We are able to see the stars in the sky because they send out light which reaches us. Light is a form of radiation.
Besides light, stars send out other forms of radiation, and one of these is radio waves. Some of these waves can be detected by special radio receivers here on earth. The radio receivers collect and magnify the radio waves, just as ordinary telescopes collect and magnify the picture the light waves give. That’s why these radio receivers are called radio telescopes, and their use to study the stars is called radio astronomy
The length of radio waves from outer space is very short compared with the wavelengths used in radio broadcasting and television. So radio astronomers must build special radio sets and antennae to be able to receive them.
Radio telescopes consist of two parts—an antenna and a radio receiver. The antenna is often a huge metal dish, mounted so it can be pointed to any part of the sky. The radio waves collected by the antenna are often very weak, so the signals have to be amplified.
Usually radio astronomers make a record of the radio waves on paper. A pen recorder writes down the signals in the form of a wavy line on a strip of paper, so the astronomers have a permanent record of their observations.
Radio telescopes can operate in all kinds of weather, unlike ordinary telescopes. Radio telescopes can also be built almost any place that is convement, and need be put on high ground or on mountains.