Mei 27, 2013
A quasi-stellar radio source ("quasar", /ˈkweɪzɑr/) is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and 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 the nature of these objects was controversial until as recently as the early 1980s, there is now a scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding its central supermassive black hole. Its size is 10–10,000 times the Schwarzschild radius of the black hole.
Quasars have a very high redshift. They seem to inhabit the centers of active young galaxies and can emit up to a thousand times the energy output of our entire galaxy. According to Hubble’s law the redshift shows that quasars are very distant and, because of their distance, much older than our universe. The most luminous quasars radiate in excess of the output of one trillion Suns. This radiation is emitted across the spectrum from X-rays to the far-infrared, but have a peak in the ultraviolet-optical bands. Some quasars are strong sources of radio emission and gamma-rays. In early optical images, quasars looked like single points of light. Infrared telescopes and the Hubble Space Telescope have identified the galaxies surrounding the quasars. These galaxies are normally too dim to be seen against the glare of the quasar otherwise.
Quasars are believed to be powered by the accretion of material into centralized supermassive black holes. This makes them luminous versions of active galaxies. Light cannot escape the super massive black holes, so the escaping energy is generated by gravitational stresses and intense friction outside of the event horizon.
