Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object (the Milky Way) was made in the 1930's, but subsequent advances (especially post-World War II) have identified a number of different sources of radio emission. These include stars and galaxies as well as entirely new classes of objects, such as Radio Galaxies, Pulsars and Masers. The discovery of the Cosmic Microwave Background Radiation was a particularly significant event. Radio astronomy is conducted either with a single telescope or with multiple linked telescopes utilising the techniques of radio interferometry and aperture synthesis. The latter has allowed radio sources to be imaged with unprecedented angular resolution.