Radio Astronomy
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== 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.
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.
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[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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
