Quasars

Quasars

Quasi-stellar radio sources (quasars) are the most distant cosmic objects observed by astronomers. Although not visible to the naked eye, quasars are also among the most energetic of cosmic phenomena. Even though some quasars may be physically smaller in size than our own solar system , some quasars are calculated to be brighter than hundreds of galaxies combined. Quasars and active galaxies appear to be related phenomena, each associated with massive rotating black holes in their central region. As a type of active galaxy, the enormous energy output of quasars can be explained using the theory of general relativity.

The great distance of quasars means that the light observed coming from them was produced when the universe was very young. Because of the finite speed of light, large cosmic distances translate to looking back in time. The observation of quasars at large distances and of their nearby scarcity argues that quasars were much more common in the early universe. Correspondingly, quasars may also represent the earliest stages of galactic evolution . This change in the universe over time (e.g., specifically the rate of quasar formation) contradicted steady-state cosmological models that relied on a universe that was the same in all directions (when averaged

over a large span of space ) and at all times. Along with the discovery of ubiquitous cosmic background radiation, the discovery of quasars tilted the cosmological argument in favor of Big Bang based cosmological models.

In 1932, American engineer Karl Jansky (1905–1945) discovered the existence of radio waves emanating from beyond the solar system. By the mid-1950s, an increasing number of astronomers using radio telescopes sought explanations for mysterious radio emissions from optically dim stellar sources.

In 1962, British radio astronomer Cyril Hazard used the moon as an occultive shield to discover strong radio emissions traceable to the constellation Virgo. Optical telescopes pinpointed a faint star-like object (subsequently designated quasar 3C273—3rd Cambridge Catalog, 273rd radio source) as the source of the emissions. Of greater interest was an unusual emission spectrum found associated with 3C273. In 1963, American astronomer Marten Schmidt explained the abnormal spectrum from 3C273 as evidence of a highly redshifted spectrum. Redshift describes the Doppler-like shift of spectral emission lines toward longer (hence, redder) wavelengths in objects moving away from an observer. Observers measure the light coming from objects moving away from them as redshifted (i.e., at longer wavelengths and at a lower frequency when the light was emitted). Conversely, observers measure the light coming from objects moving toward them as blueshifted (i.e., at shorter wavelengths and at a higher frequency when the light was emitted). Most importantly, the determination of the amount of an object's redshift allows the calculation of a recession velocity. Moreover, because the recession rate increases with distance, the recession velocity is a function (known as the Hubble relation) of the distance to the receding object. After 3C273, many other quasars were discovered with similarly redshifted spectra.

Schmidt's calculation of the redshift of the 3C273 spectrum meant that 3C273 was approximately three billion light-years away from Earth. It became immediately apparent that, if 3C273 was so distant, it had to be many thousands of times more luminous than a normal galaxy for the light to appear as bright as it did from such a great distance. Refined calculations involving the luminosity of 3C273 indicate that, although dim to optical astronomers, the quasar is actually five trillion times as bright as the Sun . The high redshift of 3C273 also implied a great velocity of recession measuring one-tenth the speed of light.

Astronomers now assert that quasars represent a class of galaxies with extremely energetic centers. Large radio emissions seem most likely associated with massive black holes with great amounts of matter available to enter the accretion disk. In fact, prior to more direct observations late in the twentieth century, the discovery of quasars provided at least tacit proof of the existence of black holes. Black holes form around a singularity (the remnant of a collapsed massive star) with a gravitational field so intense that not even light can escape. Located outside the black hole is the accretion disk, an area of intense radiation emitted as matter heats and accelerates toward the black hole's event horizon (the boundary past which nothing can escape). Further, as electrons in the accretion disk are accelerated to near light speed, they are influenced by a strong magnetic field to emit quasar-like radio waves in a process termed synchrotron radiation. Electromagnetic waves similar to the electromagnetic waves emanating from quasars are observed on Earth when physicists pass high-energy electrons through synchrotron particle accelerators. Studies of Quasar 3C273 and other quasars identified jets of radiation blasting tens of thousands of light-years into space.

In addition to radio and visible light emissions, some quasars emit light in other regions of the electromagnetic spectrum including ultraviolet, infrared, x ray, and gamma-ray regions. In 1979, an x-ray quasar was found to have a red-shift of 3.2, indicating a recession velocity equaling 97% the speed of light.

Not all quasars or active galaxies are alike. Although they seem optically similar to energetic quasars, at least 90% of active galaxies appear to be radio quiet. Accordingly, Seyfert galaxies or quasi-stellar objects (QSO) may be radio silent or emit electromagnetic radiation at greatly reduced levels. More than 1,500 quasars have now been identified as distant QSO. One hypothesis accounts for these quiet quasars by linking them to smaller black holes, or to black holes in regions of space with less matter available for consumption.

The limitations of ground-based telescopes and the need to study quasars was officially cited as one of the principal reasons to build the Hubble Space Telescope launched by the United States in 1990. In addition to direct studies of quasars, astronomers use quasars as an electromagnetic backdrop that can be used to study the primitive gas clouds found in the early universe.

See also Big bang theory; Stellar life cycle