This article is about the scientific study of celestial objects and is not to be confused withAstrology, a divinatory pseudoscience.For other uses, seeAstronomy (disambiguation).
Professional astronomy is split intoobservational andtheoretical 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 anactive role. This is especially true for the discovery and observation oftransient events.Amateur astronomers have helped with many important discoveries, such as finding new comets.
Etymology
Astronomy (from theGreekἀστρονομία fromἄστρονastron, "star" and -νομία-nomia fromνόμοςnomos, "law" or "rule") means study of celestial objects.[1] Astronomy should not be confused withastrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Thetwo fields share a common origin but became distinct, astronomy being supported byphysics while astrology is not.[2]
Use of terms "astronomy" and "astrophysics"
"Astronomy" and "astrophysics" are broadly synonymous in modern usage.[3][4][5] In dictionary definitions, "astronomy" is "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties",[6] while "astrophysics" is the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[7] Sometimes, as in the introduction of the introductory textbookThe Physical Universe byFrank Shu, "astronomy" means the qualitative study of the subject, whereas "astrophysics" is the physics-oriented version of the subject.[8] Some fields, such asastrometry, are in this sense purely astronomy rather than also astrophysics. Research departments may use "astronomy" and "astrophysics" according to whether the department is historically affiliated with a physics department,[4] and many professionalastronomers have physics rather than astronomy degrees.[5] Thus, in modern use, the two terms are often used interchangeably.[3]
The initial development of astronomy was driven by practical needs like agricultural calendars. Before recorded history archeological sites such asStonehenge provide evidence of ancient interest in astronomical observations.[12]: 15 Evidence also comes from artefacts such as theNebra sky disc which serves as an astronomical calendar, defining a year as twelvelunar months, 354 days, with intercalary months to make up the solar year. The disc is inlaid with symbols interpreted as a sun, moon, and stars includinga cluster of seven stars.[9][13][14]
Civilizations such asEgypt,Mesopotamia,Greece,India,China together – with cross-cultural influences – created astronomical observatories and developed ideas on the nature of the Universe, along with calendars and astronomical instruments.[16] A key early development was the beginning of mathematical and scientific astronomy among the Babylonians, laying the foundations for astronomical traditions in other civilizations.[17] The Babylonians discovered thatlunar eclipses recurred in thesaros cycle of 223synodic months.[18]
Following the Babylonians, significant advances were made inancient Greece and theHellenistic world. Greek astronomy sought a rational, physical explanation for celestial phenomena.[19] In the 3rd century BC,Aristarchus of Samos estimated thesize and distance of the Moon and Sun, and he proposed a model of theSolar System where the Earth and planets rotated around the Sun, now called theheliocentric model.[20] In the 2nd century BC,Hipparchus calculated the size and distance of the Moon and invented the earliest known astronomical devices such as theastrolabe.[21] He also observed the small drift in the positions of the equinoxes and solstices with respect to the fixed stars that we now know is caused byprecession.[12] Hipparchus also created a catalog of 1020 stars, and most of theconstellations of the northern hemisphere derive from Greek astronomy.[22] TheAntikythera mechanism (c. 150–80 BC) was an earlyanalog computer designed to calculate the location of theSun,Moon, andplanets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanicalastronomical clocks appeared in Europe.[23]
After the classical Greek era, astronomy was dominated by thegeocentric model of the Universe, or thePtolemaic system, named afterClaudius Ptolemy. His 13-volume astronomy work, named theAlmagest in its Arabic translation, became the primary reference for over a thousand years.[24]: 196 In this system, the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it.[25] While the system would eventually be discredited it gave the most accurate predictions for the positions of astronomical bodies available at that time.[24]
The ruins atGreat Zimbabwe andTimbuktu[35] may have housed astronomical observatories.[36] In Post-classical West Africa, astronomers studied the movement of stars and relation to seasons, crafting charts of the heavens and diagrams of orbits of the other planets based on complex mathematical calculations.[37]Songhai historianMahmud Kati documented ameteor shower in 1583.[38]
In medieval Europe,Richard of Wallingford (1292–1336) invented the first astronomical clock, theRectangulus which allowed for the measurement of angles between planets and other astronomical bodies,[39] as well as anequatorium called theAlbion which could be used for astronomical calculations such aslunar,solar andplanetarylongitudes.[40]Nicole Oresme (1320–1382) discussed evidence for the rotation of the Earth.[41]Jean Buridan (1300–1361) developed thetheory of impetus, describing motions including of the celestial bodies.[42][43]For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), theRoman 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 thedate for Easter.[44]
Early telescopic
The first sketches of the Moon's topography, fromGalileo's ground-breakingSidereus Nuncius (1610)
During theRenaissance,Nicolaus Copernicus proposed a heliocentric model of the solar system.[45] In 1610,Galileo Galilei observed phases on the planetVenus similar to those of the Moon, supporting the heliocentric model.[12] Around the same time the heliocentric model was organized quantitatively byJohannes Kepler.[46] Analyzing two decades of careful observations byTycho Brahe, Kepler devised a system that described the details of the motion of the planets around the Sun.[47]: 4 [48] While Kepler discarded the uniform circular motion of Copernicus in favor of elliptical motion,[12] he did not succeed in formulating a theory behind the laws he wrote down.[49] It wasIsaac Newton, with his invention ofcelestial dynamics and hislaw of gravitation, who finally explained the motions of the planets.[50] Newton also developed thereflecting telescope.[51]Newton, in collaboration withRichard Bentley proposed that stars are like the Sun only much further away.[47]
The new telescopes also altered ideas about stars. By 1610 Galileo discovered that the band of light crossing the sky at night that we call theMilky Way was composed of numerous stars.[12]: 48 In 1668James Gregory compared the luminosity of Jupiter toSirius to estimate its distance at over 83,000 AU.[47] The English astronomerJohn Flamsteed, Britain's firstAstronomer Royal, catalogued over 3000 stars but the data were published against his wishes in 1712.[52] The astronomerWilliam Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planetUranus, the first new planet found.[53]Friedrich Bessel developed the technique ofstellar parallax in 1838 but it was so difficult to apply that only about 100 stars were measured by 1900.[47]
Diagram of the stars, from William Herschel'sOn the construction of the heavens.[58]
In the late 1700sWilliam Herschel mapped the distribution of stars in different directions from Earth, concluding that the universe consisted of the Sun near the center of disk of stars, theMilky Way. AfterJohn Michell demonstrated that stars differ in intrinsic luminosity and after Herschel's own observations with more powerful telescopes that additional stars appeared in all directions, astronomers began to consider that some of the fuzzyspiral nebulae were distantisland Universes.[47]: 6
The existence of galaxies, including the Earth's galaxy, theMilky Way, as a group of stars was only demonstrated in the 20th century.[61] In 1912,Henrietta Leavitt discoveredCepheid variable stars with well-defined, periodic luminosity changes which can be used to fix the star's true luminosity which then becomes an accurate tool for distance estimates. Using Cepheid variable stars,Harlow Shapley constructed the first accurate map of the Milky Way.[47]: 7 Using theHooker Telescope,Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922–1923 proved conclusively thatAndromeda Nebula andTriangulum among others, were entire galaxies outside our own, thus proving that the universe consists of a multitude of galaxies.[62]
Albert Einstein's 1917 publication ofgeneral relativity began the modern era of theoretical models of the universe as a whole.[63] In 1922,Alexander Friedman published simplified models for the universe showing static, expanding and contracting solutions.[47]: 13 In 1929 Hubble published observations that the galaxies are all moving away from Earth with a velocity proportional to distance, a relation now known asHubble's law. This relation is expected if theuniverse is expanding.[47]: 13 The consequence that the universe was once very dense and hot, aBig Bang concept expounded byGeorges Lemaître in 1927,[64] was discussed but no experimental evidence was available to support it. From the 1940s on, nuclear reaction rates under high density conditions were studied leading to the development of a successful model ofbig bang nucleosynthesis in the late 1940s and early 1950s. Then in 1965cosmic microwave background radiation was discovered, cementing the evidence for the Big Bang.[47]: 16
Observational astronomy relies on many different wavelengths ofelectromagnetic radiation and the forms of astronomy are categorized according to the corresponding region of theelectromagnetic spectrum on which the observations are made.[72] Specific information on these subfields is given below.
Infrared astronomy detectsinfrared radiation with wavelengths longer than redvisible light, 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 inmolecular clouds and the cores of galaxies. Observations from theWide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous galacticprotostars and their hoststar clusters.[77][78]
With the exception of infraredwavelengths 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.[79] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space.[80]
TheJames Webb Space Telescope senses infrared radiation to detect very distant galaxies. Visible light from these galaxies was emitted billions of years ago and theexpansion of the universe shifted the light in to the infrared range. By studying these distant galaxies astronomers hope to learn about the formation of the first galaxies.[81]
Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy.[82] 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 usingcharge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 380 to 700nm[83] that same equipment can be used to observe somenear-ultraviolet andnear-infrared radiation.[84]
Ultraviolet astronomy employsultraviolet wavelengths which are absorbed by the Earth's atmosphere, requiring observations from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blueOB stars that are very bright at these wavelengths.[85]
Gamma ray astronomy observes astronomical objects at the shortest wavelengths (highest energy) of the electromagnetic spectrum.Gamma rays may be observed directly by satellites such as theCompton Gamma Ray Observatory,[88] or by specialized telescopes calledatmospheric Cherenkov telescopes. 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.[89][90]Gamma-ray astronomy provides information on the origin ofcosmic rays, possibleannihilation events fordark matter, relativistic particles outflows fromactive galactic nuclei (AGN), and, using AGN as distant sources, properties of intergalactic space.[91]Gamma-ray bursts, which radiate transiently, are extremely energetic events, and are the brightest (most luminous) phenomena in the universe.[92]
Some events originating from great distances may be observed from the Earth using systems that do not rely on electromagnetic radiation.[93][94]
Inneutrino astronomy, astronomers use heavily shieldedunderground facilities such asSAGE,GALLEX, andKamioka II/III for the detection ofneutrinos. The vast majority of the neutrinos streaming through the Earth originate from theSun, but 24 neutrinos were also detected fromsupernova 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.[93]
The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known asmulti-messenger astronomy.[98][99]
Use of opticalinterferometry to determine precise positions of stars
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects known as astrometry.[100] Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential incelestial navigation (the use of celestial objects to guide navigation) and in the making ofcalendars.[101] Careful measurement of the positions of the planets has led to a solid understanding of gravitationalperturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known ascelestial mechanics.[102] The measurement ofstellar parallax of nearby stars provides a fundamental baseline in thecosmic 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.[103] Measurements of theradial velocity[104][105] andproper motion of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy.[106]
Theoretical astronomers use several tools includinganalytical models andcomputationalnumerical 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.[107][108] Modern theoretical astronomy reflects dramatic advances in observation since the 1990s, including studies of thecosmic microwave background, distantsupernovae andgalaxy redshifts, which have led to the development of astandard model of cosmology. This model requires the universe to contain large amounts ofdark matter anddark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.[109]
Physical cosmology, the study oflarge-scale structure of the Universe, seeks to understand the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of theBig Bang, the concept that the universe begin extremely dense and hot, thenexpanded over the course of 13.8 billion years[110] to its present condition.[111] The concept of the Big Bang became widely accepted after the discovery of themicrowave background radiation in 1965.[111] Fundamental to the structure of the Universe is the existence ofdark matter anddark 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.[112]
Extragalactic
The blue, loop-shaped objects are multiple images of the same galaxy, duplicated bygravitational lensing. The cluster's gravitational field bends light, magnifying and distorting the image of a more distant object.
Galactic astronomy studies galaxies including theMilky Way, abarred spiral galaxy that is a prominent member of theLocal Group of galaxies and contains theSolar System. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is within the dusty outer arms, large portions of the Milky Way are obscured from view.[101]: 837–842, 944
Kinematic studies of matter in the Milky Way and other galaxies show there is more mass than can be accounted for by visible matter. Adark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[113]
The black spot at the top is adust devil climbing a crater wall onMars. This moving, swirling column ofMartian atmosphere (comparable to a terrestrialtornado) created the long, dark streak.
Planetary science is the study of the assemblage ofplanets,moons,dwarf planets,comets,asteroids, and other bodies orbiting the Sun, as well asexoplanets orbiting distant stars. TheSolar System has been relatively well-studied, initially through telescopes and then later by spacecraft.[123][124]
Processes studied includeplanetary differentiation; the generation of, and effects created by, a planetarymagnetic field;[125] and the creation of heat within a planet, such as by collisions, radioactive decay, andtidal heating. In turn, that heat can drive geologic processes such asvolcanism, tectonics, and surfaceerosion, studied by branches of geology.[126]
Astronomy and astrophysics have developed interdisciplinary links with other major scientific fields.Archaeoastronomy is the study of ancient or traditional astronomies in their cultural context, usingarchaeological andanthropological evidence.[135]Astrostatistics is the application of statistics to the analysis of large quantities of observational astrophysical data.[136] As "forensic astronomy", finally, methods from astronomy have been used to solve problems of art history[137][138] and occasionally of law.[139]
Amateur
Amateur astronomers can build their own equipment, and hold star parties and gatherings, such asStellafane.
Astronomy is one of the sciences to which amateurs can contribute the most.[140] Collectively, amateur astronomers observe celestial objects and phenomena, sometimes with consumer-level equipment orequipment that they build themselves. Common targets include the Sun, the Moon, planets, stars, comets,meteor showers, anddeep-sky objects such as star clusters, galaxies, and nebulae. Astronomy clubs throughout the world have programs to help their members set up and run observational programs such as to observe all the objects in the Messier (110 objects) or Herschel 400 catalogues.[141][142]Most amateurs work at visible wavelengths, but some have experimented with wavelengths outside the visible spectrum. The pioneer of amateur radio astronomy,Karl Jansky, discovered a radio source at the centre of the Milky Way.[143]Some amateur astronomers use homemade telescopes or radio telescopes originally built for astronomy research (e.g. theOne-Mile Telescope).[144][145]
Amateurs can make occultation measurements to refine the orbits of minor planets. They can discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make advances inastrophotography.[146][147][148]
^Krafft, Fritz (2009). "Astronomy". In Cancik, Hubert; Schneider, Helmuth (eds.).Brill's New Pauly.
^Berrgren, J.L.; Sidoli, Nathan (May 2007). "Aristarchus's On the Sizes and Distances of the Sun and the Moon: Greek and Arabic Texts".Archive for History of Exact Sciences.61 (3):213–54.doi:10.1007/s00407-006-0118-4.S2CID121872685.
^DeWitt, Richard (2010). "The Ptolemaic System".Worldviews: An Introduction to the History and Philosophy of Science. Chichester, England:Wiley. p. 113.ISBN978-1-4051-9563-8.
^Kennedy, Edward S. (1962). "Review:The Observatory in Islam and Its Place in the General History of the Observatory by Aydin Sayili".Isis.53 (2):237–39.doi:10.1086/349558.
^Micheau, Françoise. Rashed, Roshdi; Morelon, Régis (eds.). "The Scientific Institutions in the Medieval Near East".Encyclopedia of the History of Arabic Science.3:992–93.
^Nas, Peter J (1993).Urban Symbolism. Brill Academic Publishers. p. 350.ISBN978-90-04-09855-8.
^Kepple, George Robert; Sanner, Glen W. (1998).The Night Sky Observer's Guide. Vol. 1. Willmann-Bell, Inc. p. 18.ISBN978-0-943396-58-3.
^Murdin, Paul; Murdin, Lesley (1985).Supernovae (2 ed.). Cambridge: Cambridge University Press. p. 14.ISBN978-0-521-30038-4.
^Goldstein, Bernard R. (1967). "The Arabic version of Ptolemy's planetary hypothesis".Transactions of the American Philosophical Society.57 (pt. 4): 6.doi:10.2307/1006040.JSTOR1006040.
^Hammer, Joshua (2016).The Bad-Ass Librarians of Timbuktu And Their Race to Save the World's Most Precious Manuscripts. New York: Simon & Schuster. pp. 26–27.ISBN978-1-4767-7743-6.
^Hannam, James (2009).God's philosophers: how the medieval world laid the foundations of modern science. Icon Books. p. 180.
^Grant,The Foundations of Modern Science in the Middle Ages, (Cambridge: Cambridge University Press, 1996), pp. 114–116.
^Questions on the Eight Books of the Physics of Aristotle: Book VIII Question 12. English translation in Clagett's 1959Science of Mechanics in the Middle Ages, p. 536
^James, S. H. G. (1993). "DR Isaac Roberts (1829-1904) and his observatories".Journal of the British Astronomical Association.103: 120.Bibcode:1993JBAA..103..120J.
^Penston, Margaret J. (14 August 2002)."The electromagnetic spectrum". Particle Physics and Astronomy Research Council. Archived fromthe original on 8 September 2012. Retrieved17 November 2016.
^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. (15 June 2016). "GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence".Physical Review Letters.116 (24) 241103.arXiv:1606.04855.Bibcode:2016PhRvL.116x1103A.doi:10.1103/PhysRevLett.116.241103.PMID27367379.S2CID118651851.
^Preuss, Paul."Dark Energy Fills the Cosmos". U.S. Department of Energy, Berkeley Lab.Archived from the original on 11 August 2006. Retrieved8 September 2006.
^Pogge, Richard W. (1997)."The Once & Future Sun".New Vistas in Astronomy. Archived fromthe original(lecture notes) on 27 May 2005. Retrieved3 February 2010.
^Beatty, J.K.; Petersen, C.C.; Chaikin, A., eds. (1999).The New Solar System (4th ed.). Cambridge press. p. 70.ISBN978-0-521-64587-4.Archived from the original on 30 March 2015. Retrieved26 August 2020.
^"Astrochemistry".www.cfa.harvard.edu/. 15 July 2013. Archived fromthe original on 20 November 2016. Retrieved20 November 2016.
.jpg)
.png)
