3C 66A is what is known as a blazar, a class of highly variable quasars known also to be emitters of gamma rays. 3C 66A can vary on timescales of hours and days, a remarkable fact since it is located hundreds of millions of light-years away. Blazars and quasars are not stars, although they may appear to be in all but the most powerful telescopes. They are in reality the bright nuclei of active galaxies. Their luminosity comes from a supermassive black hole located at the center of the galaxy, having many millions or billions of solar masses. Most galaxies probably have such black holes, but when they actively accrete matter onto themselves, the energy released by this accretion can make them very bright. They may also emit relativistic jets of material, and when we look at quasars and blazars, we are seeing the jet itself. In blazars in particular, the jet is oriented right along our line of sight, and we see its most energetic parts. 3C 66A is located in the constellation of Andromeda.