Quasar

Quasars are compact objects located far outside of our galaxy. They are so bright they shine more intensely than 100 galaxies combined, but they are so distant their light takes several billion years to reach Earth. Since the 1960s, astronomers have begun to come closer to the truth about these unusual phenomena in space.

The word quasar is a combined form of quasi-stellar radio sources. These objects are so named because they have been observed through radio telescopes. However, only about 10 percent of all quasars emit radio waves. The energy coming from quasars also includes visible light, infrared and ultraviolet radiation, X rays, and possibly even gamma rays.

Words to Know

Big bang theory: Theory that explains the beginning of the universe as a tremendous explosion from a single point that occurred 12 to 15 billion years ago.

Black hole: Remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.

Gamma rays: Short-wavelength, high-energy radiation formed either by the decay of radioactive elements or by nuclear reactions.

Infrared radiation: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

Light-year: The distance light travels in one year, roughly 5.88 trillion miles (9.46 trillion kilometers).

Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.

Radio telescope: A telescope that uses radio waves to create images of celestial objects.

Radio waves: Longest form of electromagnetic radiation, measuring up to six miles from peak to peak.

Redshift: Shift of an object's light spectrum toward the red end of the visible light range—an indication that the object is moving away from the observer.

Spectrum: Range of individual wavelengths of radiation produced when light is broken down by the process of spectroscopy.

Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

In the early 1960s, American astronomer Allan Sandage photographed an area of the sky and noticed that one star had a very unusual spectrum. (A spectrum is the diagram of individual wavelengths of radiation from a star.) Most stars emit radiation consistent with the spectrum of ionized (electrically charged) hydrogen, the most abundant element on the surface of stars. This star, however, had a spectrum that seemed to reveal none of the elements known to exist in stars. The wavelengths at which it emitted radiation were heavily skewed toward the red-end range of visible light.

Such a skewed spectrum is known as redshift and is an indication of an object moving away from the point of observation. The greater the redshift, the faster the object is moving away. And as an object moves farther away, it picks up speed, increasing its redshift.

In 1963, Dutch astronomer Maarten Schmidt correctly identified the star's strange spectrum as that of a normal star with a high redshift. His calculations placed it an amazing two billion light-years away. In order to be observable from Earth at that distance, the object could not be a star, but had to be something larger, like a galaxy.

Schmidt measured the diameter of the object and learned that although it was emitting as much energy as one trillion suns, it was only about the size of the solar system. The brightest quasar to date, located in the constellation Draco, shines with the light of 1.5 quadrillion suns.

Origin of quasars

Astronomers formally believed that a quasar is found in a particular type of galaxy and is formed during the collision between two distant galaxies. When this happens, one galaxy creates a black hole in the other with the mass of about 100 million suns. (A black hole is a single point of infinite mass and gravity.) Gas, dust, and stars are continually pulled into the black hole. The temperature in the black hole then rises to hundreds of millions of degrees, and the black hole spews out tremendous quantities of radiation.

This theory was turned upside down in late 1996 when the Hubble Space Telescope (HST) took pictures of galaxies that are hosts to quasars. The pictures revealed that there was no pattern to the shapes and sizes of the galaxies. The pictures also showed that while many of the galaxies were colliding with each other as scientists had theorized, almost as many galaxies showed no signs of collision.

Quasars are the most distant, fastest, and most luminous large objects known in the universe. Because they are so far away, they give us a glimpse of the early universe. Since a light-year is a measure of the distance light travels in space in one year, viewing an object one billion lightyears away is really like looking one billion years back in time. Some quasars are so distant they are virtually at the edge of time. They are relics from the period following the big bang event that created the universe some 12 to 15 billion years ago.

The most distant quasar known was discovered in November 1999 by the National Aeronautics and Space Administration's BATSE (Burst And Transient Source Experiment) satellite. Known by the scientific name 4C 71.07, this quasar appears to be about 11 billion light-years away. We therefore see quasar 4C 70.71 as it existed perhaps as little as a billion years after the big bang. That time period seems long by human standards, but it is near infancy by standards of the universe.

[See also Big bang theory; Black hole; Galaxy; Redshift ]

quasar An object with a high redshift which looks like a star, but is actually the very luminous active nucleus of a distant galaxy. The name is a contraction of quasi-stellar, from their star-like appearance. The first quasars discovered were strong radio sources (quasi-stellar radio sources, or QSSs), but many more are now known which are relatively radio-quiet (quasi-stellar objects, or QSOs). At the distance implied by the redshift, the nucleus must be up to 100 times brighter than the whole of a normal galaxy. Yet some quasars vary in brightness on a time-scale of weeks, indicating that this huge amount of radiation originates in a volume only a few light weeks across. The source may therefore be an accretion disk around a black hole with 10⁷ or 10⁸ solar masses. Some quasars show little change in their light output, while others are much more variable: for example, 3C 279 has varied by a factor of nearly 500 in four months.

The first quasar to be identified as such in 1963 was the radio source 3C 273 at a redshift of 0.158, and it remains the optically brightest quasar as observed from Earth, at 13th magnitude. Thousands of quasars have since been found, many with high redshifts which imply that we see them as they were when the Universe was only about a tenth of its present age. The redshifted spectra of quasars show strong, often very broad, emission lines as well as continuum radiation. A rich absorption-line spectrum is also seen in the ultraviolet region (shifted into the optical region in high-redshift quasars), caused by clouds of intergalactic gas or interstellar material in galaxies between the quasar and Earth. The numerous absorption lines due to hydrogen in the clouds are together known as the Lyman-α forest.

The large distances to quasars and the dominance of light from the central regions makes it difficult to observe the surrounding galaxy with Earth-based telescopes, but the Hubble Space Telescope has clearly resolved the host galaxies, both spiral and elliptical. In many cases the host galaxies are seen to be interacting or merging with one or more neighbours. Such interactions supply the massive black-hole nucleus with gas or stars, which ultimately fuels the quasar outburst. Some apparently normal galaxies may contain the remnants of quasar activity in their nuclei, and some Seyfert galaxies and Markarian galaxies have nuclei that are intrinsically as bright as some quasars. Quasars which vary greatly in their light output are termed optically violently variable (OVV) and are classified as blazars along with BL Lacertae objects. See also active galactic nucleus.

quasar , one of a class of blue celestial objects having the appearance of stars when viewed through a telescope and currently believed to be the most distant and most luminous objects in the universe; the name is shortened from quasi-stellar radio source (QSR). Quasars were discovered as the visible counterparts of certain discrete celestial sources of radio waves (see radio astronomy ). Similar starlike objects that do not emit radio waves were subsequently discovered and named quasi-stellar objects (QSOs). Although their visible light is faint, the quasars are optically brighter than the galaxies with which radio sources had been identified before 1963. Before their spectra were studied carefully, it was believed that the quasars were stars in our galaxy. However, the lines in their spectra have enormous red shifts that seem to imply that they are receding from the Milky Way with speeds as great as 95% of the speed of light. Only shifts toward the red end of the spectrum have been observed for quasars; blue-shifted ones that would indicate a quasar approaching our galaxy have not yet been found. If quasars were simply objects being ejected from nearby galaxies at high speeds, and not the distant objects they appear to be, then some would have blue shifts. If Hubble's law for the expansion of the universe is extrapolated to include the quasars, they would be many billion light-years away and consequently as luminous intrinsically as 1,000 galaxies combined. To account for such brilliant light, astronomers believe that quasars are supermassive black holes in galactic nuclei, releasing energy by the accretion of matter through a rotating viscous disk (see cosmology ).

Bibliography: See H. L. Shipman, Black Holes, Quasars, and the Universe (2d ed. 1980).

quasar (quasi-stellar object) In astronomy, an object that appears to be a massive, highly compressed, extremely powerful source of radio and light waves, characterized by a large red shift. If such red shifts are due to the Doppler effect, it can be deduced that quasars are more remote than any other objects previously identified; many are receding at velocities greater than half the speed of light. Their energy may result from the gravitational collapse of a galaxy or from many supernovas exploding in quick succession, although there seems to be no reason why such events should be occurring.

qua·sar / ˈkwāˌzär/ • n. Astron. a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies.

quasar •César, quasar

quasar (ˈkweɪzɑː) Astronomy quasistellar object