, Encyclopedia of the History of Arabic Science, 3, 992–93, BOOK, Nas, Peter J, Urban Symbolism, 1993, Brill Academic Publishers, 978-90-04-09855-8, 350, In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian astronomer Azophi in his Book of Fixed Stars.BOOK, Kepple, George Robert, Glen W. Sanner, The Night Sky Observer's Guide, 1, Willmann-Bell, Inc., 1998, 978-0-943396-58-3, 18, 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. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Azophi, Albumasar, Biruni, Arzachel, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Astronomers during that time introduced many Arabic names now used for individual stars.BOOK, Arthur, Berry, A Short History of Astronomy From Earliest Times Through the 19th Century, Dover Publications, Inc., New York, 1961, 978-0-486-20210-5, BOOK, Hoskin, Michael, The Cambridge Concise History of Astronomy, Cambridge University Press, 1999, 978-0-521-57600-0, It is also believed that the ruins at Great Zimbabwe and TimbuktuBOOK,weblink The royal kingdoms of Ghana, Mali, and Songhay: life in medieval Africa, Pat, McKissack, McKissack, Frederick, 1995, H. Holt, 978-0-8050-4259-7, may have housed astronomical observatories.JOURNAL,weblink Eclipse brings claim of medieval African observatory, 2002, New Scientist, 3 February 2010, Clark, Stuart, Carrington, Damian, 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.WEB,weblink Cosmic Africa explores Africa's astronomy, 3 February 2002, Science in Africa, yes,weblink" title="web.archive.org/web/20031203055223weblink">weblink 3 December 2003, dmy, BOOK,weblink African Cultural Astronomy, Jarita C., Holbrook, Medupe, R. Thebe, Urama, Johnson O., Springer, 2008, 978-1-4020-6638-2, WEB,weblink Africans studied astronomy in medieval times, 30 January 2006, The Royal Society, 3 February 2010,weblink" title="web.archive.org/web/20080609112829weblink">weblink 9 June 2008, Stenger, Richard NEWS,weblink Star sheds light on African 'Stonehenge', CNN, 5 December 2002, yes,weblink" title="web.archive.org/web/20110512162930weblink">weblink 12 May 2011, dmy, . CNN. 5 December 2002. Retrieved on 30 December 2011.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.J.L. Heilbron, The Sun in the Church: Cathedrals as Solar Observatories (1999) p.3

Scientific revolution

File:Galileo moon phases.jpg|thumb|upright|Galileo's sketches and observations of the MoonMoon(File:Medieval Astronomy (f.4v).jpg|thumb|upright|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.Forbes, 1909, pp. 49–58 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.Forbes, 1909, pp. 58–64Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars,Chambers, Robert (1864) Chambers Book of Days 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.Forbes, 1909, pp. 79–81 The distance to a star was announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.Forbes, 1909, pp. 147–50During 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.Forbes, 1909, pp. 74–76Significant 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.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.BOOK, Belkora, Leila, Minding the heavens: the story of our discovery of the Milky Way, 978-0-7503-0730-7,weblink CRC Press, 2003, 1–14, 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.{{citation needed|date=March 2018}} In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.JOURNAL, Einstein's gravitational waves found at last, Nature News,weblink 11 February 2016, Castelvecchi, Davide, Witze, Witze, 10.1038/nature.2016.19361, 2016-02-11, JOURNAL, Observation of Gravitational Waves from a Binary Black Hole Merger, B.P. Abbott (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters, 2016, 116, 6, 061102, 10.1103/PhysRevLett.116.061102, 26918975, 2016PhRvL.116f1102A, 1602.03837, etal,

Observational astronomy

Our main source of information about celestial bodies and other objects is visible light, or more generally electromagnetic radiation.WEB,weblink Electromagnetic Spectrum, NASA, 17 November 2016,weblink" title="web.archive.org/web/20060905131651weblink">weblink 5 September 2006, yes, 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

File:USA.NM.VeryLargeArray.02.jpg|thumb|The Very Large Array in New Mexico, an example of a radio telescoperadio telescopeRadio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range.BOOK, Cox, A.N., Allen's Astrophysical Quantities, 2000,weblink Springer-Verlag, 124, New York
radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and Phase (waves)>phase of radio waves, whereas this is not as easily done at shorter wavelengths.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. Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21 cm, are observable at radio wavelengths.A wide variety of other objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.

Infrared astronomy

File:In Search of Space.jpg|thumb|ALMA Observatory is one of the highest observatory sites on Earth. Atacama, Chile.NEWS, In Search of Space,weblink 5 August 2014, Picture of the Week, European Southern Observatory, ]]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.WEB,weblink Wide-field Infrared Survey Explorer Mission, 30 September 2014, NASA University of California, Berkeley, 17 November 2016, Majaess, D. (2013). Discovering protostars and their host clusters via WISE, ApSS, 344, 1 (VizieR catalog)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.STAFF>DATE=11 SEPTEMBER 2003, Why infrared astronomy is a hot topic, ESA,weblink
LAST=DATE=PUBLISHER=NASA CALIFORNIA INSTITUTE OF TECHNOLOGYDEADURL=YESARCHIVEDATE=5 OCTOBER 2008, dmy-all,

Optical astronomy

File:The Keck Subaru and Infrared obervatories.JPG|thumb|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 FacilityNASA Infrared Telescope FacilityHistorically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.BOOK, Moore, P., Philip's Atlas of the Universe, 1997, George Philis Limited, Great Britain
charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 Ã…ngstrom>Ã… to 7000 Ã… (400 nm to 700 nm), that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 Ã… (10 to 320 nm). 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. However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.

X-ray astronomy

(File:B30727.jpg|thumb|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. 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.

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. 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.WEB,weblink The electromagnetic spectrum, Penston, Margaret J., 14 August 2002, Particle Physics and Astronomy Research Council,weblink" title="archive.is/20120908014227weblink">weblink 8 September 2012, yes, 17 November 2016, 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.

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. 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.THOMAS K.>LAST=GAISSER, 1990
pages=1–2isbn=978-0-521-33931-5, Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.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.JOURNAL, LIGO Scientific Collaboration and Virgo Collaboration, Abbott, Benjamin P., Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett., 116, 6, 061102, 2016, 10.1103/PhysRevLett.116.061102, 1602.03837, 2016PhRvL.116f1102A, 26918975, A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.WEB,weblink Opening new windows in observing the Universe, Tammann, G.A., F.K., 2003, Europhysics News,weblink" title="archive.is/20120906192257weblink">weblink 6 September 2012, yes, D., Thielemann, Trautmann, 17 November 2016, JOURNAL, LIGO Scientific Collaboration and Virgo Collaboration, Abbott, B.P., Abbott, R., Abbott, T.D., Abernathy, M. R., Acernese, F., Ackley, K., Adams, C., Adams, T., 2016-06-15, GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence, Physical Review Letters, 116, 24, 241103, 10.1103/PhysRevLett.116.241103, 27367379, 1606.04855, 2016PhRvL.116x1103A, The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.WEB, Planning for a bright tomorrow: Prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo,weblink LIGO Scientific Collaboration, 31 December 2015, BOOK, Neutrinos in Particle Physics, Astronomy and Cosmology, Zhizhong, Xing, Shun, Zhou, Springer, 2011, 978-3642175602, 313,weblink Extract of page 313

Astrometry and celestial mechanics

File:EmissionNebula NGC6357.jpg|upright|thumb|Star cluster Pismis 24Pismis 24One 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.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.WEB, Calvert, James B., 28 March 2003,weblink Celestial Mechanics, University of Denver, 21 August 2006,weblink" title="web.archive.org/web/20060907120741weblink">weblink 7 September 2006, no, 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 allows 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.WEB,weblink Hall of Precision Astrometry, University of Virginia Department of Astronomy, 17 November 2016,weblink" title="web.archive.org/web/20060826104509weblink">weblink 26 August 2006, yes, During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.JOURNAL, Wolszczan, A., Frail, D. A., A planetary system around the millisecond pulsar PSR1257+12, Nature, 1992, 355, 6356, 145–47, 10.1038/355145a0, harv, 1992Natur.355..145W,

Theoretical astronomy

{{Nucleosynthesis}}Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are generally better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.JOURNAL, H., Roth, A Slowly Contracting or Expanding Fluid Sphere and its Stability, Physical Review, 39, 3, 525–29, 1932, 10.1103/PhysRev.39.525, 1932PhRv...39..525R, BOOK, A.S., Eddington, Internal Constitution of the Stars, Cambridge University Press, 1926,weblink 978-0-521-33708-3, Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.Phenomena modeled by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale distribution of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.A few examples of this process:{| class="wikitable"
Physical process Experimental tool Theoretical model Explains/predicts
GravitationRadio telescopesSelf-gravitating systemEmergence of a star system
Nuclear fusion Spectroscopy Stellar evolution How the stars shine and how metals formed
The Big BangHubble Space Telescope, COBE Expanding universe Age of the Universe
Quantum fluctuations Cosmic inflation Flatness problem
Gravitational collapse X-ray astronomy General relativity Black holes at the center of Andromeda galaxy
CNO cycle in stars The dominant source of energy for massive star.
Dark matter and dark energy are the current leading topics in astronomy,WEB,weblink third paragraph, "There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is", 2 November 2009, NASA, Dark matter, 2010,weblink" title="web.archive.org/web/20091030022817weblink">weblink 30 October 2009, no, as their discovery and controversy originated during the study of the galaxies.

Specific subfields

Astrophysics

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".{{Citation | last = Keeler | first = James E. | author-link = James E. Keeler | title = The Importance of Astrophysical Research and the Relation of Astrophysics to the Other Physical Sciences | journal = The Astrophysical Journal | volume = 6 | issue = 4 | pages = 271–88 | date = November 1897 | bibcode = 1897ApJ.....6..271K |doi = 10.1086/140401| quote =[Astrophysics] is closely allied on the one hand to astronomy, of which it may properly be classed as a branch, and on the other hand to chemistry and physics.… It seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space—what they are, rather than where they are.… That which is perhaps most characteristic of astrophysics is the special prominence which it gives to the study of radiation.}}WEB, astrophysics, Merriam-Webster, Incorporated,weblink 2011-05-22,weblink" title="web.archive.org/web/20110610085146weblink">weblink 10 June 2011, no, Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background.WEB,weblink Focus Areas – NASA Science, nasa.gov, WEB,weblink astronomy, Encyclopædia Britannica, 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. 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.

Solar astronomy

File:Uvsun trace big.jpg|thumb|An ultraviolet image of the Sun's active photosphere as viewed by the TRACE space telescope. NASANASAFile:Observatórium Lomnický štít 1.jpg|thumb|Solar observatory Lomnický štít (SlovakiaSlovakia{{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.WEB,weblink The Solar FAQ, Johansson, Sverker, Sverker Johansson, 27 July 2003, Talk.Origins Archive, 11 August 2006,weblink" title="web.archive.org/web/20060907235636weblink">weblink 7 September 2006, no, 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.WEB,weblink Environmental issues : essential primary sources, Lerner, K. Lee, Brenda Wilmoth, 2006, Thomson Gale,weblink" title="archive.is/20120710152134weblink">weblink 10 July 2012, yes, Lerner, 17 November 2016, The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.WEB, Pogge, Richard W., 1997,weblink The Once & Future Sun, lecture notes, New Vistas in Astronomy, 3 February 2010, yes,weblink" title="web.archive.org/web/20050527094435weblink">weblink 27 May 2005, 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.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.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 the descend into the atmosphere.WEB, Stern, D.P., Peredo, M., 28 September 2004,weblink The Exploration of the Earth's Magnetosphere, NASA, 22 August 2006,weblink" title="web.archive.org/web/20060824003619weblink">weblink 24 August 2006, no,

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.WEB,weblink About Astrobiology, 20 October 2008, 21 January 2008, NASA Astrobiology Institute, NASA,weblink" title="web.archive.org/web/20081011192341weblink">weblink 11 October 2008, The term exobiology is similar.Mirriam Webster Dictionary entry "Exobiology" (accessed 11 April 2013)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.BOOK, The life and death of planet Earth, Ward, P.D., Brownlee, D., 2004, Owl Books, New York, 978-0-8050-7512-0, The origin and early evolution of life is an inseparable part of the discipline of astrobiology.WEB,weblink Origins of Life and Evolution of Biospheres, Journal: Origins of Life and Evolution of Biospheres, 2015-04-06, 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.NEWS,weblink Release of the First Roadmap for European Astrobiology, European Science Foundation, Astrobiology Web, March 29, 2016, 2016-04-02, NEWS, Corum, Jonathan, Mapping Saturn’s Moons,weblink 18 December 2015, The New York Times, 18 December 2015, NEWS, Cockell, Charles S., How the search for aliens can help sustain life on Earth, 4 October 2012,weblink CNN News, 8 October 2012,

Planetary science

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 this planetary system, although many new discoveries are still being made.BOOK,weblink Remote Sensing for the Earth Sciences: Manual of Remote Sensing, 2004, John Wiley & Sons, 3rd, Bell III, J. F., Campbell, B.A., Robinson, M.S., 17 November 2016,weblink" title="web.archive.org/web/20060811220029weblink">weblink 11 August 2006, yes, File:dust.devil.mars.arp.750pix.jpg|thumb|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. NASANASAThe Solar System is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus, and Neptune.WEB, Grayzeck, E., Williams, D.R., 11 May 2006,weblink Lunar and Planetary Science, NASA, 21 August 2006,weblink" title="web.archive.org/web/20060820173205weblink">weblink 20 August 2006, no, Beyond Neptune lies the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.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.JOURNAL, Montmerle, Thierry, Augereau, Jean-Charles, Chaussidon, Marc, Solar System Formation and Early Evolution: the First 100 Million Years, Earth, Moon, and Planets, 98, 1–4, 39–95, 2006, 10.1007/s11038-006-9087-5, 2006EM&P...98...39M, etal, 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.Montmerle, 2006, pp. 87–90A 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.BOOK, Beatty, J.K., Petersen, C.C., Chaikin, A., The New Solar System, Cambridge press,weblink 70edition = 4th, 1999, 978-0-521-64587-4,

Stellar astronomy

File:Ant Nebula.jpg|thumb|Mz 3Mz 3The 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.Harpaz, 1994, pp. 7–18 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.Almost all elements heavier than hydrogen and helium were created inside the cores of stars.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.Harpaz, 1994The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae;Harpaz, 1994, pp. 173–78 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.Harpaz, 1994, pp. 111–18 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.BOOK, Audouze, Jean, Israel, Guy, The Cambridge Atlas of Astronomy, 3rd, Cambridge University Press, 1994, 978-0-521-43438-6, 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.Harpaz, 1994, pp. 189–210 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.Harpaz, 1994, pp. 245–56{{see also|Solar astronomy}}

Galactic astronomy

File:Milky Way Spiral Arm.svg|right|thumb|Observed structure of the Milky WayMilky WayOur 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.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.WEB,weblink The Galactic Centre, Ott, Thomas, 24 August 2006, Max-Planck-Institut für extraterrestrische Physik, 17 November 2016,weblink" title="web.archive.org/web/20060904140550weblink">weblink 4 September 2006, yes, 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.BOOK, Michael David, Smith, 2004, 53–86, The Origin of Stars, Cloud formation, Evolution and Destruction, Imperial College Press, 978-1-86094-501-4,weblink 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.BOOK, Michael David, Smith, 2004, 185–99, The Origin of Stars, Massive starsisbn=978-1-86094-501-4,weblink 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.JOURNAL, Van den Bergh, Sidney, The Early History of Dark Matter, Publications of the Astronomical Society of the Pacific, 1999, 111, 760, 657–60, 10.1086/316369, astro-ph/9904251, 1999PASP..111..657V,

Astrochemistry

Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation.NEWS,weblink Astrochemistry, 2013-07-15, www.cfa.harvard.edu/, 2016-11-20,weblink 2016-11-20, yes, 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.

Extragalactic astronomy

File:grav.lens1.arp.750pix.jpg|thumb|This image shows several blue, loop-shaped objects that are multiple images of the same galaxy, duplicated by the gravitational lensgravitational lensThe 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.Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.WEB, Keel, Bill, 1 August 2006,weblink Galaxy Classification, University of Alabama, 8 September 2006,weblink" title="web.archive.org/web/20060901074027weblink">weblink 1 September 2006, no, 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 generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been formed through mergers of large galaxies.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.Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.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 super-massive black hole that is emitting radiation from in-falling material.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.WEB,weblink Active Galaxies and Quasars, NASA, 17 November 2016,weblink" title="web.archive.org/web/20060831033713weblink">weblink 31 August 2006, yes, 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.BOOK, Michael, Zeilik, Astronomy: The Evolving Universe, 8th, Wiley, 2002, 978-0-521-80090-7,

Physical cosmology

{{Nature timeline}}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.File:Hubble Extreme Deep Field (full resolution).png|thumb|left|upright=1.15|Hubble Extreme Deep FieldHubble Extreme Deep FieldObservations 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 yearsWEB, Cosmic Detectives,weblink The European Space Agency (ESA), 2013-04-02, 2013-04-15,
to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.BOOK, Dodelson, Scott, Modern cosmology, 2003, 978-0-12-219141-1, Academic Press, 1–22,
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. (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.WEB, Hinshaw, Gary, 13 July 2006,weblink Cosmology 101: The Study of the Universe, NASA WMAP, 10 August 2006,weblink" title="web.archive.org/web/20060813053535weblink">weblink 13 August 2006, no, 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.Dodelson, 2003, pp. 216–61Gravitational 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.WEB,weblink Galaxy Clusters and Large-Scale Structure, University of Cambridge, 8 September 2006,weblink" title="web.archive.org/web/20061010041120weblink">weblink 10 October 2006, no, Various fields of physics are crucial to studying the universe. Interdisciplinary studies involve the fields of quantum mechanics, particle physics, plasma physics, condensed matter physics, statistical mechanics, optics, and nuclear physics.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.WEB, Preuss, Paul,weblink Dark Energy Fills the Cosmos, U.S. Department of Energy, Berkeley Lab, 8 September 2006,weblink" title="web.archive.org/web/20060811215815weblink">weblink 11 August 2006, no,

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 vast amount of observational astrophysical data.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 law and history.

Amateur astronomy

File:Telescope trailer 22.jpg|right|thumb|upright|Amateur astronomers can build their own equipment, and hold star parties and gatherings, such as StellafaneStellafaneAstronomy is one of the sciences to which amateurs can contribute the most.MIMS III>FIRST = FORREST M., Amateur Science—Strong Tradition, Bright Futuredate=1999issue=5411, 55–56, 10.1126/science.284.5411.55, Astronomy has traditionally been among the most fertile fields for serious amateurs [...], 1999Sci...284...55M, Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with 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, amateur 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 which interest them.WEB,weblink The American Meteor Society, 24 August 2006,weblink" title="web.archive.org/web/20060822135040weblink">weblink 22 August 2006, no, WEB, Jerry, Lodriguss,weblink Catching the Light: Astrophotography, 24 August 2006,weblink" title="web.archive.org/web/20060901185541weblink">weblink 1 September 2006, no, Most amateurs work at visible wavelengths, but a small minority 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).WEB, Ghigo, F., 7 February 2006,weblink Karl Jansky and the Discovery of Cosmic Radio Waves, National Radio Astronomy Observatory, 24 August 2006,weblink" title="web.archive.org/web/20060831105945weblink">weblink 31 August 2006, no, WEB,weblink Cambridge Amateur Radio Astronomers, 24 August 2006, 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.WEB,weblink The International Occultation Timing Association, 24 August 2006,weblink" title="web.archive.org/web/20060821180723weblink">weblink 21 August 2006, no, WEB,weblink Edgar Wilson Award, IAU Central Bureau for Astronomical Telegrams, 24 October 2010,weblink" title="web.archive.org/web/20101024202325weblink">weblink 24 October 2010, yes, dmy, WEB,weblink American Association of Variable Star Observers, AAVSO, 3 February 2010,weblink" title="web.archive.org/web/20100202050715weblink">weblink 2 February 2010, no,

Unsolved problems in astronomy

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the Universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.
  • 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?JOURNAL
FIRST = PAVEL, The Initial Mass Function of Stars: Evidence for Uniformity in Variable Systemsdate=2002issue=5552, 82–91, 10.1126/science.1067524, 11778039bibcode = 2002Sci...295...82K, A deeper understanding of the formation of stars and planets is needed.
  • 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.WEB,weblinkweblink" title="web.archive.org/web/20110628214416weblink">weblink 28 June 2011, Rare Earth: Complex Life Elsewhere in the Universe?
accessdate =12 August 2006, 2002-07-15, WEB,weblink The Quest for Extraterrestrial Intelligence, Sagan, Carl, Cosmic Search Magazine, 12 August 2006,weblink" title="web.archive.org/web/20060818144558weblink">weblink 18 August 2006, no, Is the Solar System normal or atypical?
  • What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet their true nature remains unknown.WEB,weblink


, 11 Physics Questions for the New Century, Pacific Northwest National Laboratory
archiveurl =weblink" title="web.archive.org/web/20060203152634weblink">weblink 3 February 2006, What will be the ultimate fate of the universe?HINSHAW>FIRST = GARY, 15 December 2005,weblink What is the Ultimate Fate of the Universe?accessdate =28 May 2007weblink" title="web.archive.org/web/20070529145436weblink">weblink> archivedate= 29 May 2007, no,
  • How did the first galaxies form?WEB, FAQ – How did galaxies form?,weblink NASA, July 28, 2015, How did supermassive black holes form?WEB, Supermassive Black Hole,weblink Swinburne University, July 28, 2015,
  • What is creating the ultra-high-energy cosmic rays?JOURNAL, Annual Review of Astronomy and Astrophysics, The Origin of Ultra-High-Energy Cosmic Rays, Hillas, A.M., 22, September 1984, 10.1146/annurev.aa.22.090184.002233, 425–44, This poses a challenge to these models, because [...], 1984ARA&A..22..425H,
  • Why is the abundance of lithium in the cosmos four times lower than predicted by the standard Big Bang model?JOURNAL, Howk, J. Christopher, Lehner, Nicolas, Fields, Brian D., Mathews, Grant J., 6 September 2012, Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud,weblink Nature, en, 489, 7414, 121–23, 10.1038/nature11407, 22955622, 1207.3081, 2012Natur.489..121H,
  • What really happens beyond the event horizon?WEB,weblink What Happens When You Enter A Black Hole?, 15 December 2014, Business Insider International, Orwig, Jessica, 17 November 2016,

See also

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References

{{Reflist}}

Bibliography

  • BOOK, George, Forbes, History of Astronomy, Plain Label Books, London, 1909, 978-1-60303-159-2, Available at Project Gutenberg, Google books
  • BOOK
FIRST1 = AMOS, Stellar Evolutionisbn = 978-1-56881-012-6weblink>publisher = A K Peters, Ltd,
  • BOOK, Unsöld, A., The New Cosmos: An Introduction to Astronomy and Astrophysics, 2001, Springer, 978-3-540-67877-9, Baschek, B.,

External links

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