Searches have been undertaken for signals from other civilizations in the Galaxy , so far without success. Extra-Galactic 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 background radiation from the Big Bang explosion that marked the birth of the universe. Quasars are distant galaxies that release enormous amounts of energy primarily in the form of radio waves.
Pulsars are neutron stars that emit highly rhythmic radio pulses.
Several physical mechanisms are recognized that produce the observed radio emission. At short wavelengths the emission is dominated by thermal sources. At short radio wavelengths thermal emission sources dominate the sky, and at long radio wavelengths the sky is dominated by non-thermal emission sources. Because of the random motions of electrons, all bodies emit thermal, or heat, radiation characteristic of their temperature. Careful measurements of the intensity and spectrum of emissions are used to calculate the temperature of distant celestial bodies, such as the planets in the Earth's solar system, as well as of hot clouds of ionized gas located throughout the Galaxy.