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Quasar

Mount Palomar, Milky Way Galaxy, absorption lines, astronomical object, emission lines

Quasar, astronomical object that is very bright for its size and distance from Earth. Another important identifying feature of quasars is a shift in the light emitted by quasars that indicates they are moving away from Earth very quickly.

Quasars occur at the centers of distant galaxies. Many astronomers believe that quasars are associated with giant black holes at the centers of these galaxies. Black holes are regions of space that are so massive and dense that not even light can escape their gravitational pull. The matter that swirls around black holes releases enormous amounts of energy, which astronomers believe provides quasars’ light.

The name quasar is an acronym for quasi-stellar radio source, because quasars appear to be starlike, or pointlike objects in the sky, since they are so far away from Earth. Also, the first quasars observed (in the 1950s) were all strong emitters of radio waves. By the end of the 1990s, only about 10 percent of the total number of quasars discovered were strong emitters of radio waves. Even though the original definition of quasar is no longer correct, most astronomers still use the acronym. Some astronomers prefer to use the name quasi-stellar object (QSO) instead of quasar because most quasars are not strong radio-wave emitters. Most quasars emit much of their radiation in the infrared range. Infrared radiation has wavelengths just longer than those of visible light. Many quasars are also strong emitters of X-ray radiation.

Astronomers identified quasars as sources of intense radio-wave emission in the late 1950s. In 1960 astronomers used the 200-in (508-cm) telescope on Mount Palomar in California to observe the positions of these radio sources. Astronomers can study quasars by using an instrument to separate the radiation emitted by the quasar into its individual wavelength components, just as water vapor in the atmosphere separates sunlight into its components in a rainbow. This technique is called spectroscopy. Spectroscopy reveals that, at some wavelengths, the light emitted by an object is especially bright or especially dim. These bright and dark spots are called emission lines and absorption lines, respectively. Chemical elements in the object cause these lines. Emission lines are caused when atoms of a certain element emit a particular wavelength of radiation. Absorption lines are caused when atoms of a particular element absorb radiation of a certain wavelength.

When astronomers applied spectroscopy to quasars, they discovered emission and absorption lines that did not match those of any known elements. In 1963 Dutch-born American astronomer Maarten Schmidt discovered that these unidentified emission lines in the spectrum of a particular quasar, called 3C 273, formed a familiar pattern but occurred in unexpected wavelengths in the object’s radiation. He concluded that the radiation of the quasar as it appeared from Earth was shifted by a large amount. The shift exhibited by the quasar was greater than the shift of any other known object.

One known cause of a shift in an object’s radiation is called the Doppler effect. When an object emitting radiation such as light or sound moves away from an observer, each wave that the object emits must travel slightly farther than the wave before it traveled. This makes the distance between waves appear longer to the observer than it would appear if the object were stationary. Objects moving toward an observer appear to emit radiation with a shorter wavelength than they would if they were stationary. Austrian physicist Christian Johann Doppler discovered this effect in sound, and the name Doppler effect was applied to all types of radiation. In astronomy, the effect is often called redshift. Most astronomical objects are moving away from one another, making their radiation appear to have longer wavelengths than normal. An increase in the wavelength of visible light means the light moves toward the red end of the spectrum of visible light. A rule called Hubble’s law, named after American astronomer Edwin Hubble, relates the amount of redshift (and therefore velocity) that an object has to its distance from Earth. The redshift of the quasar 3C 273 shows that the object is 1.5 billion light-years (a light-year is the distance that light travels in a year—9.5 trillion km, or 5.9 trillion mi) from Earth. Schmidt determined that the quasar was much too distant to be part of the Milky Way Galaxy.

By the late 1990s, more than 10,000 quasars had been identified. Astronomers had measured the redshifts of a few thousand of these quasars. A small number of quasars have redshifts near 5, meaning that they must be about 12 billion light-years from Earth. Quasars are very far from Earth, yet they are bright enough for astronomers to detect. This means that their actual brightness, or the amount of radiation that they actually emit (compared to their brightness as seen from Earth), must be huge. Many quasars produce more energy than thousands of ordinary galaxies combined produce. In 1998 astronomers at the University of Washington found a quasar with an estimated brightness of about 30,000 times that of the entire Milky Way Galaxy.

Analysis of the light of quasars shows that quasars must be much smaller than ordinary galaxies. In the 1990s astronomers discovered that every quasar is surrounded by a haze of visible light. They found this light to be starlight from the quasar’s host galaxy. Measurements of the redshift of light from host galaxies showed that the redshifts of quasars are indeed cosmological and that the objects are very distant.



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