Quasar

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Quasar

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.

Although 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 and tilted the cosmological argument in favor of Big Bang based cosmological models.

The discovery of quasars

In 1932, American engineer Karl Janskey (1905–1945) discovered 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. Allan Sandage first reported several faint starlike objects as optical counterparts to radio sources in 1960. In 1963, American astronomer Marten Schmidt explained the abnormal spectrum from 3C273 as evidence of a highly redshifted spectrum. Red-shift 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.

Modern observation and interpretation of quasars

Astronomers now assert that quasars represent a class of galaxies with extremely energetic centers. Large radio emissions seem most likely associated with large black holes with large 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 stars) 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 . 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 electro-magnetic spectrum including ultraviolet, infrared, x ray, and gamma-ray regions. In 1979, an x-ray quasar was found to have a redshift 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 1500 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.

Observations have shown that quasars are extra-galactic, but many questions about their distances and nature stirred great interest among astronomers in the latter half of the twentieth century. Assuming that modern astronomical theory holds true for these bodies, quasars are the most distant, and from their brightnesses, also the most luminous objects known. The most luminous ones are thousands of times more energetic than larger, luminous galaxies such as the Milky Way and Messier 31. In spite of this, quasar brightnesses are quite variable, changing in times of hours and sometimes doubling their luminosities in as short a timespan as a week. This means that the main source or sources of their luminosity must be situated in a volume of space not much larger than a solar system, which light can cross in 12 hours. This is an enormous power source (luminosity) to fit into such a relatively small volume.

Astrophysics supplies two possible sources for such enormous energy from such small regions. They are:

Matter falling into an enormous black hole with a mass on the order of 1010 solar masses or more, where much of the gravitational energy released during the matter's infall towards the black hole is converted into light and other radiation in an accretion disk of matter surrounding the black hole.
The annihilation of ordinary matter (electrons, protons, etc.) and antimatter (positrons, etc.) as they collide at enormous rates.

The first of those is favored today by most astronomers, because there is independent evidence for the existence of such massive black holes in galaxies. The second possibility would produce enormous intensities of gamma radiation at definite energies (wavelengths); these have not been observed by the Gamma Ray Observatory (GRO) spacecraft that the NASA launched into orbit around the Earth in 1991.

Blazars are optically violently variable quasars and BL Lacertae objects that comprise a subgroup of quasars. The spectra of BL Lacertae objects make it difficult to determine the nature of these objects. BL Lacertae was found to be at the center of a giant elliptical galaxy, which Joseph Miller at Lick Observatory found in 1978.

Another interesting phenomenon has been the detection of double and multiple quasars that are very close together. The symmetric patterns of these multiple quasars are most readily explained by gravitational lensing of a very distant quasar's light by a galaxy that is too distant to be detected visually but is nevertheless between the quasar and the Milky Way. The lensing is caused by the bending of light in a strong gravitational field (as predicted by the General Theory of Relativity). Among the most recent examples is the Cloverleaf Quasar, where presumably an unseen galaxy between a quasar and the Milky Way has formed four images of the quasar.

The detection of galaxies associated with blazars and of multiple images of quasars presumably formed by gravitational lensing by galaxies too distant to be detected otherwise has favored the hypothesis that the quasars are similar to distant galaxies, conform to Hubble's law, and represent a phenomenon that was more common in earlier stages of the development of our universe than it is at present.

Big bang theory is driving the search for closer, later quasars, in order to fill in the gap in the evolution of the universe between the most distant (hence earliest) quasars now known, and the background remnant radiation from the primeval fireball of the early universe, which comes to us from the time when matter and radiation decoupled in the early evolution of the universe.

In January 2003, the Hubble Space Telescope imaged the relatively nearby quasar, 3C273. By utilizing techniques that blocked the quasar's light, astronomers were able to observe significantly more details of the quasar's host galaxy. Accordingly, in addition to identifying and studying quasars, in some cases astronomers are now able to see into regions of the cosmos these powerful beacons normally mask.

Resources

books

Hawking, Stephen. The Illustrated Brief History of Time, Updated and Expanded. New York: Bantam, 2001.

Kirshner, Robert P. The Extravagant Universe: ExplodingStars, Dark Energy, and the Accelerating Cosmos. Princeton, NJ: Princeton University Press, 2002.

Rees, Martin J. Our Cosmic Habitat. Princeton, NJ: Princeton University Press, 2001.

Sagan, Carl. Cosmos. New York: Random House, 1980.

periodicals

Meyer, A. "Quasars from a Complete Spectroscopic Survey." Monthly Notices of the Royal Astronomical Society 324 no. 2 (2001): 343-354.

Phillipps, Steven. "The proximity Effect as a Probe of Cosmological Models." Monthly Notices of the Royal Astronomical Society 336 no. 2 (2002): 587-591.