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Radio astronomy

Subfield of astronomy that studies celestial objects at radio frequencies

  • 11 min read
  • 4 chapters
The Very Large Array near Socorro, New Mexico, United States.
user:Hajor · CC BY-SA 3.0

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Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used alone, or with multiple linked telescopes utilizing the techniques of 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

Before 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 equations showed that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. Over the next few decades, several attempts were subsequently made to detect radio emissions from the Sun, but without success due to technical limitations of the instruments.

In 1896, an experiment by German astrophysicists Johannes Wilsing and Julius Scheiner attempted to detect solar radio emissions.

Between 1897 and 1900, a centimeter wave radiation apparatus set up by Oliver Lodge also attempted to detect solar radio emissions.

In 1902, upon the discovery of the radio-reflecting ionosphere, physicists concluded that the layer would bounce any solar or other astronomical radio transmissions back into space, making them undetectable.

In the early 1930s, Karl Jansky discovered the first astronomical radio source serendipitously. 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 in the constellation of Sagittarius.

In April 1933, Jansky announced his discovery at a meeting in Washington, D.C., and the field of radio astronomy was born.

In October 1933, Jansky's discovery was published in a journal article entitled "Electrical disturbances apparently of extraterrestrial origin" in the Proceedings of the Institute of Radio Engineers.

Jansky concluded that since the Sun (and therefore other stars) were not large emitters of radio noise, the strange radio interference might be generated by interstellar gas and dust in the galaxy, in particular, by "thermal agitation of charged particles".

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.

In 1937, radio amateur Grote Reber, inspired by Jansky's work, built a parabolic radio telescope 9 meters (30 ft) in diameter in his backyard in Wheaton, Illinois. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies.

On 27 February 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year, George Clark Southworth, 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. Several other people independently discovered solar radio waves, including E. Schott in Denmark and Elizabeth Alexander working on Norfolk Island.

In the 1950s, Jansky's peak radio source, one of the brightest in the sky, was designated Sagittarius A 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.

In the 1950s, 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.

During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' (1 mi (1.6 km)) and later a '5 km' (5 km (3.1 mi)) 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 (3C) Cambridge Catalogues of Radio Sources.

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