Quasars are currently a theory. They are sources of tremendous amounts of energy from the core of galaxies. The central engine of a Quasar is theorized to be a super massive Black hole which is actively consuming material at a tremendous rate. (Note that most galaxies are thought to contain a super massive Black Hole but most do not actively consume large amounts of material, as in the case of a Quasar.) As the Super massive Black Hole consumes material, the objects reach tremendous temperatures (millions of degrees) and extreme speeds before entering the Black Hole and, in the process, give up large quantities of energy in the form of x-rays and visible light, which can be detected here on earth. Quasars are millions of times more massive than our sun and would contain most of a galaxy’s nucleus. At the end of their life they would remain Black Holes.