radio astronomy

Study of radio waves emitted naturally by objects in space, by means of a radio telescope. Radio emission comes from hot gases (thermal radiation); electrons spiralling in magnetic fields (synchrotron radiation); and specific wavelengths (lines) emitted by atoms and molecules in space, such as the 21-cm/8.3-in line emitted by hydrogen gas.

Radio astronomy began in 1932 when US astronomer Karl Jansky detected radio waves from the centre of our Galaxy, but the subject did not develop until after World War II. Radio astronomy has greatly improved our understanding of the evolution of stars, the structure of galaxies, and the origin of the universe. Astronomers have mapped the spiral structure of the Milky Way from the radio waves given out by interstellar gas, and they have detected many individual radio sources within our Galaxy and beyond.

Among radio sources in our Galaxy are the remains of supernova explosions, such as the Crab nebula and pulsars. Short-wavelength radio waves have been detected from complex molecules in dense clouds of gas where stars are forming. Searches have been undertaken for signals from other civilizations in the Galaxy, so far without success.

Strong sources of radio waves beyond our Galaxy include radio galaxies and quasars. Their existence far off in the universe demonstrates how the universe has evolved with time. Radio astronomers have also detected weak cosmic background radiation, thought to be from the Big Bang explosion that marked the birth of the universe.