Radio astronomy

Radio astronomy is the study of celestial phenomena through measurement of radio waves emitted by physical processes occurring in space.

Description

Radio waves have a much greater wavelength than light waves. In order to receive signals with large signal-to-noise ratio, radio astronomy requires a large antenna or an array of smaller antennas working together (for example, the Very Large Array). Some of these radio telescopes use a parabolic dish to reflect the waves to a receiver which detects and amplifies the signal into usable data. This allows astronomers to see a region of the radio sky. If they take multiple scans of overlapping strips of the sky they can piece together an image ('mosaicing'). Radio astronomy is a relatively new field of astronomical research that still has much more to be discovered.

Astronomical sources

A 151 MHz map of the region: 140° to 180° galactic longitude; -5° to 5° galactic latitude from the CLFST at the Mullard Radio Astronomy Observatory. Just like in the visible, at low radio frequencies the sky is dominated by small bright sources, but the sources are typically active galaxies and supernova remnants rather than stars.

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 (see Vera Rubin). 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:

• 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

Observational techniques

Radio telescopes can now be found all over the world (see List of radio telescopes). Most are designed for microwave radiation. Widely separated telescopes are often combined using a technique called interferometry in order to obtain observations with much higher resolution than could be obtained using a single receiver. Initially telescopes within a few kilometers of each other were combined (see, for example, the Mullard Radio Astronomy Observatory), but since the 1970s telescopes from all over the world (and even in Earth orbit) have been combined to perform Very Long Baseline Interferometry.

The United States government has established an institution to conduct radio astronomy research in the US, titled the National Radio Astronomy Observatory (commonly abbreviated as NRAO). This institution controls various radio telescopes around the United States included the world's largest fully mobile radio telescope, the Green Bank Telescope. The United States government has also set aside a national radio quiet zone for radio astronomy research centered around Green Bank, West Virginia. As a result, Green Bank is now the home of NRAO's primary facility.

History

Early work

The history of radio astronomy developed from radio communication research, instead of from following out of conventional astronomical research by the scientific community. Heinrich Hertz developed the parabolic dish that is now used widely in radio astronomy. There were various attempts, such as in England, France, and Germany, to observe signals emitted from the Sun by the 1890s. In 1890, Thomas Edison tried to observe solar signals but failed. In 1894, Oliver Lodge, a radio pioneer, put forth the idea that the Sun indeed did radiate radio waves. Lodge was unable to detect the presence of such emission though, due to technical limitations of his apparatus. It would not be until 1945 that the scientific community en masse would come to realize that the Sun emitted radio waves and begin understanding the origin of radio waves from space. In 1899, the Nikola Tesla developed a sensitive radio telescope., known as the Teslascope, and noted repetitive signals that he deduced must be coming from a non-terrestrial source. The scientific community refused to investigate the data after the announcement of his findings. A 1996 analysis indicated Tesla was observing Jovian plasma torus signals. In 1902, Charles Nordman tried to observe solar emission in the Alps, but failed due to the deficiencies in his apparatus. Also, Henri Deslandres and Lois Decome put forth an idea of a similar experiment to that of Nordman.

Jansky and Reber

After a lull for a few decades, the development of radio astronomy began in full in the early 1930s when Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, was investigating static that interfered with short wave voice transmissions using a turntable mounted 100 ft. by 20 ft. directional antenna working at a frequency of 20.5 MHz (wavelength about 14.6 meters). By rotating the antenna the direction of a received "static" could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system. After sorting out signals from nearby and distant thunderstorms, Jansky continued to investigate a faint steady hiss of unknown origin. Jansky finally determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is typical of an astronomical source "fixed" on the celestial sphere rotating in sync with sidereal time. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.

Grote Reber helped pioneer radio astronomy when he built a large parabolic "dish" radio telescope (9m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies. 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. After World War II, substantial improvements in radio astronomy technology were made by astronomers in Europe, Australia and the United States, and the field of radio astronomy began to blossom.

Modern developments

Modern developments came in 1946 with the introduction of the technique called astronomical interferometry where many radio telescopes are combined in a large array to achieve much higher resolutions. The technology progressed when Martin Ryle inaugurate radio interferometer. Ryle's group in Cambridge obtained a Nobel Prize for this and later aperture synthesis work. The Lloyd's mirror interferometer was also developed independently in 1946 by Joseph Pawsey's group at the CSIR, (later CSIRO) in Sydney.

In the early 1950s the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 3C surveys of radio sources. Two issues, one astronomical and one technical, dominated the research in Cambridge, from the late 1940's for more than thirty years. What was the nature of the discrete radio sources, or `radio stars'? Where were they, what were they, what were their properties, how many were there, how did they work and what was their significance in the Universe? Of parallel importance was the puzzle of how to devise new kinds of radio telescope which would elucidate these astronomical questions.

In 1956, the National Radio Astronomy Observatory was formed to build and operate large radio telescopes. On October 11, 1957, one of the largest radio telescope of the time went into operation at Jodrell Bank for Manchester University. By the 1960s, radio astronomy was recognized as a useful field by mainstream science and accompanied optical astronomy in the observation of the phenomenon of the universe.

See also

General

Very Long Baseline Interferometry, Aperture synthesis

History

History of astronomical interferometry

External articles and references

Further reading

Journals

• Gart Westerhout, The early history of radio astronomy. Ann. New York Acad. Sci. 189 Education in and History of Modern Astronomy (August 1972) 211-218 doi 10.1111/j.1749-6632.1972.tb12724.x
• Hendrik Christoffel van de Hulst, The Origin of Radio Waves From Space.
• Karl G. Jansky, Directional studies of atmospherics at high frequencies. Proc. IRE 20 (1932) 1920
• Karl G. Jansky, Radio waves from outside the solar system. Nature 132 (1933) 6
• Karl G. Jansky, Electrical disturbances apparently of extraterrestrial origin. Proc. IRE 21 (1933) 1387.
• Karl G. Jansky, "Electrical phenomena that apparently are of interstellar origin". Popular Astronomy 41 (1933) 548.
• History of High-Resolution Radio Astronomy. Annual Review of Astronomy and Astrophysics, September 2001

Books

• Woodruff T. Sullivan, III, The early years of radio astronomy. 1984.
• Woodruff T. Sullivan, III, Classics in Radio Astronomy. Reidel Publishing Company, Dordrecht, 1982.
• Kristen Rohlfs, Thomas L Wilson, Tools of Radio Astronomy. Springer 2003. 461 pages. ISBN 3540403876
• Raymond Haynes, Roslynn Haynes, and Richard McGee, Explorers of the Southern Sky: A History of Australian Astronomy. Cambridge University Press 1996. 541 pages. ISBN 0521365759
• Shigeru Nakayama, A Social History of Science and Technology in Contemporary Japan: Transformation Period 1970-1979. Trans Pacific Press 2006. 580 pages. ISBN 1876843462
• David L. Jauncey, Radio Astronomy and Cosmology. Springer 1977. 420 pages. ISBN 9027708398
• Allan A. Needell, Science, Cold War and American State: Lloyd V. Berkner and the Balance of Professional Ideals. Routledge 2000. ISBN 905702621X (ed., see Chapter 10, Expanding Federal Support of Private Research: The Case of Radio Astronomy (Pages 259 - 596))
• Bruno Bertotti, Modern Cosmology in Retrospect. Cambridge University Press 1990. 446 pages. ISBN 0521372135 (ed., see essays by Robert Wilson, Discovery of the cosmic microwave background and Woodruff T. Sullivan, III, The entry of radio astronomy into cosmology: radio stars and 309 Martin Ryle's 2C survey.))
• J. S. Hey, The Evolution of Radio Astronomy. Neale Watson Academic, 1973.
• D. T. Wilkinson and P. J. E. Peebles, Serendipitous Discoveries in Radio Astronomy. National Radio Astronomy Observatory, Green Bank, WV, 1983.
• Joseph Lade Pawsey and Ronald Newbold Bracewell, Radio Astronomy. Clarendon Press, 1955. 361 pages.
• J. C.Kapteyn, P. C. v. d. Kruit, & K. v. Berkel, The legacy of J.C. Kapteyn: studies on Kapteyn and the development of modern astronomy. Astrophysics and space science library, v. 246. Dordrecht: Kluwer Academic Publishers 2000.
• Roger Clifton Jennison, Introduction to Radio Astronomy. 1967. 160 pages.
• Robin Michael Green, Spherical Astronomy. Cambridge University Press 1985. 546 pages. ISBN 0521317797
• Albrecht Krüger, Introduction to Solar Radio Astronomy and Radio Physics. Springer 1979. 356 pages. ISBN 9027709572
• N. Ben-Yehuda, The politics and morality of deviance: moral panics, drug abuse, deviant science, and reversed stigmatization. SUNY series in deviance and social control. Albany: State University of New York Press 1990. (ed., see "Deviant Sciences Early Radio Astronomy", Pages 181 - 220.)

Websites

French History

• The History of the Nancay Radio Observatory - a history of French radio astronomy

History (America, Post 1930s)

• 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

Other history

• Tesla, Nikola, "Talking with Planets". Collier's Weekly, February 19, 1901. (EarlyRadioHistory.us)
• Corum, K. L., J. F. Corum, "Nikola Tesla and the Planetary Radio Signals