Radio astronomy is a subfield of
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
that studies
celestial objects at
radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when
Karl Jansky
Karl Guthe Jansky (October 22, 1905 – February 14, 1950) was an American physicist and radio engineer who in April 1933 first announced his discovery of radio waves emanating from the Milky Way in the constellation Sagittarius. He is considered ...
at
Bell Telephone Laboratories reported radiation coming from the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
. Subsequent observations have identified a number of different sources of radio emission. These include
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s and
galaxies, as well as entirely new classes of objects, such as
radio galaxies,
quasars,
pulsars, and
masers. The discovery of the
cosmic microwave background radiation, regarded as evidence for the
Big Bang theory
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, was made through radio astronomy.
Radio astronomy is conducted using large
radio antennas
In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...
referred to as
radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of
radio interferometry
An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, ...
and
aperture synthesis. The use of interferometry allows radio astronomy to achieve high
angular resolution
Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolut ...
, 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
Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from '' radio astronomy'' in that the latter is a passive o ...
'' in that the former is a passive observation (i.e., receiving only) and the latter an active one (transmitting and receiving).
History
Before 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
James Clerk Maxwell (13 June 1831 – 5 November 1879) was a Scottish mathematician and scientist responsible for the classical theory of electromagnetic radiation, which was the first theory to describe electricity, magnetism and ligh ...
's
equations had shown that
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
is associated with
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describe ...
and
magnetism, and could exist at any
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
. 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.
Karl Jansky
Karl Guthe Jansky (October 22, 1905 – February 14, 1950) was an American physicist and radio engineer who in April 1933 first announced his discovery of radio waves emanating from the Milky Way in the constellation Sagittarius. He is considered ...
made the discovery of the first astronomical radio source
serendipitously in the early 1930s. As a newly hired radio engineer with
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 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 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.
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
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
in the
constellation of
Sagittarius.
Jansky announced his discovery at a meeting in Washington D.C. in April 1933 and the field of radio astronomy was born.
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
The ''Proceedings of the IEEE'' is a monthly peer-reviewed scientific journal published by the Institute of Electrical and Electronics Engineers (IEEE). The journal focuses on electrical engineering and computer science. According to the '' Jour ...
''. 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."
(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
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
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.)
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.
Grote Reber was inspired by Jansky's work, and built a parabolic radio telescope 9m in diameter in his backyard in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies. On February 27, 1942,
James Stanley Hey
James Stanley Hey (3 May 1909 – 27 February 2000) was an English physicist and radio astronomer. With the targeted application of radar technology for astronomical research, he laid the basis for the development of radio astronomy. While work ...
, a
British Army
The British Army is the principal land warfare force of the United Kingdom, a part of the British Armed Forces along with the Royal Navy and the Royal Air Force. , the British Army comprises 79,380 regular full-time personnel, 4,090 Gurkha ...
research officer, made the first detection of radio waves emitted by the Sun. Later that year
George Clark Southworth
George Clark Southworth (August 24, 1890 – July 6, 1972), who published as G. C. Southworth, was a prominent American radio engineer best known for his role in the development of waveguides in the early 1930s.
Biography
Southworth was born in ...
, at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mul ...
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. Several other people independently discovered solar radio waves, including
E. Schott in
Denmark
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and
Elizabeth Alexander working on
Norfolk Island.
At
Cambridge University
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Non literal: From this place, we gain enlightenment and precious knowledge.
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, where ionospheric research had taken place during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
,
J.A. Ratcliffe along with other members of the
Telecommunications Research Establishment that had carried out wartime research into
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
, 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
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
) became capable of handling the computationally intensive
Fourier transform
A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed ...
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 the
Cambridge Interferometer to map the radio sky, producing the
Second (2C) and
Third
Third or 3rd may refer to:
Numbers
* 3rd, the ordinal form of the cardinal number 3
* , a fraction of one third
* 1⁄60 of a ''second'', or 1⁄3600 of a ''minute''
Places
* 3rd Street (disambiguation)
* Third Avenue (disambiguation)
* Hi ...
(3C) Cambridge Catalogues of Radio Sources.
Techniques
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
A mosaic is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and were particularly pop ...
image. The type of instrument used depends on the strength of the signal and the amount of detail needed.
Observations from the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
'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.
Water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
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, in order to minimize the water vapor content in the line of sight. Finally, transmitting devices on Earth may cause
radio-frequency interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
. Because of this, many radio observatories are built at remote places.
Radio telescopes
Radio telescopes may need to be extremely large in order to receive signals with low
signal-to-noise ratio. Also since
angular resolution
Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolut ...
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 their
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 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).
Radio interferometry
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
Ruby Violet Payne-Scott, BSc (Phys) MSc DipEd (Syd) (28 May 1912 – 25 May 1981) was an Australian pioneer in radiophysics and radio astronomy, and was one of two Antipodean women pioneers in radio astronomy and radio physics at the end of the ...
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
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 ground-breaking 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 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
arc minutes
ARC may refer to:
Business
* Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s
* Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services
* ...
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 and
Edward Appleton with
James Stanley Hey
James Stanley Hey (3 May 1909 – 27 February 2000) was an English physicist and radio astronomer. With the targeted application of radar technology for astronomical research, he laid the basis for the development of radio astronomy. While work ...
in the UK).
Modern
radio interferometers consist of widely separated radio telescopes observing the same object that are connected together using
coaxial cable,
waveguide,
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair
Hair is a protein filament that grows ...
, or other type of
transmission line. This not only increases the total signal collected, it can also be used in a process called
aperture synthesis to vastly increase resolution. This technique works by superposing ("
interfering") the signal
waves from the different telescopes on the principle that
waves that coincide with the same
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. In order 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
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
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. Th ...
. Instead of physically connecting the antennas, data received at each antenna is paired with timing information, usually from a local
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
, 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
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
The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources usin ...
(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), and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).
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.
Astronomical sources
Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including
pulsars,
quasars
and
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
planets.
Other sources include:
*
Sun
*
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
*
Sagittarius A, the
Galactic Center of the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
, with one portion
Sagittarius A* thought to be a radio wave emitting
supermassive black hole
*
Active galactic nuclei and
pulsars have jets of charged particles which emit
synchrotron radiation
* Merging
galaxy clusters often show diffuse radio emission
*
Supernova remnants can also show diffuse radio emission;
pulsars are a type of supernova remnant that shows highly synchronous emission.
* The
cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
is
blackbody radio/microwave emission
International regulation
Radio astronomy service (also: ''radio astronomy radiocommunication service'') is, according to Article 1.58 of the
International Telecommunication Union's (ITU)
Radio Regulations
Radio regulation refers to the regulation and licensing of radio in international law, by individual governments, and by municipalities.
International regulation
The International Telecommunication Union (ITU) is a specialized agency of the Unit ...
(RR), defined as "A
radiocommunication service involving the use of radio astronomy". Subject of this radiocommunication service is to receive
radio waves transmitted by
astronomical or celestial objects.
Frequency allocation
The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012).
[''ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations'']
In order 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 with-in the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared.
*primary allocation: is indicated by writing in capital letters (see example below)
*secondary allocation: is indicated by small letters
*exclusive or shared utilization: is within the responsibility of administrations
In line to the appropriate
ITU Region
The International Telecommunication Union (ITU), in its International Radio Regulations, divides the world into three ITU regions for the purposes of managing the global radio spectrum. Each region has its own set of frequency allocations, the ...
the frequency bands are allocated (primary or secondary) to the ''radio astronomy service'' as follows.
See also
*
Atacama Large Millimeter Array
*
Channel 37
*
Gamma-ray astronomy
Gamma-ray astronomy is the astronomical observation of gamma rays,Astronomical literature generally hyphenates "gamma-ray" when used as an adjective, but uses "gamma ray" without a hyphen for the noun. the most energetic form of electromagne ...
*
Infrared astronomy
Infrared astronomy is a sub-discipline of astronomy which specializes in the observation and analysis of astronomical objects using infrared (IR) radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers, and falls in be ...
*
Radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from '' radio astronomy'' in that the latter is a passive o ...
*
Time smearing
*
X-ray astronomy
*
Waves (''Juno'') (radio instrument on the ''Juno'' Jupiter orbiter)
*
Radio Galaxy Zoo
Radio Galaxy Zoo (RGZ) is an internet crowdsourced citizen science project that seeks to locate supermassive black holes in distant galaxies. It is hosted by the web portal Zooniverse. The scientific team want to identify black hole/jet pairs a ...
*
Würzburg radar#Post-war use in astronomy
References
Further reading
; Journals
*
*
; Books
* Bruno Bertotti (ed.), ''Modern Cosmology in Retrospect''. Cambridge University Press 1990.
* James J. Condon, et al.: ''Essential Radio Astronomy.'' Princeton University Press, Princeton 2016, .
* 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.
* 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.
External links
nrao.edu National Radio Astronomy Observatory– a brief history from
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
Goddard Space Flight Center
Society of Amateur Radio Astronomers UnwantedEmissions.com A general reference for radio spectrum allocations, including radio astronomy.
What is Radio Astronomy– Radioastrolab
{{DEFAULTSORT:Radio Astronomy
Observational astronomy
Astronomical imaging
Astronomical sub-disciplines