radio astronomy

Dictionary: radio astronomy

n.
The branch of astronomy that deals with the origin and nature of emissions from extraterrestrial sources in the radio wave range of electromagnetic radiation rather than in the visible range.

radio astronomer radio astronomer n.

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Sci-Tech Encyclopedia: Radio astronomy

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The study of celestial objects by measurement and analysis of the electromagnetic radiation they emit in the wavelength range from 1 mm to 30 m (0.04 in. to 100 ft). Radio astronomers study an entire range of celestial objects, including the normal stars, planets, galaxies, and the exotic quasars, pulsars, and x-ray sources.

Radio universe

Since the late 1940s, radio telescopes have been used to map the skies and determine the positions and intensities, or fluxes, of individual sources of radio emission. Such maps have been made with increasing sensitivity and angular resolution; the latter property enables astronomers to determine the position of the radio sources accurately. Knowing the position, astronomers can refer to optical photographs of the sky and establish precisely which object is emitting radio waves. This procedure has led to the identification of radio sources with many bright galaxies and even with the most distant objects in the universe, the quasistellar objects (or quasars). However, nearly one-fifth of all radio sources are unidentified, that is, excellent photographs taken at the radio source positions show no object at all from which the radiation could arise. One concludes from this that these unidentified sources are “normal” galaxies and quasars at such great distances that they cannot be seen optically. See also Radio telescope.

It appears that there are more sources at great distances per unit volume of space than there are nearby, a result that means the universe is expanding, and that those distant sources are, in general, stronger than ones nearby. These two conclusions are fundamentally linked, and together they mean that further work on cosmological problems must await a clearer understanding of the nature and evolution of individual radio sources. See also Cosmology.

Galaxies and radio galaxies

Spiral galaxies, such as the Milky Way Galaxy, are often radio sources, although most are quite weak. Elliptical galaxies are usually not radio sources. However, the few elliptical galaxies that are radio sources are very spectacular ones, being among the most energetic radio objects in the sky. These are known as the radio galaxies.

The radio emission from spiral galaxies is typically confined to a small nuclear region supplemented by much weaker extended emission from the disk of the galaxy. In elliptical radio galaxies, on the other hand, the radio emission emanates from a small region at the center of the galaxy. Radio-emitting material is expelled from the nuclear region in two oppositely directed collimated streams, or jets, that extend out to distances many times the size of the visible galaxy. The enormous radio energy involved, 10¹⁰–10¹² times the solar luminosity, together with the very small size of the nuclear region from which this energy arises, less than 10¹⁴ km, leads to the conclusion that the source of energy is a black hole at the center of the radio galaxy. Such a black hole would be 10⁷–10⁹ times as massive as the Sun. See also Black hole.

The radio emission from galaxies and radio galaxies is generated by the electron synchrotron process, in which relativistic electrons spiral around magnetic field lines and emit a continuous radiation spectrum throughout the band accessible to radio astronomers. See also Galaxy, external.

Solar system astronomy

The Sun is an intense radio source, but only because it is so close to Earth. If it were at the distance of the nearest stars, its radio emission could not be detected. Solar radio emission tends to be intermittent: solar flares that produce cosmic rays and plasma streams that interact with Earth are visible as radio bursts; these are most frequent during the peak of the 11-year solar cycle. See also Cosmic rays; Sun.

Jupiter is a much stronger radio source than had been expected from estimates of its surface temperature by optical astronomers. Most of its radio emission is caused by electron synchrotron emission in its very strong magnetic field. The very-long-wavelength emission of Jupiter is impulsive, and its strength depends upon the position of Io, one of Jupiter's moons. Similar impulsive long-wavelength bursts have also been discovered from Saturn. See also Jupiter; Saturn.

Radio stars

Several nearby, apparently normal stars are detectable at radio wavelengths. Such stars as Algol, β Persei, and AR Lacerta are multiple star systems that have been found to be radio sources. The radio emission from these stars is dominated by radio bursts in which the radio fluxes may increase by a factor of 100 or more. It is clear that the radio bursts are initiated by or are a product of mass exchange processes going on between (at least) two closely bound stars.

An extreme example of radio emission from stars comes from stars that are also x-ray sources. Again, these objects are usually binary systems in which mass exchange plays a deciding role in their continuing evolution, but with x-ray sources one of the component stars appears to be a star that has exhausted its reservoir of nuclear fuel and is collapsing to its final state. See also Binary star; X-ray astronomy.

Supernova remnants

During the formation of a supernova, the atmospheric envelope of a star is ejected and in this process becomes a rapidly expanding cloud of relativistic particles, magnetic field, and filaments of ionized gas. These conditions are precisely those necessary for the generation of radio emission through electron synchrotron radiation, and very intense radio sources indeed exist at the positions of old supernovae chronicled by ancient astronomers. One of the most interesting of the radio sources associated with a supernova remnant is the Crab Nebula. See also Crab Nebula; Supernova.

Hll regions

An Hll region (a region of ionized hydrogen) is a large cloud of interstellar gas that has been ionized and heated by one or more bright, hot stars located within. These nebulae are sources of both continuum and line energy at radio and optical wavelengths. Since cosmic matter consists mostly of hydrogen, the ionized gas consists mainly of protons and electrons that emit continuum energy by bremsstrahlung. See also Nebula.

Hydrogen line

Study of the 21-cm line of neutral atomic hydrogen has been exceptionally rewarding in its contribution to the knowledge of galactic structure and of the physical characteristics of interstellar gas. Line intensity normally reflects the amount of gas in the line of sight; line wavelength and width indicate the line-of-sight velocity of the gas and the state of internal motion, just as with recombination lines in HII regions. If the gas overlies a strong radio source, the gas temperature can be inferred by observing the 21-cm line in absorption.

The structure of the Galaxy has been elucidated by the study of the amount and velocity of the hydrogen within it. A prime advantage of this method is that the very distant gas is just as visible as nearby gas, whereas optical studies of the whole Galaxy are impossible because the very distant stars are made invisible by intervening clouds of dust. The results of these radio studies indicate that the Milky Way Galaxy is a spiral. See also Milky Way Galaxy.

Molecular lines

It has long been believed that simple molecules could not exist in the tenuous gas between stars, because the starlight radiation field would be sufficiently intense to break apart even the simplest molecular species. In spite of these arguments, by 1968 radio astronomers had found rotational transitions of three simple molecules, OH, H₂O, and NH₃. These molecules were found in dark clouds of gas and dust in the interstellar medium. Such dark clouds are believed to be the sites of recent and continuing star formation. Since 1968, more than 87 molecular species have been detected in interstellar space principally by observations of millimeter rotational lines or lines arising from the interaction of the rotation of the molecule's nuclei with the spin of its electrons. More complicated organic molecules containing as many as 12 atoms have been discovered. Observations of these molecular species may make it possible to establish the chemistry and thermodynamics in the interstellar clouds from which, ultimately, stars, planets, and life itself must form. See also Molecular structure and spectra; Radio telescope.

Columbia Encyclopedia: radio astronomy

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radio astronomy, study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally.

Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes, whose use corresponds to that of the optical telescope in observing visible light. In the most common design, a parabolic "dish" replaces the mirror of the reflecting optical telescope. This dish serves to focus the radio waves into a concentrated signal that is then filtered, amplified, and finally analyzed using a computer. The radio signals received from outer space are extremely weak, and long observing times are required to collect a useful amount of energy. Therefore, most radio telescopes are mounted so that they can automatically track a given object as its position changes because of the rotation of the earth.

Galactic Sources of Radio Waves

Naturally occurring radio emission from the sky was accidentally discovered in 1931 by Karl Jansky. An inexplicable source of radio noise was identified in 1940 by Gröte Reber, using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky Way. This radiation is spread over a wide band of radio frequencies and originates in the ionized interstellar gases surrounding hot, bright stars. In these so-called H II regions, free electrons emit radio waves when they are scattered by collisions with the heavier ions. Other sources of radio waves within our galaxy are the remnants of supernovas, or exploding stars. The most famous example of a supernova remnant is the Crab Nebula in Taurus.

Because there are strong magnetic fields (see magnetism) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiation emitted by energetic electrons as they rapidly spiral around the magnetic lines of force, instead of simply being deflected by collisions with ions.

A third source of radio waves within our own galaxy consists of the atoms and molecules in the interstellar matter. This radiation is at discrete frequencies instead of over a broad band, or continuum, of frequencies. The first of these "radio lines" to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms). The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there. Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy. Thus, before the advent of radio astronomy, the spiral structure of the Milky Way had not actually been observed but was only inferred from comparison with the Andromeda Galaxy and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H₂CO), have been discovered in the interstellar medium by means of their radio lines.

Extragalactic Sources of Radio Waves

Radio waves also come from outside the Milky Way. These extragalactic radio sources have great implications for cosmology, the theory of the overall structure of the universe. Spiral galaxies like the Milky Way are only weak sources of radio waves, but certain giant elliptical and irregular galaxies emit more than a million times as much radio energy as ordinary galaxies. Such galaxies are usually marked by dust lanes, which are unusual for galaxies lacking spiral arms. Some of these objects can be detected only by their radio emission, but in other cases the position of the radio source has been determined accurately enough to allow astronomers to identify the radio source with a galaxy visible in an image taken with a large optical telescope.

Other radio sources were optically identified with what at first appeared to be faint blue stars. However, it was discovered that these "stars" had enormous red shifts (shifting of the spectral lines toward the red end of the spectrum) that implied, according to Hubble's law, that they were the most remote objects ever detected and that their intrinsic intensities were about 1000 times greater than an entire galaxy. These extraordinary objects were named quasi-stellar radio sources, which was soon shortened to quasars. Their nature is still not completely understood.

Many thousands of extragalactic radio sources are known. Of those optically identified radio sources, roughly one third are quasars, and the remainder are radio galaxies. In addition to these localized radio sources, there is uniform low-level radio noise from every direction in the sky. This cosmic background radiation is believed to be an indication that the universe began with an explosive big bang rather than having always existed in an unchanging steady state. More recently radio astronomy has discovered pulsars, thought to be rapidly spinning neutron stars that radiate bursts of energy on and off regularly between 1 and 30 times a second.

Bibliography

See J. D. Kraus, Radio Astronomy (1966); G. Verschuur, The Invisible Universe Revealed (1987).

WordNet: radio astronomy

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Note: click on a word meaning below to see its connections and related words.

The noun has one meaning:

Meaning #1: the branch of astronomy that detects and studies the radio waves emitted by celestial bodies

Wikipedia: Radio astronomy

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The Very Large Array, a radio interferometer in New Mexico, USA

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object (the Milky Way) was made in the 1930s, but subsequent advances (especially post-World War II) have identified a number of different sources of radio emission. These include stars and galaxies as well as entirely new classes of objects, such as Radio Galaxies, Pulsars and Masers. The discovery of the Cosmic Microwave Background Radiation was a particularly significant event. Radio astronomy is conducted using large radio antenna referred to as radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis. The latter has allowed radio sources to be imaged with unprecedented angular resolution.

1 History
2 Techniques
- 2.1 Radio telescopes
- 2.2 Radio interferometry
  - 2.2.1 Very Long Baseline Interferometry
3 Astronomical sources
4 See also
5 Notes
6 References
7 Further reading
- 7.1 Journals
- 7.2 Books
8 External links

History

The idea that celestial bodies may be emitting radio waves had been suspected some time before its discovery. In the 1860s James Clerk Maxwell's equations had shown that electromagnetic radiation from stellar sources could exist with any wavelength, not just optical. Several notable scientists and experimenters such as Nikola Tesla, Oliver Lodge, and Max Planck predicted that the sun should be emitting radio waves. Lodge tried to observe solar signals but was unable to detect them due to technical limitations of his apparatus.^[1]

The first identified astronomical radio source was one discovered serendipitously in the early 1930s when Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, was investigating static that interfered with short wave transatlantic voice transmissions. Using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis 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 and teacher Albert Melvin Skellett, who pointed out that signal seemed to be typical of an astronomical source "fixed" in relationship to the stars on the celestial sphere rotating in sync with sidereal time.^[2] By comparing his observations with optical astronomical maps, Jansky eventually concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius ^[3]. He also concluded that since he was unable to detect radio noise from the Sun, the strange radio interference may be generated by interstellar gas and dust in the galaxy (which later proved correct).^[4] He announced his discovery in 1933. Jansky wanted to investigate the radio waves from the Milky Way in further detail but Bell Labs re-assigned him to another project, so he did no further work in the field of astronomy. However, his pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of radio flux density, the Jansky (Jy), after him.

Grote Reber also helped pioneer radio astronomy when he built a large parabolic "dish" radio telescope (9m in diameter) in 1937. He was instrumental in repeating Karl Jansky's pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies ^[5]. On February 27, 1942, J.S. Hey, a British Army research officer, helped progress radio astronomy further, when he discovered that the sun emitted radio waves ^[6]. By the early 1950s Martin Ryle and Antony Hewish at Cambridge University had used the Cambridge Interferometer to map the radio sky, producing the famous 2C and 3C surveys of radio sources.

Techniques

Radio astronomers use different types of techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze what type of emissions it makes. To “image” a region of the sky in more detail, multiple overlapping scans can be recorded and piece together in an image ('mosaicing'). The types of instruments being used depends on the weakness of the signal and the amount of detail needed.

Radio telescopes

Main article: Radio telescope

An optical image of the galaxy M87 (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.

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" 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-born engineer, radiophysicist, and radio astronomer Joseph Lade Pawsey and Ruby Payne-Scott in 1946. Surprisingly 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 WWII 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 their ground breaking paper submitted in mid 1946 and published in 1947. The use of a sea-cliff interferometer had been demonstrated by numerous groups in Australia 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 a 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 min 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 J. Stanley Hey 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, 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. 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 milliarcsecond are possible.

The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes located across the 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 is also a VLBI network, the Long Baseline Array, operating in Australia.

Since its inception, recording data onto hard media has been the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth optical fibre networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was pioneered by the EVN (European VLBI Network) who now perform an increasing number of scientific e-VLBI projects per year.^[7]

Astronomical sources

Main article: Astronomical radio source

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 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.

Radio astronomy is also partly responsible for the idea that dark matter is an important component of our universe; radio measurements of the rotation of galaxies suggest that there is much more mass in galaxies than has been directly observed. 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
Sagittarius A, the galactic center of the Milky Way
Active galactic nuclei and pulsars have jets of charged particles which emit synchrotron radiation
Merging galaxy clusters often show diffuse radio emission[1]
Supernova remnants can also show diffuse radio emission
The Cosmic microwave background is blackbody radio emission
Jupiter

Notes

^ http://www.nrao.edu/whatisra/hist_prehist.shtml NRAO.org, "Pre-History of Radio Astronomy" Compiled by F. Ghigo
^ World of Scientific Discovery on Karl Jansky
^ Karl G. Jansky, "Radio waves from outside the solar system", Nature, 132, p.66. 1933
^ World of Scientific Discovery on Karl Jansky
^ Grote Reber
^ J. S. Hey. The Radio Universe, 2nd Ed., Pergamon Press, Oxford-New York (1975),
^ A technological breakthrough for radio astronomy - Astronomical observations via high-speed data link

References

External links

History of Radio Astronomy, National Radio Astronomy Observatory
The History of Radio Astronomy - Haystack Observatory, MIT
Hanes, Dave, "Physics 014: The Course Notes, Radio Astronomy". Astronomy Group and Department of Physics, Queen's University. 2000-2001.
Reber Radio Telescope - National Park Services
Radio Telescope Developed - a brief history from NASA Goddard Space Flight Center
Society of Amateur Radio Astronomers
Visualization of Radio Telescope Data Using Google Earth
Podcast Discussion on Radio Astronomy and the ALMA observatory

Radio astronomy

Main articles

Astronomical interferometer / History · Very Long Baseline Interferometry · Radio telescope · Radio window · Astronomical radio source

Notable radio telescopes

List of radio telescopes

Single dish	Arecibo Observatory (Puerto Rico) · Effelsberg Telescope (Germany) · Green Bank Telescope (West Virginia, USA) · Lovell Telescope (UK) · Parkes Observatory (Australia)

Interferometers	Combined Array for Research in Millimeter-wave Astronomy (Cedar Flat, CA) · European VLBI Network (Europe) · Giant Metrewave Radio Telescope (India) · MERLIN (UK) · Molonglo Observatory Synthesis Telescope (Canberra, Australia) · One-Mile Telescope (UK) · Very Large Array (New Mexico, USA) · Very Long Baseline Array (USA) · Westerbork Synthesis Radio Telescope (Netherlands)

Proposed/in-progress
telescopes

Five hundred meter Aperture Spherical Telescope (China) · Allen Telescope Array (California, USA) · Atacama Large Millimeter Array (Chile) · LOFAR (Netherlands) · Murchison Widefield Array (Australia) · Square Kilometre Array (TBD)

Observatories

Haystack Observatory (US) · Jodrell Bank Observatory (UK) · Mullard Radio Astronomy Observatory (UK) · National Radio Astronomy Observatory (USA) · Special Astrophysical Observatory of the Russian Academy of Science (Russia)

People

Antony Hewish · Karl Guthe Jansky · Bernard Lovell · Jan Oort · Groete Reber · Martin Ryle

Cosmic microwave background radiation · SETI · Interferometry · Radio propagation · Aperture synthesis · Wow! signal
Other types of astronomical observation: Optical astronomy · Submillimetre astronomy · Infrared astronomy · High-energy astronomy

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radio astronomy

Contents

History

Techniques

Radio telescopes

Radio interferometry

Very Long Baseline Interferometry

Astronomical sources

See also

Notes

References

Further reading

Journals

Books

External links