Editing Astronomy (section)

== Observational astronomy ==
{{Main|Observational astronomy}}
[[File:Openstax Astronomy EM spectrum and atmosphere.jpg|thumb|upright=1.6|Overview of types of observational astronomy by observed wavelengths and their observability]]
The main source of information about [[celestial body|celestial bodies]] and other objects is [[visible light]], or more generally [[electromagnetic radiation]].<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html|title=Electromagnetic Spectrum|publisher=NASA|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060905131651/http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html|archive-date=5 September 2006 }}</ref> 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 telescope]]]]
{{Main|Radio astronomy}}
Radio astronomy uses radiation with [[wavelength]]s greater than approximately one millimeter, outside the visible range.<ref name="cox2000">{{cite book
|editor=Cox, A.N.
|title=Allen's Astrophysical Quantities
|date=2000
|url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA124
|publisher=Springer-Verlag
|page=124
|location=New York
|isbn=978-0-387-98746-0
|access-date=26 August 2020
|archive-date=19 November 2020
|archive-url=https://web.archive.org/web/20201119200822/https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA124
|url-status=live
}}</ref> Radio astronomy is different from most other forms of observational astronomy in that the observed [[radio wave]]s can be treated as [[wave]]s rather than as discrete [[photon]]s. 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.<ref name="cox2000"/>

Although some [[radio wave]]s are emitted directly by astronomical objects, a product of [[black-body radiation|thermal emission]], most of the radio emission that is observed is the result of [[synchrotron radiation]], which is produced when [[electron]]s orbit [[magnetic field]]s.<ref name="cox2000"/> Additionally, a number of [[spectral line]]s produced by [[interstellar gas]], notably the [[hydrogen]] spectral line at 21&nbsp;cm, are observable at radio wavelengths.<ref name="shu1982"/><ref name="cox2000"/>

A wide variety of other objects are observable at radio wavelengths, including [[supernova]]e, interstellar gas, [[pulsar]]s, and [[active galactic nuclei]].<ref name="shu1982"/><ref name="cox2000"/>

=== Infrared astronomy ===
[[File:In Search of Space.jpg|thumb|[[Atacama Large Millimeter Array|ALMA]] Observatory is one of the highest observatory sites on Earth. Atacama, Chile.<ref>{{cite news|title=In Search of Space|url=http://www.eso.org/public/images/potw1431a/|access-date=5 August 2014|work=Picture of the Week|agency=European Southern Observatory|archive-date=13 August 2020|archive-url=https://web.archive.org/web/20200813090738/https://www.eso.org/public/images/potw1431a/|url-status=live}}</ref>]]
{{Main|Infrared astronomy}}
Infrared astronomy is founded on the detection and analysis of [[infrared]] radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, [[circumstellar disk]]s 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 cloud]]s and the cores of galaxies. Observations from the [[Wide-field Infrared Survey Explorer]] (WISE) have been particularly effective at unveiling numerous galactic [[protostar]]s and their host [[star clusters]].<ref name="wright">{{cite web|url=http://wise.ssl.berkeley.edu/|title=Wide-field Infrared Survey Explorer Mission|date=30 September 2014|publisher=[[NASA]] [[University of California, Berkeley]]|access-date=17 November 2016|archive-url=https://web.archive.org/web/20100112144939/http://wise.ssl.berkeley.edu/|archive-date=12 January 2010}}</ref><ref name=ma2013>{{Cite journal |bibcode = 2013Ap&SS.344..175M|title = Discovering protostars and their host clusters via WISE|last1 = Majaess|first1 = D.|journal = Astrophysics and Space Science|volume = 344|issue = 1|pages = 175–186|year = 2013|arxiv = 1211.4032|doi = 10.1007/s10509-012-1308-y|s2cid = 118455708}}</ref>
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.<ref>{{cite news
|author=Staff
|date=11 September 2003
|title=Why infrared astronomy is a hot topic
|publisher=ESA
|url=http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html
|access-date=11 August 2008
|archive-date=30 July 2012
|archive-url=https://archive.today/20120730/http://www.esa.int/esaCP/SEMX9PZO4HD_FeatureWeek_0.html
|url-status=live
}}</ref> Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.<ref>{{cite news|url=http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html|title=Infrared Spectroscopy – An Overview|publisher=[[NASA]] [[California Institute of Technology]]|access-date=11 August 2008|archive-url=https://web.archive.org/web/20081005031543/http://www.ipac.caltech.edu/Outreach/Edu/Spectra/irspec.html|archive-date=5 October 2008}}</ref>

=== 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 Facility]] (right) is an example of a telescope that operates only at near-infrared wavelengths.]]
{{Main|Optical astronomy}}
Historically, optical astronomy, which has been also called visible light astronomy, is the oldest form of astronomy.<ref name="moore1997">{{cite book
|author=Moore, P.
|title=Philip's Atlas of the Universe
|date=1997
|publisher=George Philis Limited
|location=Great Britain
|isbn=978-0-540-07465-5}}</ref> Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using [[charge-coupled device]]s (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 [[Ångstrom|Å]] to 7000 Å (400 [[nanometre|nm]] to 700&nbsp;nm),<ref name="moore1997"/> that same equipment can be used to observe some [[near-ultraviolet]] and [[near-infrared]] radiation.{{cn|date=March 2025}}

=== Ultraviolet astronomy ===
{{Main|Ultraviolet astronomy}}

Ultraviolet astronomy employs [[ultraviolet]] wavelengths between approximately 100 and 3200&nbsp;Å (10 to 320&nbsp;nm).<ref name="cox2000"/> 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 [[star]]s ([[OB star]]s) 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 nebula]]e, [[supernova remnant]]s, and active galactic nuclei.<ref name="cox2000"/> However, as ultraviolet light is easily absorbed by [[interstellar dust]], an adjustment of ultraviolet measurements is necessary.<ref name="cox2000"/>

=== X-ray astronomy ===
{{Main|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-radiation|X-ray wavelengths]]. Typically, X-ray radiation is produced by [[synchrotron emission]] (the result of electrons orbiting magnetic field lines), [[bremsstrahlung radiation|thermal emission from thin gases]] above 10<sup>7</sup> (10&nbsp;million) [[kelvin]]s, and [[blackbody radiation|thermal emission from thick gases]] above 10<sup>7</sup> Kelvin.<ref name="cox2000"/> Since X-rays are absorbed by the [[Earth's atmosphere]], all X-ray observations must be performed from [[high-altitude balloon]]s, [[rocket]]s, or [[X-ray astronomy satellite]]s. Notable [[Astrophysical X-ray source|X-ray sources]] include [[X-ray binaries]], [[pulsar]]s, [[supernova remnant]]s, [[elliptical galaxies]], [[clusters of galaxies]], and [[active galactic nuclei]].<ref name="cox2000"/>

=== Gamma-ray astronomy ===
{{Main|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 telescope]]s.<ref name="cox2000"/> 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.<ref name="spectrum">{{cite web|url=http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature|title=The electromagnetic spectrum|last=Penston|first=Margaret J.|date=14 August 2002|publisher=Particle Physics and Astronomy Research Council|archive-url=https://archive.today/20120908014227/http://www.pparc.ac.uk/frontiers/latest/feature.asp?article=14F1&style=feature|archive-date=8 September 2012|access-date=17 November 2016}}</ref>

Most [[Gamma ray|gamma-ray]] emitting sources are actually [[gamma-ray burst]]s, 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 star]]s, and [[black hole]] candidates such as active galactic nuclei.<ref name="cox2000"/>

=== 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.{{cn|date=March 2025}}

In [[neutrino astronomy]], astronomers use heavily shielded [[Neutrino observatory|underground facilities]] such as [[SAGE (ruSsian American Gallium Experiment)|SAGE]], [[GALLEX]], and [[Kamioka Observatory|Kamioka II/III]] for the detection of [[neutrino]]s. The vast majority of the neutrinos streaming through the Earth originate from the [[Sun]], but 24 neutrinos were also detected from [[supernova 1987A]].<ref name="cox2000"/> [[Cosmic ray]]s, 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.<ref>{{cite book
|first=Thomas K.|last=Gaisser|date=1990
|title=Cosmic Rays and Particle Physics|url=https://archive.org/details/cosmicrayspartic0000gais|url-access=registration|pages=[https://archive.org/details/cosmicrayspartic0000gais/page/1 1–2]
|publisher=Cambridge University Press|isbn=978-0-521-33931-5}}</ref> Some future [[neutrino detector]]s may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.<ref name="cox2000"/>

[[Gravitational-wave astronomy]] is an emerging field of astronomy that employs [[gravitational-wave detector]]s 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 observation of gravitational waves|first detection]] on 14 September 2015, observing gravitational waves from a [[binary black hole]].<ref name="PRL-20160211">{{cite journal |collaboration=LIGO Scientific Collaboration and Virgo Collaboration |last1=Abbott |first1=Benjamin P. |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=[[Physical Review Letters]] |volume=116 |issue=6 |pages=061102 |year=2016 |doi=10.1103/PhysRevLett.116.061102 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |pmid=26918975 |s2cid=124959784 }}</ref> A second [[gravitational wave]] was detected on 26 December 2015 and additional observations should continue but [[gravitational wave]]s require extremely sensitive instruments.<ref>{{cite web |url=http://www.europhysicsnews.org/index.php?option=article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html |title=Opening new windows in observing the Universe |last1=Tammann |first1=Gustav-Andreas <!-- Gustav Alfred Andreas -->|author-link=Gustav Andreas Tammann |first2=Friedrich-Karl |last2=Thielemann |author-link2=Friedrich-Karl Thielemann |first3=Dirk |last3=Trautmann |date=2003 |publisher=Europhysics News |archive-url=https://archive.today/20120906192257/http://www.europhysicsnews.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epn/abs/2003/02/epn03208/epn03208.html |archive-date=6 September 2012 |access-date=17 November 2016 }}</ref><ref>{{Cite journal |author1=LIGO Scientific Collaboration and Virgo Collaboration |last2=Abbott |first2=B. P. |last3=Abbott |first3=R. |last4=Abbott |first4=T. D.|last5=Abernathy |first5=M. R. |last6=Acernese |first6=F. |last7=Ackley |first7=K. |last8=Adams |first8=C. |last9=Adams |first9=T. |date=15 June 2016 |title=GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence |journal=Physical Review Letters |volume=116 |issue=24 |pages=241103 |doi=10.1103/PhysRevLett.116.241103 |pmid=27367379 |arxiv=1606.04855 |bibcode=2016PhRvL.116x1103A |s2cid=118651851 }}</ref>

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as [[multi-messenger astronomy]].<ref>{{cite web|title=Planning for a bright tomorrow: Prospects for gravitational-wave astronomy with Advanced LIGO and Advanced Virgo|url=http://www.ligo.org/science/Publication-ObservingScenario/index.php|publisher=[[LIGO Scientific Collaboration]]|access-date=31 December 2015|archive-date=23 April 2016|archive-url=https://web.archive.org/web/20160423031110/http://www.ligo.org/science/Publication-ObservingScenario/index.php|url-status=live}}</ref><ref>{{cite book |title=Neutrinos in Particle Physics, Astronomy and Cosmology |first1=Zhizhong |last1=Xing |first2=Shun |last2=Zhou |publisher=Springer |date=2011 |isbn=978-3-642-17560-2 |page=313 |url=https://books.google.com/books?id=6QXqlCHLjJkC&pg=PA313 |access-date=20 June 2015 |archive-date=3 February 2021 |archive-url=https://web.archive.org/web/20210203012300/https://books.google.com/books?id=6QXqlCHLjJkC&pg=PA313 |url-status=live }}</ref>

=== Astrometry and celestial mechanics ===
{{Main|Astrometry|Celestial mechanics}}
[[File:EmissionNebula NGC6357.jpg|thumb|Star cluster [[Pismis 24]] with a nebula]]
One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in [[celestial navigation]] (the use of celestial objects to guide navigation) and in the making of [[calendar]]s.<ref name=":0">{{Cite book |last=Fraknoi |first=Andrew |url=https://openstax.org/details/books/astronomy-2e |title=Astronomy 2e |date=2022 |display-authors=etal |publisher=OpenStax |isbn=978-1-951693-50-3 |edition=2e |oclc=1322188620 |access-date=16 March 2023 |archive-date=23 February 2023 |archive-url=https://web.archive.org/web/20230223211041/https://openstax.org/details/books/astronomy-2e |url-status=live }}</ref>{{rp|39}}

Careful measurement of the positions of the planets has led to a solid understanding of gravitational [[Perturbation theory|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 object]]s will allow for predictions of close encounters or potential collisions of the Earth with those objects.<ref>{{cite web|last = Calvert|first = James B.|date = 28 March 2003|url = http://www.du.edu/~jcalvert/phys/orbits.htm|title = Celestial Mechanics|publisher = University of Denver|access-date = 21 August 2006|archive-url = https://web.archive.org/web/20060907120741/http://www.du.edu/~jcalvert/phys/orbits.htm|archive-date = 7 September 2006}}</ref>

The measurement of [[stellar parallax]] of nearby stars provides a fundamental baseline in the [[cosmic distance ladder]] that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the [[radial velocity]] and [[proper motion]] of stars allow astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated [[dark matter]] in the galaxy.<ref>{{cite web|url=http://www.astro.virginia.edu/~rjp0i/museum/engines.html|title=Hall of Precision Astrometry|publisher=[[University of Virginia]] Department of Astronomy|access-date=17 November 2016|archive-url=https://web.archive.org/web/20060826104509/http://www.astro.virginia.edu/~rjp0i/museum/engines.html|archive-date=26 August 2006 }}</ref>

During the 1990s, the measurement of the [[stellar wobble]] of nearby stars was [[Methods of detecting extrasolar planets#Astrometry|used to detect]] large [[extrasolar planet]]s orbiting those stars.<ref name="Wolszczan">{{cite journal| author=Wolszczan, A.| author2=Frail, D. A.| title=A planetary system around the millisecond pulsar PSR1257+12| journal=Nature| date=1992| volume=355| issue=6356|pages=145–47| doi= 10.1038/355145a0| bibcode=1992Natur.355..145W| s2cid=4260368}}</ref>