In 1960, very strong radio emissions were observed by an American astronomer, Allan R. Sandage to be coming from certain localized direction in the sky. When viewed on the photographic plate, they appeared like stars. But they were not stars, as proved by their other characteristics including a large red shift. The accurate position measurement of these star like objects on optical photographs, led to the discovery of a new class of objects in the universe, the quasars (quasi-stellar sources).
They appear star like on the photograph because their angular diameters are less than about 1 second of an arc, which is the resolution limit of ground-based optical telescopes. Since stars also have angular diameters much less than this, they too appear unresolved or point-like on a photograph.
In 1962 a much brighter star like object 3C273 was identified by Maarten Schmidt with the help of a radio telescope in Australia. Its red shift was found to be 0.158. This red shift turned out to be far larger than any other that had been detected for ordinary galaxies. These observations established the existence of quasars beyond doubt.
Quasars are generally much bluer than most of the stars, except white dwarf stars. The blueness of quasars, as an identifying characteristic, led to the discovery that many blue star like objects have a large red shift, and are therefore quasars. Till today scientists have studied more than 1000 quasars but their nature and distance from earth remain a puzzle.
Quasars consist of a massive nucleus with a total size of less than a light year, which is surrounded by an extended halo of gas excited by the energy radiated by the central object. The central object emits radiation over a wide spectral range. Some quasars emit significant amount of energy at radio frequencies ranging from about 30 MHz to 100 GHz. It is believed that the energy emitted by quasars is gravitational and not thermonuclear in origin. More than ergs of energy are released in quasars over their life-time.
Till to day scientists have not been able to measure the exact distance of quasars from the earth. Various similarities of quasars with radio galaxies strongly suggest that quasars are also active nuclei of galaxies might be associated with the birth of some galaxies. Studies have shown that quasars must have been much more common in the universe about many years ago.