ABSTRACT
The development of radio astronomy in the mid-twentieth century opened up a new window on the universe. Prior to this development, astronomical observations were limited to the narrow range of visible wavelengths and consequently a limited range of astronomical phenomena. e sky at radio wavelengths is much dierent from the sky at visible wavelengths. Objects bright at visible wavelengths, such as stars, are not what dominate the emission in the radio sky. At radio wavelengths, we can detect the thermal continuum and spectral-line emission from objects too cold to produce visible light, permitting studies of the cold interstellar medium of our Galaxy and others, as well as the cosmic microwave background, the relic radiation from the early universe. A form of nonthermal radiation called synchrotron emission produces prominent radio emission and is seen from a host of interesting astronomical sources such as supernova remnants and quasars. Two of the brightest sources in the radio sky, Cassiopeia A and Cygnus A, are synchrotron-emitting sources and are relatively faint at visible wavelengths; thus, observations at radio wavelengths complement those at optical observations.
