An artist rendition of an accretion disk in ULAS J1120+0641.
A quasar (also known as a QSO or quasi-stellar object) is an extremely luminous active galactic nucleus (AGN). It has been theorized that most large galaxies contain a supermassive black hole in their center, with mass ranging from millions to billions of times that of the Sun. In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk. As gas falls toward the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous: the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way.
Understanding[]
it is now known that quasars are distant but extremely luminous objects, so any light that reaches Earth is redshifted, due to the metric expansion of space.
Quasars inhabit the center of active galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200-400 billion stars. This radiation is emitted across the electromagnetic spectrum, almost uniformly, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays. With high-resolution imaging from ground-based telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been detected in some cases. These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars, with the exception of 3C 273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes.
Properties[]
Over 200,000 quasars are known. Most of these are from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.056 and 7.085. Using Hubble's law, it can be estimated that these quasars are approximately 600 million to 28.85 billion light years away from Earth (in terms of comoving distance). Because of the great distances to the farthest quasars and the finite velocity of light, they and their surrounding space appear as they existed in the very early universe.
The power of quasars originates from supermassive black holes that are believed to exist at the core of most galaxies. The Doppler shifts of stars near the cores of galaxies indicate that they are rotating around tremendous masses with very steep gravity gradients, suggesting black holes.
Although quasars appear faint when they are viewed from Earth, they are visible from extreme distances, being the most luminous objects in the known universe. The brightest quasar in the sky is 3C 273, which has an average apparent magnitude of 12.8, which is bright enough to be seen through a medium-size amateur telescope. From a distance of about 33 light-years, 3C 273 would appear as bright as the Sun. This quasar's luminosity is, therefor, about 4 trillion (4 x 1012) times that of the Sun, or about 100 times that of the total light of giant galaxies like the Milky Way. This assumes the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet. In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward the Earth, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at the Earth. Such quasars are called blazars.
A hyperluminous quasar, APM 08279+5255, was detected in 1998. It has an absolute magnitude of -32.2. High resolution imaging with the Hubble Space Telescope and the 10 m Keck Telescope revealed that this system is gravitationally lensed. A study of the gravitational lensing of this system suggests that the light emitted has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.
Quasars were far more common in the early universe than they are today.