Radio astronomy
Large baseline array of radiotelescopes.
credits: National Radio Astronomy Observatory
While optical astronomy, conducted using the naked eye or telescopes, has been around for, respectively, millennia and centuries, radio astronomy is a rather recent development (i.e., mostly after World War II).
In radio astronomy giant antennas are pointed at the sky to collect radio waves (with wavelengths of the order of meters and centimeters). Radio waves are produced by the acceleration of electrons. In radio towers, electrons oscillate back and force in a wire (antenna) and in doing so they emit electromagnetic waves that have a wavelength of the order of the lenght of the antenna, i.e., meters or centimeters.
Goldstone-Apple Valley Radio Telescope (GAVRT).
Credits: George Stephan
In space, there are similar phenomena that produce radio waves. Charged particles, such as electrons, feel the magnetic force. Electrons in the interstellar medium start spiraling along the lines of the magnetic force. Whenever free electrons (not bound to a nucleus) are constrained to make a circular path, i.e. they hey have to bend around and deviate from a straight path, they emit electromagnetic radiation. Ultra-relativistic electrons (electrons present in certain centers of galaxies that move at a speed close to the speed of light) produce synchrotron radiation, that is radiation focused in one direction and in the cm-meter wavelength range. Radio sources have been found in ours and other galaxies; these sources have been associated with stars, and some outshine the most powerful stars by factors of 1,000 and much more.
A radio telescope has construction similarities with an optical
telescope. It has a dish focusing the electromagnetic radiation to a
pick-up antenna placed in the focus of the dish. The signal is then
processed and visualized in false colors, each color representing a
different intensity of radiation.
Radiotelescopes are sometimes linked together so as to make a larger virtual "dish".
For more details take a look at the following external link about
Radio Astronomy .