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AstronomyAstronomy is a natural science that is the study ofcelestial objects (suchas moons,planets,stars, nebulae, and galaxies), the physics, chemistry, mathematics,and evolutionof such objects, and phenomena that originate outside the atmosphere of Earth,includingsupernovae explosions, gamma ray bursts, and cosmic background radiation. A related butdistinct subject, cosmology, is concerned with studying the universe as a whole.[1]

Astronomy is one of the oldest sciences. Prehistoric cultures left behind astronomical artifacts suchas the Egyptian monuments and Nubian monuments, and early civilizations such as

the Babylonians, Greeks, Chinese, Indians, Iranians and Maya performed methodical observations ofthe night sky. However, the invention of the telescope was required before astronomy was able todevelop into a modern science. Historically, astronomy has included disciplines as diverseas astrometry, celestial navigation, observational astronomy, and the making ofcalendars, butprofessional astronomy is nowadays often considered to be synonymous with astrophysics.[2]

During the 20th century, the field of professional astronomy split into observational and theoreticalbranches. Observational astronomy is focused on acquiring data from observations of astronomicalobjects, which is then analyzed using basic principles of physics. Theoretical astronomy is orientedtoward the development of computer or analytical models to describe astronomical objects andphenomena. The two fields complement each other, with theoretical astronomy seeking to explain theobservational results and observations being used to confirm theoretical results.

Amateur astronomers have contributed to many important astronomical discoveries, and astronomy isone of the few sciences where amateurs can still play an active role, especially in the discovery andobservation of transient phenomena.

Astronomy is not to be confused with astrology, the belief system which claims that human affairs arecorrelated with the positions of celestial objects. Although the two fields share a common origin theyare now entirely distinct.[3]

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Lexicology[edit source | editbeta]

The word astronomy(from the Greek words astron (), "star" and -nomy from nomos (),"law" or "culture") literally means "law of the stars" (or "culture of the stars" depending on the

translation).Use of terms "astronomy" and "astrophysics"[edit source | editbeta]Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.[4][5][6] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outsidethe Earth's atmosphere and of their physical and chemical properties" [7] and "astrophysics" refers tothe branch of astronomy dealing with "the behavior, physical properties, and dynamic processes ofcelestial objects and phenomena".[8] In some cases, as in the introduction of the introductorytextbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitativestudy of the subject, whereas "astrophysics" is used to describe the physics-oriented version of thesubject.[9] However, since most modern astronomical research deals with subjects related to physics,modern astronomy could actually be called astrophysics. [4] Few fields, such as astrometry, are purely

astronomy rather than also astrophysics. Various departments in which scientists carry out researchon this subject may use "astronomy" and "astrophysics," partly depending on whether the departmentis historically affiliated with a physics department, [5] and many professional astronomers have physicsrather than astronomy degrees.[6]One of the leading scientific journals in the field is the European

journal namedAstronomy and Astrophysics. The leading American journals are The AstrophysicalJournal and The Astronomical Journal.History[edit source | editbeta]

Main article: History of astronomyIn early times, astronomy only comprised the observation and predictions of the motions of objectsvisible to the naked eye. In some locations, such as Stonehenge, early cultures assembled massiveartifacts 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 knowingwhen to plant crops, as well as in understanding the length of the year. [10]

Before tools such as the telescope were invented, early study of the stars had to be conducted fromthe only vantage points available, namely tall buildings and high ground using the naked eye. As
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civilizations developed, most notably in Mesopotamia,China,Egypt, Greece, India, and CentralAmerica, astronomical observatories were assembled, and ideas on the nature of the universe beganto be explored. Most of early astronomy actually consisted of mapping the positions of the stars andplanets, a science now referred to asastrometry. From these observations, early ideas about themotions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universewere 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, orthe Ptolemaic system, named afterPtolemy.[11]

A particularly important early development was the beginning of mathematical and scientificastronomy, which began among the Babylonians, who laid the foundations for the later astronomicaltraditions that developed in many other civilizations.[12] TheBabylonians discovered that lunareclipses recurred in a repeating cycle known as a saros.[13]

Following the Babylonians, significant advances in astronomy were made in ancient Greece andtheHellenistic world. Greek astronomy is characterized from the start by seeking a rational, physicalexplanation for celestial phenomena.[14] In the 3rd century BC,Aristarchus of Samos estimatedthesize and distance of the Moon and Sun, and was the first to propose a heliocentric model of thesolar system.[15] In the 2nd century BC, Hipparchus discovered precession, calculated the size anddistance of the Moon and invented the earliest known astronomical devices such as the astrolabe.[16]Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of

the northern hemisphere derive from Greek astronomy. [17] TheAntikythera mechanism (c. 15080BC) was an early analog computerdesigned to calculate the location of the Sun, Moon,and planets for a given date. Technological artifacts of similar complexity did not reappear until the14th century, when mechanical astronomical clocks appeared inEurope.[18]

During the Middle Ages, astronomy was mostly stagnant in medieval Europe, at least until the 13thcentury. However, astronomy flourished in the Islamic world and other parts of the world. This led tothe emergence of the first astronomical observatories in the Muslim world by the early 9th century.[19][20][21]In 964, theAndromeda Galaxy, the largest galaxy in the Local Group, was discovered by thePersian astronomerAzophi and first described in his Book of Fixed Stars.[22]TheSN 1006supernova,the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian

Arabic astronomerAli ibn Ridwan and the Chinese astronomers in 1006. Some of the prominent

Islamic (mostly Persian and Arab) astronomers who made significant contributions to the scienceincludeAl-Battani, Thebit,Azophi,Albumasar, Biruni,Arzachel,Al-Birjandi, and the astronomers ofthe Maragheh and Samarkand observatories. Astronomers during that time introduced manyArabicnames now used for individual stars.[23][24] It is also believed that the ruins at GreatZimbabweand Timbuktu[25] may have housed an astronomical observatory. [26] Europeans hadpreviously believed that there had been no astronomical observation in pre-colonial Middle Ages sub-Saharan Africa but modern discoveries show otherwise.[27][28][29][30]

Scientific revolution[edit source | editbeta]During the Renaissance,Nicolaus Copernicus proposed a heliocentric model of the solar system. Hiswork was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileoinnovated by using telescopes to enhance his observations. [31]

Kepler was the first to devise a system that described correctly the details of the motion of the planetswith the Sun at the center. However, Kepler did not succeed in formulating a theory behind the lawshe wrote down.[32] It was left to Newton's invention ofcelestial dynamics and his law of gravitation tofinally explain the motions of the planets. Newton also developed thereflecting telescope.[31]

Further discoveries paralleled the improvements in the size and quality of the telescope. Moreextensive star catalogues were produced by Lacaille. The astronomerWilliam Herschel made adetailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first newplanet found.[33] The distance to a star was first announced in 1838 when the parallax of61 Cygniwasmeasured by Friedrich Bessel.[34]

During the 1819th centuries, attention to the three body problem by Euler, Clairaut,and D'Alembertled to more accurate predictions about the motions of the Moon and planets. This

work was further refined by Lagrangeand Laplace, allowing the masses of the planets and moons tobe estimated from their perturbations.[35]

Significant advances in astronomy came about with the introduction of new technology, includingthe spectroscope and photography.Fraunhoferdiscovered about 600 bands in the spectrum of theSun in 181415, which, in 1859, Kirchhoffascribed to the presence of different elements. Stars were
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proven to be similar to the Earth's own Sun, but with a wide range oftemperatures,masses, andsizes.[23]

The existence of the Earth's galaxy, the Milky Way, as a separate group of stars, was only proved inthe 20th century, along with the existence of "external" galaxies, and soon after, the expansion ofthe Universe, seen in the recession of most galaxies from us. [36]Modern astronomy has alsodiscovered many exotic objects such as quasars, pulsars,blazars, and radio galaxies, and has usedthese observations to develop physical theories which describe some of these objects in terms ofequally exotic objects such as black holesand neutron stars. Physical cosmology made huge

advances during the 20th century, with the model of the Big Bang heavily supported by the evidenceprovided by astronomy and physics, such as the cosmic microwave background radiation, Hubble'slaw, andcosmological abundances of elements.Space telescopes have enabled measurements inparts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.Observational astronomy[edit source | editbeta]

Main article: Observational astronomyIn astronomy, the main source of information about celestial bodies and other objects is thevisible light or more generallyelectromagnetic radiation.[37] Observational astronomy may be dividedaccording to the observed region of the electromagnetic spectrum. Some parts of the spectrum canbe observed from the Earth's surface, while other parts are only observable from either high altitudesor space. Specific information on these subfields is given below.Radio astronomy[edit source | editbeta]Radio astronomy studies radiation with wavelengthsgreater than approximately one millimeter.[38] Radio astronomy is different from most other forms of observational astronomy in that theobserved radio waves can be treated as waves rather than as discrete photons. Hence, it is relativelyeasier to measure both the amplitudeand phase of radio waves, whereas this is not as easily done atshorter wavelengths.[38]

Although some radio waves are produced by astronomical objects in the form ofthermal emission,most of the radio emission that is observed from Earth is seen in the form ofsynchrotron radiation,which is produced when electronsoscillate around magnetic fields.[38]Additionally, a numberofspectral lines produced by interstellar gas, notably the hydrogenspectral line at 21 cm, areobservable at radio wavelengths.[9][38]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellargas, pulsars, and active galactic nuclei.[9][38]

Infrared astronomy[edit source | editbeta]Main article: Infrared astronomyInfrared astronomy deals with the detection and analysis ofinfrared radiation (wavelengths longerthan red light). Except at wavelengthsclose to visible light, infrared radiation is heavily absorbed bythe atmosphere, and the atmosphere produces significant infrared emission. Consequently, infraredobservatories have to be located in high, dry places or in space. The infrared spectrum is useful forstudying objects that are too cold to radiate visible light, such as planets and circumstellar disks.Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowingobservation of young stars in molecular clouds and the cores of galaxies.[39]Some molecules radiate

strongly in the infrared. This can be used to study chemistry in space; more specifically it can detectwater in comets.[40]

Optical astronomy[edit source | editbeta]Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[41] Optical images were originally drawn by hand. In the late 19th century and most of the 20thcentury, images were made using photographic equipment. Modern images are made using digitaldetectors, particularly detectors using charge-coupled devices(CCDs). Although visible light itselfextends from approximately 4000 to 7000 (400 nm to 700 nm),[41] the same equipment used atthese wavelengths is also used to observe somenear-ultraviolet and near-infrared radiation.Ultraviolet astronomy[edit source | editbeta]Main article: Ultraviolet astronomy

Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths betweenapproximately 100 and 3200 (10 to 320 nm).[38]Light at these wavelengths is absorbed by theEarth's atmosphere, so observations at these wavelengths must be performed from the upperatmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation andspectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This
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//en.wikipedia.org/wiki/Hubble's_lawhttp://en.wikipedia.org/wiki/Big_Bang_nucleosynthesishttp://en.wikipedia.org/wiki/Space_telescopehttp://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=5http://en.wikipedia.org/wiki/Observational_astronomyhttp://en.wikipedia.org/wiki/Celestial_bodyhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Astronomy#cite_note-37http://en.wikipedia.org/wiki/Electromagnetic_spectrumhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=6http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Wavehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Black_body_radiationhttp://en.wikipedia.org/wiki/Synchrotron_radiationhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Spectral_linehttp://en.wikipedia.org/wiki/Interstellar_gashttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Astronomy#cite_note-shu1982-9http://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Supernovahttp://en.wikipedia.org/wiki/Pulsarhttp://en.wikipedia.org/wiki/Active_galactic_nucleihttp://en.wikipedia.org/wiki/Astronomy#cite_note-shu1982-9http://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=7http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=7http://en.wikipedia.org/wiki/Infrared_astronomyhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Wavelengthshttp://en.wikipedia.org/wiki/Circumstellar_diskhttp://en.wikipedia.org/wiki/Molecular_cloudhttp://en.wikipedia.org/wiki/Astronomy#cite_note-39http://en.wikipedia.org/wiki/Astronomy#cite_note-40http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=8http://en.wikipedia.org/wiki/Astronomy#cite_note-moore1997-41http://en.wikipedia.org/wiki/Charge-coupled_deviceshttp://en.wikipedia.org/wiki/%C3%85ngstromhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Astronomy#cite_note-moore1997-41http://en.wikipedia.org/wiki/Near-ultraviolethttp://en.wikipedia.org/wiki/Near-infraredhttp://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=9http://en.wikipedia.org/wiki/Ultraviolet_astronomyhttp://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/OB_star


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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.[38] However, as ultraviolet light is easily absorbed by interstellar dust, anappropriate adjustment of ultraviolet measurements is necessary. [38]

X-ray astronomy[edit source | editbeta]Main article:X-ray astronomyX-ray astronomy is the study ofastronomical objectsat X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field

lines), thermal emission from thin gases above 107 (10 million)kelvins, and thermal emission fromthick gases above 107 Kelvin.[38] Since X-rays are absorbed by the Earth's atmosphere, all X-rayobservations must be performed from high-altitude balloons, rockets, orspacecraft. Notable X-raysources include X-ray binaries,pulsars,supernova remnants, elliptical galaxies, clusters of galaxies,and active galactic nuclei.[38]

According to NASA's official website, X-rays were first observed and documented in 1895 by WilhelmConrad Rntgen, a Germanscientist who found them quite by accident when experimentingwith vacuum tubes. Through a series of experiments, including the infamous X-ray photograph hetook of his wife's hand with a wedding ring on it, Rntgen was able to discover the beginningelements of radiation. The "X", in fact, holds its own significance, as it represents Rntgen's inabilityto identify exactly what type of radiation it was.

Gamma-ray astronomy[edit source |editbeta]Main article: Gamma ray astronomyGamma ray astronomy is the study of astronomical objects at the shortest wavelengths of theelectromagnetic spectrum. Gamma rays may be observed directly by satellites such as theComptonGamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[38] The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect theflashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere. [42]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gammaradiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-raysources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars,and black hole candidates such as active galactic nuclei. [38]

Fields not based on the electromagnetic spectrum[edit source | editbeta]In addition to electromagnetic radiation, a few other events originating from great distances may beobserved from the Earth.In neutrino astronomy, astronomers use special underground facilities such as SAGE, GALLEX,and Kamioka II/III for detectingneutrinos. These neutrinos originate overwhelmingly from the Sun, but24 neutrinos were also detected from supernova 1987A.[38] Cosmic rays, which consist of very highenergy particles that can decay or be absorbed when they enter the Earth's atmosphere, result in acascade of particles which can be detected by current observatories. [43] Additionally, somefuture neutrino detectorsmay also be sensitive to the particles produced when cosmic rays hit theEarth's atmosphere.[38]Gravitational wave astronomy is an emerging new field of astronomy whichaims to use gravitational wave detectors to collect observational data about compact objects. A few

observatories have been constructed, such as the Laser Interferometer GravitationalObservatoryLIGO, but gravitational waves are extremely difficult to detect.[44]

Planetary astronomers have directly observed many of these phenomena through spacecraft andsample return missions. These observations include fly-by missions with remote sensors, landingvehicles that can perform experiments on the surface materials, impactors that allow remote sensingof buried material, and sample return missions that allow direct laboratory examination.Astrometry and celestial mechanics[edit source | editbeta]Main articles:AstrometryandCelestial mechanicsOne of the oldest fields in astronomy, and in all of science, is the measurement of the positions ofcelestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and starshas been essential in celestial navigation and in the making ofcalendars.

Careful measurement of the positions of the planets has led to a solid understanding ofgravitational perturbations, and an ability to determine past and future positions of the planets withgreat accuracy, a field known as celestial mechanics. More recently the tracking ofnear-Earthobjects will allow for predictions of close encounters, and potential collisions, with the Earth. [45]
http://en.wikipedia.org/wiki/Planetary_nebulahttp://en.wikipedia.org/wiki/Supernova_remnanthttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Interstellar_dusthttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=10http://en.wikipedia.org/wiki/X-ray_astronomyhttp://en.wikipedia.org/wiki/Astronomical_objecthttp://en.wikipedia.org/wiki/Astronomical_objecthttp://en.wikipedia.org/wiki/X-radiationhttp://en.wikipedia.org/wiki/Synchrotron_emissionhttp://en.wikipedia.org/wiki/Bremsstrahlung_radiationhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Blackbody_radiationhttp://en.wikipedia.org/wiki/Blackbody_radiationhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Earth's_atmospherehttp://en.wikipedia.org/wiki/High-altitude_balloonhttp://en.wikipedia.org/wiki/Rockethttp://en.wikipedia.org/wiki/X-ray_astronomy_satelliteshttp://en.wikipedia.org/wiki/Astrophysical_X-ray_sourcehttp://en.wikipedia.org/wiki/Astrophysical_X-ray_sourcehttp://en.wikipedia.org/wiki/X-ray_binarieshttp://en.wikipedia.org/wiki/Pulsarhttp://en.wikipedia.org/wiki/Supernova_remnanthttp://en.wikipedia.org/wiki/Supernova_remnanthttp://en.wikipedia.org/wiki/Elliptical_galaxieshttp://en.wikipedia.org/wiki/Clusters_of_galaxieshttp://en.wikipedia.org/wiki/Active_galactic_nucleihttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Wilhelm_Conrad_R%C3%B6ntgenhttp://en.wikipedia.org/wiki/Wilhelm_Conrad_R%C3%B6ntgenhttp://en.wikipedia.org/wiki/Germanyhttp://en.wikipedia.org/wiki/Vacuum_tubeshttp://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=11http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=11http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=11http://en.wikipedia.org/wiki/Gamma_ray_astronomyhttp://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatoryhttp://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatoryhttp://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatoryhttp://en.wikipedia.org/wiki/Atmospheric_Cherenkov_telescopehttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Astronomy#cite_note-spectrum-42http://en.wikipedia.org/wiki/Gamma-rayhttp://en.wikipedia.org/wiki/Gamma-ray_bursthttp://en.wikipedia.org/wiki/Neutron_starhttp://en.wikipedia.org/wiki/Black_holehttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=12http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=12http://en.wikipedia.org/wiki/Neutrino_astronomyhttp://en.wikipedia.org/wiki/Neutrino_observatoryhttp://en.wikipedia.org/wiki/SAGE_(ruSsian_American_Gallium_Experiment)http://en.wikipedia.org/wiki/GALLEXhttp://en.wikipedia.org/wiki/Kamioka_Observatoryhttp://en.wikipedia.org/wiki/Neutrinohttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Supernova_1987Ahttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Cosmic_rayhttp://en.wikipedia.org/wiki/Astronomy#cite_note-43http://en.wikipedia.org/wiki/Neutrino_detectorhttp://en.wikipedia.org/wiki/Neutrino_detectorhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Gravitational_wave_astronomyhttp://en.wikipedia.org/wiki/Gravitational_wave_detectorhttp://en.wikipedia.org/wiki/LIGOhttp://en.wikipedia.org/wiki/Gravitational_wavehttp://en.wikipedia.org/wiki/Astronomy#cite_note-44http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=13http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=13http://en.wikipedia.org/wiki/Astrometryhttp://en.wikipedia.org/wiki/Celestial_mechanicshttp://en.wikipedia.org/wiki/Celestial_navigationhttp://en.wikipedia.org/wiki/Calendarhttp://en.wikipedia.org/wiki/Perturbation_theoryhttp://en.wikipedia.org/wiki/Celestial_mechanicshttp://en.wikipedia.org/wiki/Near-Earth_objecthttp://en.wikipedia.org/wiki/Near-Earth_objecthttp://en.wikipedia.org/wiki/Astronomy#cite_note-45http://en.wikipedia.org/wiki/Planetary_nebulahttp://en.wikipedia.org/wiki/Supernova_remnanthttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Interstellar_dusthttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=10http://en.wikipedia.org/wiki/X-ray_astronomyhttp://en.wikipedia.org/wiki/Astronomical_objecthttp://en.wikipedia.org/wiki/X-radiationhttp://en.wikipedia.org/wiki/Synchrotron_emissionhttp://en.wikipedia.org/wiki/Bremsstrahlung_radiationhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Blackbody_radiationhttp://en.wikipedia.org/wiki/Blackbody_radiationhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Earth's_atmospherehttp://en.wikipedia.org/wiki/High-altitude_balloonhttp://en.wikipedia.org/wiki/Rockethttp://en.wikipedia.org/wiki/X-ray_astronomy_satelliteshttp://en.wikipedia.org/wiki/Astrophysical_X-ray_sourcehttp://en.wikipedia.org/wiki/Astrophysical_X-ray_sourcehttp://en.wikipedia.org/wiki/X-ray_binarieshttp://en.wikipedia.org/wiki/Pulsarhttp://en.wikipedia.org/wiki/Supernova_remnanthttp://en.wikipedia.org/wiki/Elliptical_galaxieshttp://en.wikipedia.org/wiki/Clusters_of_galaxieshttp://en.wikipedia.org/wiki/Active_galactic_nucleihttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Wilhelm_Conrad_R%C3%B6ntgenhttp://en.wikipedia.org/wiki/Wilhelm_Conrad_R%C3%B6ntgenhttp://en.wikipedia.org/wiki/Germanyhttp://en.wikipedia.org/wiki/Vacuum_tubeshttp://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=11http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=11http://en.wikipedia.org/wiki/Gamma_ray_astronomyhttp://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatoryhttp://en.wikipedia.org/wiki/Compton_Gamma_Ray_Observatoryhttp://en.wikipedia.org/wiki/Atmospheric_Cherenkov_telescopehttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Astronomy#cite_note-spectrum-42http://en.wikipedia.org/wiki/Gamma-rayhttp://en.wikipedia.org/wiki/Gamma-ray_bursthttp://en.wikipedia.org/wiki/Neutron_starhttp://en.wikipedia.org/wiki/Black_holehttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=12http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=12http://en.wikipedia.org/wiki/Neutrino_astronomyhttp://en.wikipedia.org/wiki/Neutrino_observatoryhttp://en.wikipedia.org/wiki/SAGE_(ruSsian_American_Gallium_Experiment)http://en.wikipedia.org/wiki/GALLEXhttp://en.wikipedia.org/wiki/Kamioka_Observatoryhttp://en.wikipedia.org/wiki/Neutrinohttp://en.wikipedia.org/wiki/Sunhttp://en.wikipedia.org/wiki/Supernova_1987Ahttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Cosmic_rayhttp://en.wikipedia.org/wiki/Astronomy#cite_note-43http://en.wikipedia.org/wiki/Neutrino_detectorhttp://en.wikipedia.org/wiki/Astronomy#cite_note-cox2000-38http://en.wikipedia.org/wiki/Gravitational_wave_astronomyhttp://en.wikipedia.org/wiki/Gravitational_wave_detectorhttp://en.wikipedia.org/wiki/LIGOhttp://en.wikipedia.org/wiki/Gravitational_wavehttp://en.wikipedia.org/wiki/Astronomy#cite_note-44http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=13http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=13http://en.wikipedia.org/wiki/Astrometryhttp://en.wikipedia.org/wiki/Celestial_mechanicshttp://en.wikipedia.org/wiki/Celestial_navigationhttp://en.wikipedia.org/wiki/Calendarhttp://en.wikipedia.org/wiki/Perturbation_theoryhttp://en.wikipedia.org/wiki/Celestial_mechanicshttp://en.wikipedia.org/wiki/Near-Earth_objecthttp://en.wikipedia.org/wiki/Near-Earth_objecthttp://en.wikipedia.org/wiki/Astronomy#cite_note-45


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The measurement ofstellar parallax of nearby stars provides a fundamental baseline in the cosmicdistance ladderthat is used to measure the scale of the universe. Parallax measurements of nearbystars provide an absolute baseline for the properties of more distant stars, because their propertiescan be compared. Measurements ofradial velocityand proper motion show the kinematics of thesesystems through the Milky Way galaxy. Astrometric results are also used to measure the distributionofdark matterin the galaxy.[46]

During the 1990s, the astrometric technique of measuring the stellar wobble was used todetect large extrasolar planets orbiting nearby stars.[47]

Theoretical astronomy[edit source |editbeta]Theoretical astronomers use a wide variety of tools which include analytical models (forexample,polytropes to approximate the behaviors of a star) and computationalnumerical simulations.Each has some advantages. Analytical models of a process are generally better for giving insight intothe heart of what is going on. Numerical models can reveal the existence of phenomena and effectsthat would otherwise not be seen.[48][49]

Theorists in astronomy endeavor to create theoretical models and figure out the observationalconsequences of those models. This helps observers look for data that can refute a model or help inchoosing between several alternate or conflicting models.Theorists also try to generate or modify models to take into account new data. In the case of aninconsistency, the general tendency is to try to make minimal modifications to the model to fit the

data. In some cases, a large amount of inconsistent data over time may lead to total abandonment ofa model.Topics studied by theoretical astronomers include: stellar dynamics and evolution; galaxyformation;large-scale structure ofmatterin the Universe; origin ofcosmic rays; generalrelativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysicalrelativity serves as a tool to gauge the properties of large scale structures for which gravitation playsa significant role in physical phenomena investigated and as the basis forblackhole (astro)physics and the study ofgravitational 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 ofphysics.

A few examples of this process:

Physicalprocess

Experimental toolTheoreticalmodel

Explains/predicts

Gravitation Radio telescopesSelf-gravitatingsystem

Emergence of a star system

Nuclear fusion Spectroscopy Stellar evolutionHow the stars shine and how metalsformed

The Big BangHubble SpaceTelescope, COBE

Expandinguniverse

Age of the Universe

Quantum

fluctuationsCosmic inflation Flatness problem

Gravitationalcollapse

X-ray astronomy General relativityBlack holes at the centerofAndromeda galaxy

CNOcycle in stars

The dominant source of energy formassive star.

Dark matterand dark energy are the current leading topics in astronomy,[50] as their discovery andcontroversy originated during the study of the galaxies.Specific subfields[edit source | editbeta]

Solar astronomy[edit source | editbeta]At a distance of about eight light-minutes, the most frequently studied star is the Sun, a typical main-

sequence dwarf starofstellar class G2 V, and about 4.6 Gyr old. The Sun is not considereda variable star, but it does undergo periodic changes in activity known as thesunspot cycle. This is an11-year fluctuation in sunspot numbers. Sunspots are regions of lower-than- average temperaturesthat are associated with intense magnetic activity.[51]
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The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it firstbecame a main-sequence star. The Sun has also undergone periodic changes in luminosity that canhave a significant impact on the Earth.[52] The Maunder minimum, for example, is believed to havecaused the Little Ice Age phenomenon during the Middle Ages.[53]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region knownas the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures,then by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure

fornuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys theenergy flux by means of radiation. The outer layers form a convection zone where the gas materialtransports energy primarily through physical displacement of the gas. It is believed that thisconvection zone creates the magnetic activity that generates sun spots. [51]

A solar wind of plasma particles constantly streams outward from the Sun until it reachesthe heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allenradiation belts, as well as the aurorawhere the lines of the Earth's magnetic field descend intothe atmosphere.[54]

Planetary science[edit source | editbeta]Main articles: Planetary science andPlanetary geologyThis astronomical field examines the assemblage ofplanets, moons, dwarf

planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The SolarSystem has been relatively well-studied, initially through telescopes and then later by spacecraft. Thishas provided a good overall understanding of the formation and evolution of this planetary system,although many new discoveries are still being made. [55]

streak. NASA image.The Solar System is subdivided into the inner planets, the asteroid belt, and the outer planets. Theinnerterrestrial planets consist ofMercury, Venus, Earth, and Mars. The outergas giant planetsare Jupiter, Saturn, Uranus, and Neptune.[56] Beyond Neptune lies theKuiper Belt, and finally the OortCloud, which may extend as far as a light-year.The planets were formed in the protoplanetary diskthat surrounded the early Sun. Through a processthat 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 theunaccreted 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 intensebombardment, evidenced by the many impact craters on the Moon. During this period, some of theprotoplanets may have collided, theleading hypothesisfor how the Moon was formed.[57]

Once a planet reaches sufficient mass, the materials with different densities segregate within,during planetary differentiation. This process can form a stony or metallic core, surrounded by amantle and an outer surface. The core may include solid and liquid regions, and some planetarycores generate their own magnetic field, which can protect their atmospheres from solar windstripping.[58]

A planet or moon's interior heat is produced from the collisions that created the body, radioactive

materials (e.g.uranium, thorium, and26

Al), ortidal heating. Some planets and moons accumulateenough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate orretain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, withouttidal heating, cool more quickly; and their geological activity ceases with the exception of impactcratering.[59]

Stellar astronomy[edit source | editbeta]The study ofstars and stellar evolution is fundamental to our understanding of the universe. Theastrophysics of stars has been determined through observation and theoretical understanding; andfrom computer simulations of the interior.[60]

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. Whendestabilized, cloud fragments can collapse under the influence of gravity, to form aprotostar. A

sufficiently dense, and hot, core region will triggernuclear fusion, thus creating a main-sequence star.[61]

Almost all elements heavier than hydrogen andhelium were created inside the cores of stars.[60]

The characteristics of the resulting star depend primarily upon its starting mass. The more massivethe star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over
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pherehttp://en.wikipedia.org/wiki/Astronomy#cite_note-54http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=17http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=17http://en.wikipedia.org/wiki/Planetary_sciencehttp://en.wikipedia.org/wiki/Planetary_geologyhttp://en.wikipedia.org/wiki/Planethttp://en.wikipedia.org/wiki/Natural_satellitehttp://en.wikipedia.org/wiki/Dwarf_planethttp://en.wikipedia.org/wiki/Dwarf_planethttp://en.wikipedia.org/wiki/Comethttp://en.wikipedia.org/wiki/Asteroidhttp://en.wikipedia.org/wiki/Solar_Systemhttp://en.wikipedia.org/wiki/Solar_Systemhttp://en.wikipedia.org/wiki/Astronomy#cite_note-geology-55http://en.wikipedia.org/wiki/NASAhttp://en.wikipedia.org/wiki/Asteroid_belthttp://en.wikipedia.org/wiki/Terrestrial_planethttp://en.wikipedia.org/wiki/Mercury_(planet)http://en.wikipedia.org/wiki/Venushttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Marshttp://en.wikipedia.org/wiki/Gas_gianthttp://en.wikipedia.org/wiki/Jupiterhttp://en.wikipedia.org/wiki/Saturnhttp://en.wikipedia.org/wiki/Uranushttp://en.wikipedia.org/wiki/Neptunehttp://en.wikipedia.org/wiki/Astronomy#cite_note-planets-56http://en.wikipedia.org/wiki/Kuiper_Belthttp://en.wikipedia.org/wiki/Oort_Cloudhttp://en.wikipedia.org/wiki/Oort_Cloudhttp://en.wikipedia.org/wiki/Protoplanetary_diskhttp://en.wikipedia.org/wiki/Radiation_pressurehttp://en.wikipedia.org/wiki/Solar_windhttp://en.wikipedia.org/wiki/Impact_craterhttp://en.wikipedia.org/wiki/Giant_impact_hypothesishttp://en.wikipedia.org/wiki/Astronomy#cite_note-Montmerle2006-57http://en.wikipedia.org/wiki/Planetary_differentiationhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Astronomy#cite_note-58http://en.wikipedia.org/wiki/Uraniumhttp://en.wikipedia.org/wiki/Thoriumhttp://en.wikipedia.org/wiki/Aluminium#Isotopeshttp://en.wikipedia.org/wiki/Tidal_accelerationhttp://en.wikipedia.org/wiki/Volcanismhttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Erosionhttp://en.wikipedia.org/wiki/Astronomy#cite_note-new_solar_system-59http://en.wikipedia.org/w/index.php?title=Astronomy&action=edit&section=18http://en.wikipedia.org/w/index.php?title=Astronomy&veaction=edit&section=18http://en.wikipedia.org/wiki/Starhttp://en.wikipedia.org/wiki/Stellar_evolutionhttp://en.wikipedia.org/wiki/Astronomy#cite_note-Amos7-60http://en.wikipedia.org/wiki/Star_formationhttp://en.wikipedia.org/wiki/Dark_nebulahttp://en.wikipedia.org/wiki/Protostarhttp://en.wikipedia.org/wiki/Nuclear_fusionhttp://en.wikipedia.org/wiki/Main-sequence_starhttp://en.wikipedia.org/wiki/Astronomy#cite_note-Smith2004-61http://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Heliumhttp://en.wikipedia.org/wiki/Nucleosynthesishttp://en.wikipedia.org/wiki/Astronomy#cite_note-Amos7-60


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time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusionof helium requires a higher core temperature, so that the star both expands in size, and increases incore density. The resulting red giant enjoys a brief life span, before the helium fuel is in turnconsumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as theyfuse increasingly heavier elements. [62]

The final fate of the star depends on its mass, with stars of mass greater than about eight times theSun becoming core collapsesupernovae;[63] while smaller stars form planetary nebulae, and evolveinto white dwarfs.[64] The remnant of a supernova is a denseneutron star, or, if the stellar mass was at

least three times that of the Sun, a black hole.[65]Close binary stars can follow more complexevolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause asupernova.[66] Planetary nebulae and supernovae are necessary for the distribution ofmetals to theinterstellar medium; without them, all new stars (and their planetary systems) would be formed fromhydrogen and helium alone.[67]

See also: Solar astronomyGalactic astronomy[edit source | editbeta]Oursolar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member ofthe Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held togetherby mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are largeportions 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 beasupermassive black hole at the center. This is surrounded by four primary arms that spiral from thecore. This is a region of active star formation that contains many younger,population I stars. The diskis surrounded by a spheroid halo of older, population II stars, as well as relatively denseconcentrations of stars known as globular clusters.[68]

Between the stars lies the interstellar medium, a region of sparse matter. In the densestregions, molecular clouds ofmolecular hydrogenand other elements create star-forming regions.These begin as a compact pre-stellar core ordark nebulae, which concentrate and collapse (involumes determined by the Jeans length) to form compact protostars.[61]

As the more massive stars appear, they transform the cloud into an H II region of glowing gas andplasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the

cloud, often leaving behind one or more young open clusters of stars. These clusters graduallydisperse, and the stars join the population of the Milky Way. [69]

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is moremass 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.[70]

Extragalactic astronomy[edit source | editbeta]This image shows several blue, loop-shaped objects that are multiple images of the same galaxy,duplicated by thegravitational lens effect of the cluster of yellow galaxies near the middle of thephotograph. The lens is produced by the cluster's gravitational field that bends light to magnify anddistort 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 theformation andevolution of Galaxies; their morphology and classification; and the examination ofactive galaxies, andthe groups and clusters of galaxies. The latter is important for the understanding of the large-scalestructure of the cosmos.Most galaxies are organized into distinct shapes that allow for classification schemes. They arecommonly divided into spiral, ellipticaland Irregulargalaxies.[71]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The starsmove along random orbits with no preferred direction. These galaxies contain little or no interstellardust; few star-forming regions; and generally older stars. Elliptical galaxies are more commonly foundat the core of galactic clusters, and may be 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 formationwhere massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo ofolder stars. Both the Milky Way and theAndromeda Galaxy are spiral galaxies.
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