Longer-wavelength radio radiation reveals the processes occurring in gaseous nebulas, pulsars (spinning neutron stars), supernova remnants, and the active cores of distant galaxies. A radio telescope consists of an antenna, or antenna-array, connected to a receiver.
The antenna may be as simple as a dipoletype antenna, which looks something like a TV antenna. More often, though, it is a parabolic reflector dish, or an array of dishes. The radio waves are focused and collected by a secondary antenna, known as a feed. They are then transferred to a receiver for amplification and analysis. As with an optical telescope, the sensitivity of the radio telescope increases with the surface area of the antenna.
When two or more radio telescopes are linked so that their signals are fed to a common receiver, they are jointly termed an interferometer. The greater the distance between the radio telescopes, called the baseline, the finer the detail that can be resolved. The Very Large Array, near Socorro in New Mexico, has baselines up to 22 miles (36 km) long, while the Very Long Baseline Array consists of 10 dishes spread over about 5,000 miles (8,000 km)—from Hawaii across the USA to the Virgin Islands in the Caribbean Sea.