An compact, extragalactic object which is highly luminous and looks like a star. Their redshifts can be large and their brightness varies. Quasars have an intrinsic luminosity which can reach some 100 times that of bright galaxies. They are thought to be active galactic nuclei with a size a little larger than the solar system. The first quasar to be identified as such, in 1963, was the radio source 3C 273 at a redshift of 0.158. With its 13th magnitude, it is the optically brightest quasar as observed from Earth. Some quasars are strong radio sources.