Radio astronomy: Wikis

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Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. It differs from radar astronomy in that the latter utilizes artificial, rather than natural, radio sources. 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.

Contents

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).[2] 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 [4]. 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 [5]. 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.

Observations from the earth's surface are limited to those wavelengths that can pass through the atmosphere. Since water vapor is one of the main interfering components of the atmosphere, radio astronomy at higher frequencies must be conducted from very high and dry sites, to minimize the water vapor content in the line of sight.

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Radio telescopes

M87 optical image.jpg
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.[6]

Astronomical sources

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.

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:

See also

  • History of astronomical interferometry

Notes

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

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.
  • History of High-Resolution Radio Astronomy. Annual Review of Astronomy and Astrophysics, September 2001

Books

  • Goss, W. M. and McGee, Richard X., "Under the Radar The First Woman in Radio Astronomy, Ruby Payne-Scott". Springer, 2009.
  • 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. (Also, Watson Pub Intl, 1975 ISBN 0882020307)
  • 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

External links


Wikibooks

Up to date as of January 23, 2010
(Redirected to Radio Astronomy article)

From Wikibooks, the open-content textbooks collection

== What is Radio Astronomy? ==

You can read this screen because your eyes detect light. Light consists of electromagnetic waves. The different colors of light are electromagnetic waves of different lengths.

File:Http://www.nrao.edu/whatisra/images/emschafghfrt.gif

For more info go to: http://imagers.gsfc.nasa.gov/ems/waves3.html

Visible light, however, covers only a small part of the range of wavelengths in which electromagnetic waves can be produced. Radio waves are electromagnetic waves of much greater wavelength than those of light.

For centuries, astronomers learned about the sky by studying the light coming from astronomical objects, first by simply looking at the objects, and later by making photographs. Many astronomical objects emit radio waves, but that fact wasn't discovered until 1932. Since then, astronomers have developed sophisticated systems that allow them to make pictures from the radio waves emitted by astronomical objects.

Contents

Astronomical bodies emit radio waves by one of several processes, including:

Thermal radiation from solid bodies such as the planets Thermal, or "bremsstrahlung," radiation from hot gas in the interstellar medium Synchrotron radiation from relativistic electrons in weak magnetic fields Spectral line radiation from atomic and molecular transitions that occur in the interstellar medium or in the gaseous envelopes around stars Pulsed radiation resulting from the rapid rotation of neutron stars surrounded by an intense magnetic field and energetic electrons Click here for more information on how radio waves are produced.


Solar flares and sunspots are strong sources of radio emission. Their study has led to increased understanding of the complex phenomena near the surface of the Sun (image at left), and provides advanced warning of dangerous solar flares that can interrupt radio communications on the Earth and endanger sensitive equipment in satellites and even the health of astronauts. Radio telescopes are used to measure the surface temperatures of all the planets in our solar system and as well as some of the moons of Jupiter and Saturn. Radio observations have revealed the existence of intense Van Allen Belts surrounding Jupiter (image at right), powerful radio storms in the Jovian atmosphere and an internal heating source deep within the interiors of Jupiter, Saturn, Uranus, and Neptune.

Broadband continuum emission throughout the radio-frequency spectrum is observed from a variety of stars (especially binary, X-ray, and other active stars), from supernova remnants, and from magnetic fields and relativistic electrons in the interstellar medium.


Radio waves penetrate much of the gas and dust in space as well as the clouds of planetary atmospheres and pass through the terrestrial atmosphere with little distortion. Radio astronomers can therefore obtain a much clearer picture of stars and galaxies than is possible by means of optical observation.

Utilizing radio telescopes equipped with sensitive spectrometers, radio astronomers have discovered more than 100 separate molecules, including familiar chemical compounds like water vapor, formaldehyde, ammonia, methanol, ethanol, and carbon dioxide. The important spectral line of atomic hydrogen at 1421.405 MHz (21 centimeters) is used to determine the motions of hydrogen clouds in the Milky Way Galaxy and other galaxies. This is done by measuring the change in the wavelength of the observed lines arising from Doppler shift. It has been established from such measurements that the rotational velocities of the hydrogen clouds vary with distance from the galactic center. The mass of a spiral galaxy can, in turn, be estimated using this velocity data (Click on the picture of the spiral galaxy M33 at right for more details). In this way radio telescope gave some of the first hints for the presence of so called "dark matter" in where the amount of starlight is insufficient to account for the large mass inferred from the rapid rotation curves.

A number of celestial objects emit more strongly at radio wavelengths than at those of light, so radio astronomy has produced many surprises in the last half-century. By studying the sky with both radio and optical telescopes, astronomers can gain much more complete understanding of the processes at work in the universe.

How did radio astronomy get started?

The first radio astronomy observations were made in 1932 by the Bell Labs physicist Karl Jansky who detected cosmic radio noise from the center of the Milky Way Galaxy while investigating radio disturbances interfering with transoceanic telephone service. A few years later, the young radio engineer and amateur radio operator, Grote Reber (W8GFZ) built the first radio telescope (image at left) at his home in Wheaton, Illinois, and found that the radio radiation came from all along the plane of the Milky Way and from the Sun.

During the 1940s and 1950s, Australian and British radio scientists were able to locate a number of discrete sources of celestial radio emission. They associated these sources with old supernovae and active galaxies, which later became to be known as radio galaxies. The construction of ever larger antenna systems and radio interferometers (see radio telescopes), improved radio receivers and data-processing methods have allowed radio astronomers to study fainter radio sources with increased resolution and image quality.


Radio galaxies are surrounded by huge clouds of relativistic electrons that move in weak magnetic fields to produce synchrotron radiation, which can be observed throughout the radio spectrum. The electrons are thought to be accelerated by material falling into a massive black hole at the center of the galaxy and are then propelled out along a thin jet to form the radio emitting clouds that are found up to millions of light-years from the parent galaxy. The study of radio galaxies led astronomer Maarten Schmidt to discover quasars in 1963. Quasars are found in the central regions of galaxies and may shine with the luminosity of a hundred ordinary galaxies. Like radio galaxies, they are thought to be powered by a super-massive black hole up to a thousand-million times more massive than the Sun, but contained within a volume less than the size of the solar system. Although, radio galaxies and quasars are powerful sources of radio emission, they are located at great distances from the Earth, and so the signals that reach the Earth are very weak.

Measurements made in 1965 by Arno Penzias and Robert W. Wilson using an experimental communications antenna at 7 centimeter wavelength located at Bell Telephone Laboratories detected the existence of a microwave cosmic background radiation at a temperature of 3 K. This radiation, which comes from all parts of the sky, is thought to be the remaining radiation from the hot big bang, the primeval explosion from which the universe presumably originated some 15 billion years ago. Satellite and ground-based radio telescopes are used to measure the very small deviations from isotropy of the cosmic microwave background. This work has lead to refined determination of the size and geometry of the Universe.


Radio observations of quasars led to the discovery of pulsars by Jocylen Bell and Tony Hewish in Cambridge, England in 1967. Pulsars are neutron stars that have lost all their electrons and have shrunk to a diameter of a few kilometers following the explosion of the parent star in a supernova. Because they have retained the angular moment of the much larger original star, neutron stars spin very rapidly, up to 641 times per second, and contain magnetic fields as strong as a thousand-billion Gauss or more. (The Earth's magnetic field is on the order of half a Gauss.) The radio emission from pulsars is concentrated along a thin cone, which produces a series of pulses corresponding to the rotation of the neutron star, much like to beacon from a rotating lighthouse lamp.

What is the National Radio Astronomy Observatory?

The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation, operated by Associated Universities, Inc., a nonprofit research organization. The NRAO provides state-of-the-art radio telescope facilities for use by the scientific community. We conceive, design, build, operate and maintain radio telescopes used by scientists from around the world. Scientists use our facilities to study virtually all types of astronomical objects known, from planets and comets in our own Solar System to galaxies and quasars at the edge of the observable universe.

The headquarters of NRAO is in Charlottesville, Virginia, and the Observatory operates major radio telescope facilities in Socorro, New Mexico and Green Bank, West Virginia.

What do Radio Astronomers listen to?

Actually, nothing! While everyday experience and Hollywood movies make people think of sounds when they see the words "radio telescope," radio astronomers do not actually listen to noises.

First, sound and radio waves are different phenomena. Sound consists of pressure variations in matter, such as air or water. Sound will not travel through a vacuum. Radio waves, like visible light, infrared, ultraviolet, X-rays and gamma rays, are electromagnetic waves that do travel through a vacuum. When you turn on a radio you hear sounds because the transmitter at the radio station has converted the sound waves into electromagnetic waves, which are then encoded onto an electromagnetic wave in the radio frequency range (generally in the range of 500-1600 kHz for AM stations, or 86-107 MHz for FM stations). Radio electromagnetic waves are used because they can travel very large distances through the atmosphere without being greatly attenuated due to scattering or absorption. Your radio receives the radio waves, decodes this information, and uses a speaker to change it back into a sound wave. An animated gif of this process can be found here.

Radio telescopes often produce images of celestial bodies. Just as photographic film records the different amount of light coming from different parts of the scene viewed by a camera's lens, our radio telescope systems record the different amounts of radio emission coming from the area of the sky we observe. After computer processing of this information, astronomers can make a picture.

No scientific knowledge would be gained by converting the radio waves received by our radio telescopes into audible sound. If one were to do this, the sound would be "white noise," random hiss such as that you hear when you tune your FM radio between stations.

For more information see: http://www.nrao.edu


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