Naturally occurring radio emission from the sky was accidentally discovered in 1931 by Karl Jansky. An inexplicable source of radio noise was identified in 1940 by Gröte Reber, using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky Way. This radiation is spread over a wide band of radio frequencies and originates in the ionized interstellar gases surrounding hot, bright stars. In these so-called H II regions, free electrons emit radio waves when they are scattered by collisions with the heavier ions. Other sources of radio waves within our galaxy are the remnants of supernovas, or exploding stars. The most famous example of a supernova remnant is the Crab Nebula in Taurus.
Because there are strong magnetic fields (see magnetism) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiation emitted by energetic electrons as they rapidly spiral around the magnetic lines of force, instead of simply being deflected by collisions with ions.
A third source of radio waves within our own galaxy consists of the atoms and molecules in the interstellar matter. This radiation is at discrete frequencies instead of over a broad band, or continuum, of frequencies. The first of these "radio lines" to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms). The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there. Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy. Thus, before the advent of radio astronomy, the spiral structure of the Milky Way had not actually been observed but was only inferred from comparison with the Andromeda Galaxy and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H2CO), have been discovered in the interstellar medium by means of their radio lines.