Frequently Asked Questions about Radio Astronomy

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.


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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.

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

  1. Thermal radiation from solid bodies such as the planets
  2. Thermal, or "bremsstrahlung," radiation from hot gas in the interstellar medium
  3. Synchrotron radiation from relativistic electrons in weak magnetic fields
  4. Spectral line radiation from atomic and molecular transitions that occur in the interstellar medium or in the gaseous envelopes around stars
  5. 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 are you listening 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.

What have you discovered lately?

A lot! For some of the highlights, look at NRAO's press releases about recent research results. Every year, hundreds of scientists use NRAO's radio telescopes, and they report their results in numerous papers in scientific journals. Almost any introductory astronomy textbook will contain images and tell of research results from NRAO's various radio telescopes.

GBT? VLA? VLBA? How did you come up with these names?

The GBT stands for Green Bank Telescope. Its full name is the Robert C. Byrd Green Bank Telescope, which honors Senator Robert C. Byrd for enabling NRAO to build the telescope.

The VLA got its name because it is an array of radio telescopes and it is very large. In its very early conceptual and planning stages, "Very Large Array" was a working title, probably not intended to be the final name for the facility. However, after a few years, the name stuck.

At the VLA, all of its 27 dish antennas work together as a single instrument. The signals from all antennas are brought together in real time through a microwave communication system that uses buried waveguide. As radio astronomers sought to increase their resolving power, or ability to see fine detail, by separating their antennas by even greater distances, it became impractical to bring the signals together in real time. Instead, tape recorders and precise atomic clocks were installed at each antenna, and the signals are combined after the observation is completed. This technique is called Very Long Baseline Interferometry (VLBI). When astronomers wanted to build a continent-wide radio telescope system to implement this technique, the name Very Long Baseline Array (VLBA) was the natural working title. Again, it stuck.

Is NRAO doing SETI work?

Radio observations as part of a Search for Extraterrestrial Intelligence (SETI) have been done by different groups of researchers for a number of years, and sometimes on NRAO telescopes. In the 1990s the SETI Institute, a privately-funded organization, acquired observing time on the 140-foot radio telescope in Green Bank. SETI research had its beginning at NRAO in Green Bank. Read more here.

Can I build a radio telescope at home?

Many electronic experimenters have built their own radio telescopes. In fact, the world's second radio telescope was built by an amateur radio operator, Grote Reber, in 1937. Amateurs use a variety of equipment, sometimes modified satellite receivers and dishes, to build their radio telescopes. For more detailed information about amateur radio telescopes, contact the Society of Amateur Radio Astronomers.

Amateur radio operators pursue a number of activities that are somewhat related to radio astronomy, including communicating by bouncing radio signals off the Moon and the ionized trails of meteors in the Earth's atmosphere. There also are a number of amateur radio satellites in orbit. Both the Space Shuttle and the Mir space station carry amateur radio equipment that frequently is used by the astronauts to communicate to "ham" operators and classrooms around the world. For general information about amateur radio, including how to obtain your own license, contact the American Radio Relay League.

I think I'd like to become an astronomer. What training do I need?

The typical training for a research astronomer includes earning a Bachelor's degree in Physics, Astronomy or Mathematics, followed by graduate school and a Ph.D. in Astronomy or Astrophysics. The American Astronomical Society, the professional association for astronomers in North America, offers a free brochure on careers in astronomy. High school students interested in astronomy should take as many courses in science and mathematics as possible in preparation for college.

Can I visit NRAO?

Yes. At Green Bank, West Virginia, and at the Very Large Array, near Socorro, New Mexico, we have Visitor Centers open to the public.

Can I get images or data from NRAO's radio telescopes?

Yes, there are some images on the NRAO Web site that you can browse. In addition, images and data from two surveys of the sky made by the VLA are available on the Internet. Check out the NRAO VLA Sky Survey and the FIRST Survey Web pages. You also may download software for viewing these image files.

I saw Contact. Was it filmed at the VLA? Where is the canyon I saw in that movie?

Many scenes in Contact were filmed at the VLA in September of 1996. About 200 filmmakers, including stars Jodie Foster, Tom Skerritt and James Woods, came to the VLA for the filming. The beautiful canyon seen near the VLA in the movie, however, actually is Canyon de Chelly in Arizona, "moved" to New Mexico by the magic of Hollywood special effects!

Where can I find more information about astronomy?

The World Wide Web is a gold mine of information on astronomy and space science. This includes everything from on-line astronomy courses to archives of thousands of astronomical images. An excellent starting point is the AstroWeb site.

Other sources of astronomical information include your public or school library and monthly magazines such as Sky & Telescope and Astronomy. There may be an amateur astronomy club in your community, and if there is, it is a good place to meet others who are interested in astronomy and to join activities such as observing with telescopes and hearing lectures on astronomical topics. Your community also may have a planetarium, public observatory or science museum that can provide information. The Web sites of the two magazines listed above have listings of clubs, observatories, planetaria and astronomical events for communities throughout the United States.

Modified on Friday, 26-Sep-2008 11:56:49 EDT