Radio astronomy is conducted using largeradio antennas referred to asradio telescopes, that are either used alone, or with multiple linked telescopes utilizing the techniques ofradio interferometry andaperture synthesis. The use of interferometry allows radio astronomy to achieve highangular 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 fromradar astronomy in that the former is a passive observation (i.e., receiving only) and the latter an active one (transmitting and receiving).
BeforeKarl 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'sequations had shown thatelectromagnetic radiation is associated withelectricity andmagnetism, and could exist at anywavelength. Several attempts were made to detect radio emission from theSun, including an experiment by German astrophysicistsJohannes Wilsing andJulius Scheiner in 1896 and a centimeter wave radiation apparatus set up byOliver 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-reflectingionosphere in 1902 led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable.[1]
Karl Jansky made the discovery of the first astronomical radio sourceserendipitously in the early 1930s. As a newly hired radio engineer withBell Telephone Laboratories, he was assigned the task to investigate static that might interfere withshort wave transatlantic voice transmissions. Using a largedirectional antenna, Jansky noticed that hisanalog 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 theSun 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 asidereal 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.[2] 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 theMilky Way in theconstellation ofSagittarius.[3]
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.[4] In October 1933, his discovery was published in a journal article entitled "Electrical disturbances apparently of extraterrestrial origin" in theProceedings of the Institute of Radio Engineers.[5] 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."[2][6] (Jansky's peak radio source, one of the brightest in the sky, was designatedSagittarius A in the 1950s and was later hypothesized to be emitted byelectrons in a strong magnetic field. Current thinking is that these are ions in orbit around a massiveblack 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.)[7][8][9][10]
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 offlux density, thejansky (Jy), after him.[11]
Radio amateurGrote 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.[12] On February 27, 1942,James Stanley Hey, aBritish Army research officer, made the first detection of radio waves emitted by the Sun.[13] Later that year,George Clark Southworth,[14] atBell 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.[15] Several other people independently discovered solar radio waves, includingE. Schott inDenmark[16] andElizabeth Alexander working onNorfolk Island.[17][18][19][20]
AtCambridge University, where ionospheric research had taken place duringWorld War II,J. A. Ratcliffe along with other members of theTelecommunications Research Establishment that had carried out wartime research intoradar, 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 andAntony Hewish at theCavendish Astrophysics Group developed the technique of Earth-rotationaperture synthesis. The radio astronomy group in Cambridge went on to found theMullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as theTitan) became capable of handling the computationally intensiveFourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used theCambridge Interferometer to map the radio sky, producing theSecond (2C) andThird (3C) Cambridge Catalogues of Radio Sources.[21]
Window of radio waves observable from Earth, on rough plot of Earth's atmospheric absorption and scattering (oropacity) of variouswavelengths 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 amosaic image. The type of instrument used depends on the strength of the signal and the amount of detail needed.
Observations from theEarth's surface are limited to wavelengths that can pass through the atmosphere. At low frequencies or long wavelengths, transmission is limited by theionosphere, which reflects waves with frequencies less than its characteristicplasma frequency. Thus far, radio observations have been made at frequencies as low as 15 MHz[22].Watervapor interferes with radio astronomy at higher frequencies, which has led to building radio observatories that conduct observations atmillimeter wavelengths at very high and dry sites to minimize the water vapor content in the line of sight. Finally, transmitting devices on Earth may causeradio-frequency interference. Because of this, many radio observatories are built at remote places.
Radio telescopes may need to be extremely large in order to receive signals with lowsignal-to-noise ratio. Also sinceangular resolution is a function of the diameter of the "objective" in proportion to the wavelength of the electromagnetic radiation being observed,radio telescopes have to be much larger in comparison to theiroptical 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 0.3arc seconds, 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).
TheAtacama Large Millimeter Array (ALMA), many antennas linked together in a radio interferometer An optical image of the galaxyM87 (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 ablack hole in the center of the galaxy.
The difficulty in achieving high resolutions with single radio telescopes led to radiointerferometry, developed by British radio astronomerMartin Ryle and Australian engineer, radiophysicist, and radio astronomerJoseph Lade Pawsey andRuby Payne-Scott in 1946. The first use of a radio interferometer for an astronomical observation was carried out by Payne-Scott, Pawsey andLindsay McCready on 26 January 1946 using asingle converted radar antenna (broadside array) at200 MHz nearSydney, 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 largesunspot group. The Australia group laid out the principles ofaperture synthesis in a groundbreaking paper published in 1947. The use of a sea-cliffinterferometer 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 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 10arc minutes in size and also detected circular polarization in the Type I bursts. Two other groups had also detected circular polarization at about the same time (David Martyn in Australia andEdward Appleton withJames Stanley Hey in the UK).
Modernradio interferometers consist of widely separated radio telescopes observing the same object that are connected together usingcoaxial cable,waveguide,optical fiber, or other type oftransmission line. This not only increases the total signal collected, but it can also be used in a process calledaperture synthesis to vastly increase resolution. This technique works by superposing ("interfering") the signalwaves from the different telescopes on the principle thatwaves that coincide with the samephase 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, theVery Large Array has 27 telescopes giving 351 independent baselines at once.
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 performvery-long-baseline interferometry. Instead of physically connecting the antennas, data received at each antenna is paired with timing information, usually from a localatomic 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 1milliarcsecond are possible.
The pre-eminent VLBI arrays operating today are theVery Long Baseline Array (with telescopes located across North America) and theEuropean 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),[23] and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).[24]
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.[25]
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 sourceGCRT J1745-3009.
Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, includingpulsars,quasars[26] andradio 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.
Thecosmic 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 theSun and solar activity, and radar mapping of theplanets.
Earth's radio signal is mostly natural and stronger than for example Jupiter's but is produced by Earth'sauroras and bounces at theionosphere back into space.[28]
The allocation of radio frequencies is provided according toArticle 5 of the ITU Radio Regulations (edition 2012).[30]
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 appropriateITU Region, the frequency bands are allocated (primary or secondary) to theradio astronomy service as follows.
^Hirshfeld, Alan (2018)."Karl Jansky and the Discovery of Cosmic Radio Waves". American Astronomical Society.Archived from the original on 29 September 2021. Retrieved21 September 2021.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.
^"This Month in Physics History May 5, 1933: The New York Times Covers Discovery of Cosmic Radio Waves".aps.org. American Physical Society (May 2015) Volume 24, Number 5.Archived from the original on 14 September 2021. Retrieved21 September 2021.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).
^Orchiston, W. (2005). "Dr Elizabeth Alexander: First Female Radio Astronomer".The New Astronomy: Opening the Electromagnetic Window and Expanding Our View of Planet Earth. Astrophysics and Space Science Library. Vol. 334. pp. 71–92.doi:10.1007/1-4020-3724-4_5.ISBN978-1-4020-3723-8.
^Groeneveld, C.; van Weeren, R. J.; Osinga, E.; Williams, W. L.; Callingham, J. R.; de Gasperin, F.; Botteon, A.; Shimwell, T.; Sweijen, F.; de Jong, J. M. G. H. J.; Jansen, L. F.; Miley, G. K.; Brunetti, G.; Brüggen, M.; Röttgering, H. J. A. (6 May 2024). "Characterization of the decametre sky at subarcminute resolution".Nature Astronomy.8 (6):786–795.doi:10.1038/s41550-024-02266-z.
^ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.58, definition: radio astronomy service / radio astronomy radiocommunication service
^ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations
Hendrik Christoffel van de Hulst (1945). "Radiostraling uit het wereldruim. II. Herkomst der radiogolven".Nederlands Tijdschrift voor Natuurkunde (in Dutch).11:210–221.
Books
Gerrit VerschuurThe 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,ISBN9780691137797.
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.
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.
