Quasar

A quasar (from quasi-stellar radio object) is an astronomical object that looks like a star in optical telescopes (i.e. it is a point source),

but has a very high redshift. According to the Hubble law, their redshift indicates that they are very distant from Earth and that we observe them as they were several billions of years ago. Since we can see them despite their distance, they must emit more energy than dozens of normal galaxies. Some quasars display rapid changes in luminosity, which implies that they are small (an object cannot change faster than the time it takes light to travel from one end to the other). The first quasar was discovered in 1961 by Allen Rex Sandage at Mt Palomar, California. Nowadays they are usually called QSOs (quasi-stellar objects) since it has been discovered that most do not emit much in radio.

The nature of quasars is not well understood. They appear to be a particular class of active galaxy nucleus, and a general concensus has emerged that what differentiates them from the other classes (blasars and Seyfer galaxies) is what angle we are viewing them at. What drives the immense energy generation, though, is not so clear. Part of the problem is we do not know what galaxies hosting quasars would look like in the present. The most widely accepted possibility is they are fueled by supermassive black holes.