A Quasar (contraction of QUASi-stellAR radio source) is an extremely bright and distant active galactic nucleus. They were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light that were point-like, similar to stars, rather than extended sources similar to galaxies. While there was initially some controversy over the nature of these objects, there is now a scientific consensus that a quasar is a compact halo of matter surrounding the central supermassive black hole of a young galaxy.