The study of celestial bodies by means of the Radio Waves they emit and absorb naturally. These waves are received by specially
constructed Antennas called radio telescopes. The most common design consists of a parabolic dish of open Metal latticework that focuses the radio waves into a concentrated signal that is
filtered, amplified (see Amplifier), and, finally, analyzed using a Computer. Groups of antennas can also be linked by computer to form a single radio
telescope, such as the Very Large Array and Very Long Baseline Array of the
National Radio Astronomy Observatory. The radio signals received from outer space are
extremely weak, and long observing Times are required to collect a useful amount of Energy. There are several basic types of galactic radio emissions. The type first
discovered, accidentally in 1931 by Karl Jansky, is spread over a wide band of
radio Frequencys. It is produced when free Electrons are scattered by collisions with heavier Ions in the ionized interstellar Gases (see States Of Matter) surrounding hot, bright Stars. A second type, called synchrotron Radiation, is emitted by energetic electrons as they rapidly spiral within the strong Magnetic Fields existing in the vicinity of Supernova remnants. A third type, originating in
Interstellar Matter, radiates at discrete frequencies characteristic of the
quantum jumps (see Quantum Theory) made by electrons in the Atoms and Molecules, e.g., atomic Hydrogen and formaldehyde, in the interstellar medium. Pulsars, stellar radio sources
that regularly radiate periodic bursts of energy, were discovered in 1968.
Radio waves also come from outside our Milky Way galaxy. Some extragalactic sources
are detected only by their radio emission, but others have been identified
with optically observed galaxies and quasars. In addition to localized radio
sources, there is uniform low-level radio Noise (see Interference) from every direction in the sky.