View source for Radio astronomy

{{Short description|Subfield of astronomy that studies celestial objects at radio frequencies}}
{{more citations needed|date=August 2025}}
[[File:USA.NM.VeryLargeArray.02.jpg|thumb|350px|The [[Karl G. Jansky]] [[Very Large Array]], a radio [[interferometry|interferometer]] in [[New Mexico]], United States]]
'''Radio astronomy''' is a subfield of [[astronomy]] that studies [[Astronomical object|celestial objects]] using [[radio wave]]s. It started in 1933, when [[Karl Jansky]] at [[Bell Telephone Laboratories]] reported radiation coming from the [[Milky Way]]. Subsequent observations have identified a number of different sources of radio emission. These include [[star]]s and [[galaxy|galaxies]], as well as entirely new classes of objects, such as [[Radio galaxy|radio galaxies]], [[quasar]]s, [[pulsar]]s, and [[Astrophysical maser|masers]]. The discovery of the [[cosmic microwave background radiation]], regarded as evidence for the [[Big Bang|Big Bang theory]], was made through radio astronomy.

Radio astronomy is conducted using large [[Antenna (radio)|radio antennas]] referred to as ''[[radio telescope]]s'', that are either used alone, or with multiple linked telescopes utilizing the techniques of [[Astronomical interferometer|radio interferometry]] and [[aperture synthesis]]. The use of interferometry allows radio astronomy to achieve high [[angular resolution]], as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.

Radio astronomy differs from ''[[radar astronomy]]'' in that the former is a passive observation (i.e., receiving only) and the latter an active one (transmitting and receiving).

== History ==

[[File:JanskyatAntenna hi.tif|thumb|[[Karl Jansky]] and his rotating [[directional antenna]] (early 1930s) in [[Holmdel, New Jersey]], the world's first radio telescope, which was used to discover radio emissions from the [[Milky Way]]]]

Before [[Karl Guthe Jansky|Karl Jansky]] observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources. In the 1860s, [[James Clerk Maxwell]]'s [[Maxwell's equations|equations]] had shown that [[electromagnetic radiation]] is associated with [[electricity]] and [[magnetism]], and could exist at any [[wavelength]]. Several attempts were made to detect radio emission from the [[Sun]], including an experiment by German astrophysicists [[Johannes Wilsing]] and [[Julius Scheiner]] in 1896 and a centimeter wave radiation apparatus set up by [[Oliver Lodge]] between 1897 and 1900. These attempts were unable to detect any emission due to technical limitations of the instruments. The discovery of the radio-reflecting [[ionosphere]] in 1902 led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable.<ref>{{cite web | url = http://www.nrao.edu/whatisra/hist_prehist.shtml | title = Pre-History of Radio Astronomy | author = F. Ghigo | publisher = [[National Radio Astronomy Observatory]] | access-date = 2010-04-09 | archive-date = 2020-06-15 | archive-url = https://web.archive.org/web/20200615213814/http://www.nrao.edu/whatisra/hist_prehist.shtml | url-status = live }}</ref>

[[Karl Jansky]] made the discovery of the first astronomical radio source [[Serendipity|serendipitously]] in the early 1930s. As a newly hired radio engineer with [[Bell Labs|Bell Telephone Laboratories]], he was assigned the task to investigate static that might interfere with [[short wave]] transatlantic voice transmissions. Using a large [[directional antenna]], Jansky noticed that his [[analog signal|analog]] pen-and-paper recording system kept recording a persistent repeating signal or "hiss" of unknown origin. Since the signal peaked about every 24 hours, Jansky first suspected the source of the interference was the [[Sun]] crossing the view of his directional antenna. Continued analysis, however, showed that the source was not following the 24-hour daily cycle of the Sun exactly but instead repeating on a cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend, astrophysicist Albert Melvin Skellett, who pointed out that the observed time between the signal peaks was the exact length of a [[sidereal time|sidereal day]]: the time it took for "fixed" astronomical objects, such as a star, to pass in front of the antenna every time the Earth rotated.<ref name="bookrags.com">{{cite book | url = http://www.bookrags.com/biography/karl-jansky-wsd/ | title = World of Scientific Discovery on Karl Jansky | access-date = 2010-04-09 | archive-date = 2012-01-21 | archive-url = https://web.archive.org/web/20120121112928/http://www.bookrags.com/biography/karl-jansky-wsd/ | url-status = live }}</ref> By comparing his observations with optical astronomical maps, Jansky eventually concluded that the radiation source peaked when his antenna was aimed at the densest part of the [[Milky Way]] in the [[constellation]] of [[Sagittarius (constellation)|Sagittarius]].<ref>{{cite journal | doi = 10.1038/132066a0 | author = Jansky, Karl G. |  title = Radio waves from outside the solar system | journal = Nature | volume = 132 | issue = 3323 | page = 66 | date = 1933 | bibcode = 1933Natur.132...66J| s2cid = 4063838 | doi-access = free }}</ref>

[[File:JanskywGraph.jpg|thumb|Jansky with a rough map of the night sky and pointing to the constellation of Cassiopeia. The wavy lines track the radio emissions he discovered on the chart paper, which also line up with the disk of the Milky Way.]]

Jansky announced his discovery at a meeting in Washington, D.C., in April 1933 and the field of radio astronomy was born.<ref name="aas">{{cite web |last1=Hirshfeld |first1=Alan |title=Karl Jansky and the Discovery of Cosmic Radio Waves |url=https://aas.org/posts/news/2018/07/month-astronomical-history-1 |publisher=American Astronomical Society |access-date=21 September 2021 |date=2018 |quote=In April 1933, closing in on nearly two years of study, Jansky read his breakthrough paper, “Electrical Disturbances Apparently of Extraterrestrial Origin,” before a meeting of the International Scientific Radio Union in Washington, DC. The strongest of the extraterrestrial waves, he found, emanate from a region in Sagittarius centered around right ascension 18 hours and declination — 20 degrees — in other words, from the direction of the galactic center. Jansky’s discovery made the front page of the New York Times on 5 May 1933, and the field of radio astronomy was born. |archive-date=29 September 2021 |archive-url=https://web.archive.org/web/20210929185905/https://aas.org/posts/news/2018/07/month-astronomical-history-1 |url-status=live }}</ref> In October 1933, his discovery was published in a journal article entitled "Electrical disturbances apparently of extraterrestrial origin" in the ''[[Proceedings of the Institute of Radio Engineers]]''.<ref>{{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |date=October 1933 |title=Electrical disturbances apparently of extraterrestrial origin |journal=Proc. IRE |volume=21 |issue=10 |page=1387 |doi=10.1109/JRPROC.1933.227458 |bibcode=1933PIRE...21.1387J }} Reprinted 65 years later as {{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |title=Electrical disturbances apparently of extraterrestrial origin |doi=10.1109/JPROC.1998.681378 |journal=[[Proc. IEEE]] |volume=86 |issue=7 |date=July 1998 |pages=1510–1515|bibcode=1998IEEEP..86.1510J |s2cid=47549559 }} along with an explanatory preface in {{cite journal|last1=Imbriale |first1=W.A. |title=Introduction to "Electrical Disturbances Apparently of Extraterrestrial Origin" |journal=Proc. IEEE |date=1998 |volume=86 |issue=7 |pages=1507–1509 |doi=10.1109/JPROC.1998.681377 |bibcode=1998IEEEP..86.1507I }}.</ref> Jansky concluded that since the Sun (and therefore other stars) were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy, in particular, by "thermal agitation of charged particles."<ref name="bookrags.com"/><ref>{{cite journal |last=Jansky |first=Karl Guthe |author-link=Karl Guthe Jansky |date=October 1935 |title=A note on the source of interstellar interference |journal=Proc. IRE |volume=23 |issue=10 |page=1158|doi=10.1109/JRPROC.1935.227275 |bibcode=1935PIRE...23.1158J |s2cid=51632813 }}</ref> (Jansky's peak radio source, one of the brightest in the sky, was designated [[Sagittarius A]] in the 1950s and was later hypothesized to be emitted by [[electrons]] in a strong magnetic field. Current thinking is that these are ions in orbit around a massive [[black hole]] at the center of the galaxy at a point now designated as Sagittarius A*. The asterisk indicates that the particles at Sagittarius A are ionized.)<ref>{{cite book |title=Relativity, Astrophysics and Cosmology: Volume 1 |last=Belusević |first=R. |year=2008 |url=https://books.google.com/books?id=WeICTHIxP2MC&pg=PA163|page=163 |publisher=Wiley-VCH |isbn=978-3-527-40764-4}}</ref><ref>{{Cite book |last=Kambič |first=B. |title=Viewing the Constellations with Binoculars |date=6 October 2009 |url=https://books.google.com/books?id=3vxLNPNHOcwC&pg=PA131|pages=131–133 |publisher=[[Springer (publisher)|Springer]] |isbn=978-0-387-85355-0}}</ref><ref>{{cite journal | last1 = Gillessen | first1 = S. | last2 = Eisenhauer | first2 = F. | last3 = Trippe | first3 = S. |display-authors=etal  | year = 2009 | title = Monitoring Stellar Orbits around the Massive Black Hole in the Galactic Center | journal = The Astrophysical Journal | volume = 692 | issue = 2| pages = 1075–1109 | doi = 10.1088/0004-637X/692/2/1075 | arxiv=0810.4674 | bibcode=2009ApJ...692.1075G| s2cid = 1431308 }}</ref><ref>{{cite journal | last1 = Brown | first1 = R.L. | year = 1982 | title = Precessing jets in Sagittarius A – Gas dynamics in the central parsec of the galaxy | bibcode =  1982ApJ...262..110B| journal = Astrophysical Journal | volume = 262 | pages = 110–119 | doi = 10.1086/160401| doi-access = free }}</ref>

After 1935, Jansky wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of [[flux density]], the [[jansky]] (Jy), after him.<ref name="aps">{{cite web |title=This Month in Physics History May 5, 1933: The New York Times Covers Discovery of Cosmic Radio Waves |url=https://www.aps.org/publications/apsnews/201505/physicshistory.cfm |website=aps.org |publisher=American Physical Society (May 2015) Volume 24, Number 5 |access-date=21 September 2021 |quote=Jansky died in 1950 at the age of 44, the result of a massive stroke stemming from his kidney disease. When that first 1933 paper was reprinted in Proceedings of the IEEE in 1984, the editors noted that Jansky’s work would mostly likely have won a Nobel prize, had the scientist not died so young. Today the “jansky” is the unit of measurement for radio wave intensity (flux density). |archive-date=14 September 2021 |archive-url=https://web.archive.org/web/20210914000424/https://www.aps.org/publications/apsnews/201505/physicshistory.cfm |url-status=live }}</ref>

[[File:Grote Antenna Wheaton.gif|left|thumb|[[Grote Reber]]'s 9 meter antenna in his backyard in [[Wheaton, Illinois]], the world's first parabolic radio telescope]]
[[Radio amateur]] [[Grote Reber]] was inspired by Jansky's work, and built a parabolic radio telescope 9 meters in diameter in his backyard in Wheaton, Illinois in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies.<ref>{{cite web | url = http://www.nrao.edu/whatisra/hist_reber.shtml | title = Grote Reber | access-date = 2010-04-09 | archive-date = 2020-08-07 | archive-url = https://web.archive.org/web/20200807095454/https://www.nrao.edu/whatisra/hist_reber.shtml | url-status = live }}</ref> On February 27, 1942, [[James Stanley Hey]], a [[British Army]] research officer, made the first detection of radio waves emitted by the Sun.<ref>{{cite book |last=Hey |first=J.S. |title=Radio Universe |edition=2nd |publisher=[[Pergamon Press]] |year=1975 |isbn= 978-0080187617 }}</ref> Later that year, [[George Clark Southworth]],<ref>{{cite journal |last=Southworth |first=G.C. |year=1945 |title=Microwave radiation from the Sun |journal=Journal of the Franklin Institute |volume=239 |issue=4 |pages=285–297 |doi=10.1016/0016-0032(45)90163-3 |bibcode=1945FrInJ.239..285S }}</ref> at [[Bell Labs]] like Jansky, also detected radiowaves from the Sun. Both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first.<ref>{{cite journal|last1=Kellerman|first1=K. I.|title=Grote Reber's Observations on Cosmic Static|journal=Astrophysical Journal|date= 1999|volume=525C|page=371 |bibcode=1999ApJ...525C.371K}}</ref> Several other people independently discovered solar radio waves, including [[E. Schott]] in [[Denmark]]<ref>{{cite journal |last=Schott |first=E. |year=1947 |title=175 MHz-Strahlung der Sonne |journal=Physikalische Blätter |volume=3 |issue=5 |pages=159–160 |doi=10.1002/phbl.19470030508 |language=de|doi-access=free }}</ref> and [[Elizabeth Alexander (scientist)|Elizabeth Alexander]] working on [[Norfolk Island]].<ref>{{cite book |last1=Alexander |first1=F.E.S. |date=1945 |title=Long Wave Solar Radiation |publisher=[[Department of Scientific and Industrial Research (New Zealand)|Department of Scientific and Industrial Research]], Radio Development Laboratory }}</ref><ref>{{cite book |last1=Alexander |first1=F.E.S. |date=1945 |title=Report of the Investigation of the "Norfolk Island Effect" |publisher=[[Department of Scientific and Industrial Research (New Zealand)|Department of Scientific and Industrial Research]], Radio Development Laboratory |bibcode=1945rdlr.book.....A }}</ref><ref>{{cite journal |last1=Alexander |first1=F.E.S. |date=1946 |title=The Sun's radio energy |journal=Radio & Electronics |volume=1 |issue=1 |pages=16–17}} (see [http://nlnzcat.natlib.govt.nz/vwebv/holdingsInfo?bibId=405978 ''R&E'' holdings at NLNZ] {{Webarchive|url=https://archive.today/20160723215554/http://nlnzcat.natlib.govt.nz/vwebv/holdingsInfo?bibId=405978 |date=2016-07-23 }}.)</ref><ref name=Orchiston>{{cite book |last=Orchiston |first=W. |year=2005 |chapter=Dr Elizabeth Alexander: First Female Radio Astronomer |title=The New Astronomy: Opening the Electromagnetic Window and Expanding Our View of Planet Earth |series=Astrophysics and Space Science Library |volume=334 |pages=71–92 |doi=10.1007/1-4020-3724-4_5 |isbn=978-1-4020-3723-8}}</ref>

[[File:Chart Showing Radio Signal of First Identified Pulsar.jpg|thumb|upright|Chart on which [[Jocelyn Bell Burnell]] first recognised evidence of a [[pulsar]], in 1967 (exhibited at [[Cambridge University Library]]) ]]
At [[Cambridge University]], where ionospheric research had taken place during [[World War II]], [[J. A. Ratcliffe]] along with other members of the [[Telecommunications Research Establishment]] that had carried out wartime research into [[radar]], created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied.
This early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. [[Martin Ryle]] and [[Antony Hewish]] at the [[Cavendish Astrophysics Group]] developed the technique of Earth-rotation [[aperture synthesis]]. The radio astronomy group in Cambridge went on to found the [[Mullard Radio Astronomy Observatory]] near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the [[Titan (1963 computer)|Titan]]) became capable of handling the computationally intensive [[Fourier transform]] inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5&nbsp;km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used the [[Cambridge Interferometer]] to map the radio sky, producing the [[Second Cambridge Catalogue of Radio Sources|Second]] (2C) and [[Third Cambridge Catalogue of Radio Sources|Third]] (3C) Cambridge Catalogues of Radio Sources.<ref>{{cite web|url=http://www.phy.cam.ac.uk/history/years/radioast.php|archive-url=https://web.archive.org/web/20131110022209/http://www.phy.cam.ac.uk/history/years/radioast.php|archive-date=2013-11-10|title=Radio Astronomy|publisher=Cambridge University: Department of Physics}}</ref>

==Techniques==
[[File:Atmospheric electromagnetic opacity.svg|thumb|upright=2.00|Window of radio waves observable from Earth, on rough plot of Earth's atmospheric absorption and scattering (or [[opacity (optics)|opacity]]) of various [[wavelength]]s of electromagnetic radiation]]

Radio astronomers use different techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze its emission. To "image" a region of the sky in more detail, multiple overlapping scans can be recorded and pieced together in a [[mosaic]] image. The type of instrument used depends on the strength of the signal and the amount of detail needed.

Observations from the [[Earth]]'s surface are limited to wavelengths that can pass through the atmosphere. At low frequencies or long wavelengths, transmission is limited by the [[ionosphere]], which reflects waves with frequencies less than its characteristic [[plasma frequency]]. Thus far, radio observations have been made at frequencies as low as 15&nbsp;MHz.<ref>{{cite journal |last1=Groeneveld |first1=C. |last2=van Weeren |first2=R. J. |last3=Osinga |first3=E. |last4=Williams |first4=W. L. |last5=Callingham |first5=J. R. |last6=de Gasperin |first6=F. |last7=Botteon |first7=A. |last8=Shimwell |first8=T. |last9=Sweijen |first9=F. |last10=de Jong |first10=J. M. G. H. J. |last11=Jansen |first11=L. F. |last12=Miley |first12=G. K. |last13=Brunetti |first13=G. |last14=Brüggen |first14=M. |last15=Röttgering |first15=H. J. A. |title=Characterization of the decametre sky at subarcminute resolution |journal=Nature Astronomy |date=6 May 2024 |volume=8 |issue=6 |pages=786–795 |doi=10.1038/s41550-024-02266-z|arxiv=2405.05311 |bibcode=2024NatAs...8..786G }}</ref> [[Water]] [[vapor]] interferes with radio astronomy at higher frequencies, which has led to building radio observatories that conduct observations at [[millimeter]] wavelengths at very high and dry sites to minimize the water vapor content in the line of sight. Finally, transmitting devices on Earth may cause [[radio-frequency interference]]. Because of this, many radio observatories are built at remote places.

===Radio telescopes===
{{Main|Radio telescope}}

Radio telescopes may need to be extremely large in order to receive signals with low [[signal-to-noise ratio]]. Also since [[angular resolution]] is a function of the diameter of the "[[Objective (optics)|objective]]" in proportion to the wavelength of the electromagnetic radiation being observed, ''[[radio telescope]]s'' have to be much larger in comparison to their [[Optical telescope|optical]] counterparts. For example, a 1-meter diameter optical telescope is two million times bigger than the wavelength of light observed giving it a resolution of roughly 0.3 [[arc second]]s, whereas a radio telescope "dish" many times that size may, depending on the wavelength observed, only be able to resolve an object the size of the full moon (30 minutes of arc).

===Radio interferometry===
{{main|Astronomical interferometry}}
{{see also|Radio telescope#Radio interferometry}}
[[File:The Atacama Compact Array.jpg|thumb|The [[Atacama Large Millimeter Array]] (ALMA), many antennas linked together in a radio interferometer]]
[[File:M87 optical image.jpg|thumb|300px]] [[File:M87 VLA VLBA radio astronomy.jpg|thumb|300px| An optical image of the galaxy [[Messier 87|M87]] ([[Hubble Space Telescope|HST]]), a radio image of same galaxy using interferometry ([[Very Large Array]], VLA), and an image of the center section (VLBA) using a Very Long Baseline Array (Global VLBI) consisting of antennas in the US, Germany, Italy, Finland, Sweden and Spain. The jet of particles is suspected to be powered by a [[black hole]] in the center of the galaxy.]]

The difficulty in achieving high resolutions with single radio telescopes led to radio [[interferometry]], developed by British radio astronomer [[Martin Ryle]] and Australian engineer, radiophysicist, and radio astronomer [[Joseph Lade Pawsey]] and [[Ruby Payne-Scott]] in 1946. The first use of a radio interferometer for an astronomical observation was carried out by Payne-Scott, Pawsey and [[Lindsay McCready]] on 26 January 1946 using a ''single'' converted radar antenna (broadside array) at [[Very high frequency|200 MHz]] near [[Sydney, Australia]]. This group used the principle of a sea-cliff interferometer in which the antenna (formerly a World War II radar) observed the Sun at sunrise with interference arising from the direct radiation from the Sun and the reflected radiation from the sea. With this baseline of almost 200 meters, the authors determined that the solar radiation during the burst phase was much smaller than the solar disk and arose from a region associated with a large [[sunspot]] group. The Australia group laid out the principles of [[aperture synthesis]] in a groundbreaking paper published in 1947. The use of a sea-cliff [[interferometer]] had been demonstrated by numerous groups in Australia, Iran and the UK during World War II, who had observed interference fringes (the direct radar return radiation and the reflected signal from the sea) from incoming aircraft.

The Cambridge group of Ryle and Vonberg observed the Sun at 175&nbsp;MHz for the first time in mid-July 1946 with a Michelson interferometer consisting of two radio antennas with spacings of some tens of meters up to 240 meters. They showed that the radio radiation was smaller than 10 [[Minute of arc|arc minutes]] in size and also detected circular polarization in the Type&nbsp;I bursts. Two other groups had also detected circular polarization at about the same time ([[David Martyn (scientist)|David Martyn]] in Australia and [[Edward Victor Appleton|Edward Appleton]] with [[James Stanley Hey]] in the UK).

Modern [[radio telescope#radio interferometry|radio interferometers]] consist of widely separated radio telescopes observing the same object that are connected together using [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission line]]. This not only increases the total signal collected, but it can also be used in a process called [[aperture synthesis]] to vastly increase resolution. This technique works by superposing ("[[Interference (wave propagation)|interfering]]") the signal [[wave]]s from the different telescopes on the principle that [[wave]]s that coincide with the same [[phase (waves)|phase]] will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is the size of the antennas furthest apart in the array. To produce a high-quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a "baseline") – as many different baselines as possible are required in order to get a good quality image. For example, the [[Very Large Array]] has 27 telescopes giving 351 independent baselines at once.

====Very-long-baseline interferometry====
{{main|Very-long-baseline interferometry}}

Beginning in the 1970s, improvements in the stability of radio telescope receivers permitted telescopes from all over the world (and even in Earth orbit) to be combined to perform [[very-long-baseline interferometry]]. Instead of physically connecting the antennas, data received at each antenna is paired with timing information, usually from a local [[atomic clock]], and then stored for later analysis on magnetic tape or hard disk. At that later time, the data is correlated with data from other antennas similarly recorded, to produce the resulting image. Using this method, it is possible to synthesise an antenna that is effectively the size of the Earth. The large distances between the telescopes enable very high angular resolutions to be achieved, much greater in fact than in any other field of astronomy. At the highest frequencies, synthesised beams less than 1 [[Minute of arc|milliarcsecond]] are possible.

The pre-eminent VLBI arrays operating today are the [[Very Long Baseline Array]] (with telescopes located across North America) and the [[European VLBI Network]] (telescopes in Europe, China, South Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed together producing increased sensitivity. This is referred to as Global VLBI. There are also a VLBI networks, operating in Australia and New Zealand called the LBA (Long Baseline Array),<ref>{{Cite web| url=http://www.atnf.csiro.au/vlbi/| title=VLBI at the ATNF| date=7 December 2016| access-date=16 June 2015| archive-date=1 May 2021| archive-url=https://web.archive.org/web/20210501051105/https://www.atnf.csiro.au/vlbi/| url-status=live}}</ref> and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).<ref>{{Cite web | url=http://www.astro.sci.yamaguchi-u.ac.jp/eavn/index.html | title=East Asia VLBI Network and Asia Pacific Telescope | access-date=2015-06-16 | archive-date=2021-04-28 | archive-url=https://web.archive.org/web/20210428080543/http://astro.sci.yamaguchi-u.ac.jp/eavn/index.html | url-status=live }}</ref>

Since its inception, recording data onto hard media was the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was originally pioneered in Japan, and more recently adopted in Australia and in Europe by the EVN (European VLBI Network) who perform an increasing number of scientific e-VLBI projects per year.<ref>{{Cite web |url=http://www.innovations-report.com/html/reports/physics_astronomy/report-25117.html |title=A technological breakthrough for radio astronomy – Astronomical observations via high-speed data link<!-- Bot generated title --> |date=26 January 2004 |access-date=2008-07-22 |archive-date=2008-12-03 |archive-url=https://web.archive.org/web/20081203145055/http://www.innovations-report.com/html/reports/physics_astronomy/report-25117.html |url-status=live }}</ref>

==Astronomical sources==
{{Main|Astronomical radio source}} {{See also|Radio object with continuous optical spectrum}}
[[File:GCRT J1745-3009 2.jpg|thumb|A radio image of the central region of the Milky Way galaxy. The arrow indicates a supernova remnant which is the location of a newly discovered transient, bursting low-frequency radio source [[GCRT J1745-3009]].]]
Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including [[pulsar]]s, [[quasar]]s<ref name=Shields>{{cite journal|last1=Shields|first1=Gregory A.|title=A brief history of AGN|journal=The Publications of the Astronomical Society of the Pacific|date=1999|volume=111|issue=760|pages=661–678|access-date=3 October 2014|url=http://ned.ipac.caltech.edu/level5/Sept04/Shields/Shields3.html|doi=10.1086/316378|arxiv=astro-ph/9903401|bibcode=1999PASP..111..661S|s2cid=18953602|archive-date=12 September 2009|archive-url=https://web.archive.org/web/20090912025415/http://nedwww.ipac.caltech.edu/level5/Sept04/Shields/Shields3.html|url-status=live}}</ref> and [[radio galaxy|radio galaxies]]. This is because radio astronomy allows us to see things that are not detectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.

The [[cosmic microwave background radiation]] was also first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the [[Sun]] and solar activity, and radar mapping of the [[Solar System|planets]].

Other sources include:
* [[Sun]]
* [[Jupiter]]
* [[Sagittarius A]], the [[Galactic Center]] of the [[Milky Way]], with one portion [[Sagittarius A*]] thought to be a radio wave–emitting [[supermassive black hole]]
* [[Active galactic nucleus|Active galactic nuclei]] and [[pulsar]]s have jets of charged particles which emit [[synchrotron radiation]]
* Merging [[galaxy cluster]]s often show diffuse radio emission<ref>{{cite web |url=http://www.arcetri.astro.it/~buttery/thesis/node69.html |title=Conclusion |access-date=2006-03-29 |url-status=dead |archive-url=https://web.archive.org/web/20060128231925/http://www.arcetri.astro.it/~buttery/thesis/node69.html |archive-date=2006-01-28 }}</ref>
* [[Supernova remnant]]s can also show diffuse radio emission; [[pulsar]]s are a type of supernova remnant that shows highly synchronous emission.
* The [[cosmic microwave background]] is [[blackbody]] radio/microwave emission

Earth's radio signal is mostly natural and stronger than for example Jupiter's but is produced by Earth's [[aurora]]s and bounces at the [[ionosphere]] back into space.<ref name="Geophysical Institute 1983 r818">{{cite web | title=The Earth is a Strong Radio Source even without Man's Tinkering | website=Geophysical Institute | date=June 23, 1983 | url=https://www.gi.alaska.edu/alaska-science-forum/earth-strong-radio-source-even-without-mans-tinkering | access-date=May 2, 2024}}</ref>

==International regulation==
[[File:Goldstone DSN antenna.jpg|thumb|200px|Antenna 70 m of the [[Goldstone Deep Space Communications Complex]], [[California]]]]
[[File:Green Bank Telescope.jpg|thumb|200px|Antenna 110m of the [[Green Bank Telescope|Green Bank radio telescope]], US]]
[[File:Sbursts1.ogg|thumb|200px|Jupiter radio-bursts]]

'''Radio astronomy service''' (also: ''radio astronomy radiocommunication service'') is, according to Article 1.58 of the [[International Telecommunication Union|International Telecommunication Union's]] (ITU) [[ITU Radio Regulations|Radio Regulations]] (RR),<ref>ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.58, definition: '' radio astronomy service / radio astronomy radiocommunication service''</ref> defined as "A [[radiocommunication service]] involving the use of radio astronomy". Subject of this radiocommunication service is to receive [[radio wave]]s transmitted by [[astronomical object|astronomical]] or celestial objects.

===Frequency allocation===
The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012).<ref>''ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations''</ref>

To improve harmonisation in spectrum utilisation, the majority of service-allocations stipulated in this document were incorporated in national Tables of Frequency Allocations and Utilisations which is within the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared.
*primary allocation: indicated by writing in capital letters (see example below)
*secondary allocation: indicated by small letters
*exclusive or shared utilization: within the responsibility of administrations

In line to the appropriate [[International Telecommunication Union region|ITU Region]], the frequency bands are allocated (primary or secondary) to the ''radio astronomy service'' as follows.

{| class=wikitable
|- bgcolor="#CCCCCC" align="center"
|align="center" colspan="3"| '''Allocation to services'''
|- align="center"
| &nbsp;&nbsp;&nbsp;&nbsp; Region 1 &nbsp;&nbsp;&nbsp;&nbsp;
| &nbsp;&nbsp;&nbsp;&nbsp; Region 2 &nbsp;&nbsp;&nbsp;&nbsp;
| &nbsp;&nbsp;&nbsp;&nbsp; Region 3 &nbsp;&nbsp;&nbsp;&nbsp; 
|-
|colspan="3"|13 360–13 410 '''kHz'''&nbsp; FIXED<br />
:::::&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY'''
|-
|colspan="3"|25 550–25 650 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY'''
|-
|colspan="3"|37.5–38.25 '''MHz'''&nbsp; FIXED<br />
::::: MOBILE<br />'''Radio astronomy'''
|-
|colspan="3"|322–328.6 &nbsp; &nbsp; FIXED<br />
:::: MOBILE<br />'''RADIO ASTRONOMY'''
|-
|colspan="3"|406.1–410 &nbsp; &nbsp; FIXED<br />
:::: MOBILE except aeronautical mobile<br />'''RADIO ASTRONOMY'''
|-
|colspan="3"|1 400–1 427&nbsp;&nbsp; EARTH EXPLORATION-SATELLITE (passive)<br />
:::: '''RADIO ASTRONOMY'''<br />SPACE RESEARCH (passive)
|-
|colspan="1"|1 610.6–1 613.8<br />
MOBILE-SATELLITE <br />
:(Earth-to-space)
'''RADIO ASTRONOMY'''<br />AERONAUTICAL <br />
:RADIONAVIGATION
<br />
<br />
 |colspan="1"|1 610.6–1 613.8<br />
MOBILE-SATELLITE <br />
:(Earth-to-space)
'''RADIO ASTRONOMY''' <br />AERONAUTICAL <br />
:RADIONAVIGATION
RADIODETERMINATION-<br />
:SATELLITE (Earth-to-space) 
 |colspan="1"|1 610.6–1 613.8<br />
MOBILE-SATELLITE <br />
:(Earth-to-space)
'''RADIO ASTRONOMY''' <br />AERONAUTICAL <br />
:RADIONAVIGATION
Radiodetermination-<br />
:satellite (Earth-to-space) 
|-
|colspan="3"|10.6–10.68 '''GHz''' &nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|10.68–10.7 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|14.47–14.5 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|15.35–15.4 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|22.21–22.5 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|23.6–24 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; '''RADIO ASTRONOMY''' and other services
|-
|colspan="3"|31.3–31.5 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; '''RADIO ASTRONOMY''' and other services
|-
|}

==See also==
{{Portal|Radio|Astronomy}}
* [[Atacama Large Millimeter Array]]
* [[Submillimeter Array]]
* [[Channel 37]]
* [[Gamma-ray astronomy]]
* [[Infrared astronomy]]
* [[Radar astronomy]]
* [[Time smearing]]
* [[X-ray astronomy]]
* [[Waves (Juno)|Waves (''Juno'')]] (radio instrument on the ''Juno'' Jupiter orbiter)
* [[Radio Galaxy Zoo]]
* [[Würzburg radar#Post-war use in astronomy]]
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== References ==
{{Reflist}}

== Further reading ==

{{Refbegin}}
; Journals
*  {{cite journal |author=Gart Westerhout |title=The early history of radio astronomy |journal=[[Annals of the New York Academy of Sciences]] |volume=189 |year=1972 |issue=1 |pages=211–218 |doi=10.1111/j.1749-6632.1972.tb12724.x |bibcode=1972NYASA.198..211W |s2cid=56034495 |ref=none}}
* {{cite journal |author=Hendrik Christoffel van de Hulst |year=1945 |title=Radiostraling uit het wereldruim. II. Herkomst der radiogolven |journal=Nederlands Tijdschrift voor Natuurkunde |volume=11 |pages=210–221 |language=nl |ref=none}}

; Books
*  [[Gerrit Verschuur]] ''The Invisible Universe: The Story of Radio Astronomy'' Springer 2015
* Bruno Bertotti (ed.), ''Modern Cosmology in Retrospect''. Cambridge University Press 1990.
* James J. Condon, et al.: ''Essential Radio Astronomy.'' Princeton University Press, Princeton 2016, {{ISBN|9780691137797}}.
* Robin Michael Green, ''Spherical Astronomy''. Cambridge University Press, 1985.
* Raymond Haynes, Roslynn Haynes, and Richard McGee, ''Explorers of the Southern Sky: A History of Australian Astronomy''. Cambridge University Press 1996.
* J.S. Hey, ''The Evolution of Radio Astronomy.'' Neale Watson Academic, 1973.
* David L. Jauncey, ''Radio Astronomy and Cosmology.'' Springer 1977.
* [[Roger Clifton Jennison]], ''Introduction to Radio Astronomy''. 1967.
* Jobn D. Kraus, Martt; E. Tiuri, and Antti V. Räisänen, ''Radio Astronomy'', 2nd ed, Cygnus-Quasar Books, 1986.
* Albrecht Krüger, ''Introduction to Solar Radio Astronomy and Radio Physics.'' Springer 1979.
* David P.D. Munns, ''A Single Sky: How an International Community Forged the Science of Radio Astronomy.'' Cambridge, MA: MIT Press, 2013.
* Allan A. Needell, ''Science, Cold War and American State: Lloyd V. Berkner and the Balance of Professional Ideals''. Routledge, 2000.
* Joseph Lade Pawsey and Ronald Newbold Bracewell, ''Radio Astronomy.'' Clarendon Press, 1955.
* Kristen Rohlfs, Thomas L Wilson, ''Tools of Radio Astronomy''. Springer 2003.
* D.T. Wilkinson and P.J.E. Peebles, ''Serendipitous Discoveries in Radio Astronomy.'' Green Bank, WV: National Radio Astronomy Observatory, 1983.
* Woodruff T. Sullivan III, ''The Early Years of Radio Astronomy: Reflections Fifty Years after Jansky's Discovery.'' Cambridge, England: Cambridge University Press, 1984.
* Woodruff T. Sullivan III, ''Cosmic Noise: A History of Early Radio Astronomy.'' Cambridge University Press, 2009.
* Woodruff T. Sullivan III, ''Classics in Radio Astronomy''. Reidel Publishing Company, Dordrecht, 1982.
{{Refend}}

== External links ==
{{Commons category|Radio astronomy}}
* [https://public.nrao.edu/ nrao.edu National Radio Astronomy Observatory]
* [https://archive.today/19990202131951/http://web.haystack.mit.edu/education/radiohist.html The History of Radio Astronomy] * [https://web.archive.org/web/20020328074733/http://www.cr.nps.gov/history/online_books/butowsky5/astro4o.htm Reber Radio Telescope – National Park Services]
* [https://web.archive.org/web/20040307044635/http://edmall.gsfc.nasa.gov/aacps/news/Radio_Telescope.html Radio Telescope Developed] – a brief history from [[NASA]] [[Goddard Space Flight Center]]
* [https://www.radio-astronomy.org/ Society of Amateur Radio Astronomers] 
* [http://www.ogleearth.com/2007/09/xml_or_the_comi.html Visualization of Radio Telescope Data Using Google Earth]
* [http://www.unwantedemissions.com UnwantedEmissions.com A general reference for radio spectrum allocations, including radio astronomy.]
* [http://www.levanda.co.il/ronny/RadioAstronomy_SignalProcessing.html Improving Radio Astronomy Images by Array Processing] {{Webarchive|url=https://web.archive.org/web/20110404054247/http://www.levanda.co.il/ronny/RadioAstronomy_SignalProcessing.html |date=2011-04-04 }}
* [https://archive.today/20130704122042/http://www.radioastrolab.it/en/radio-astronomy/what-is-radio-astronomy/ What is Radio Astronomy] – Radioastrolab
* [https://www.pictortelescope.com/ PICTOR: A free-to-use radio telescope]
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