QUASARS

Quasar, acronym for quasi-stellar radio source, any of a class of blue, star-like objects that have spectra which exhibit a strong red shift and are apparently very remote and emit enormous amounts of energy. The earliest quasars to be discovered were identified as sources of intense radio emission in the late 1950s (see Radio Astronomy). In 1960, using the 200-in. (508-cm) telescope on Mount Palomar in California to observe the positions of these radio sources, astronomers discovered objects the spectra of which showed emission lines that could not be identified. In 1963 the Dutch-American astronomer Maarten Schmidt discovered that these unidentified emission lines in the spectrum of quasar 3C 273 were known lines that exhibited a far stronger red shift than in any other known object.

One known cause of red shift is the Doppler effect, which shifts the wavelength of emitted light of celestial objects toward the red (longer wavelengths) when the objects are moving away from the Earth. Distant objects, such as galaxies, are receding from the Earth because of the expansion of the universe. From the amount of red shift astronomers can calculate the recession velocity. Hubble's law (see Cosmology), which states that recession velocity caused by the expansion of the universe is directly proportional to the distance of the object, indicates that quasar 3C 273 is 1.5 billion light years from the Earth.

By the end of the 1980s, several thousand quasars had been identified and the red shifts of a few hundred determined; in a small number of these, the shift factor is greater than 4. If the red shift is assumed to be cosmological, these quasars would have velocities greater than 93 per cent of that of light. According to Hubble's law, their distances would thus be greater than 10 billion light years, and their observed light would have been travelling practically as long as the age of the universe. In 1991 a quasar 12 billion light years distant was discovered by observers at Palomar Observatory, and in 1998 a team from Princeton University found three more at around this distance during the first few months of the Sloan Digital Sky Survey. Judging from the energy received on Earth from such distant objects, some quasars produce more energy than 2,000 ordinary galaxies—one, S50014 + 81, may be 60,000 times as bright as our Milky Way galaxy. Radio measurements, however, combined with the fact that electromagnetic waves emitted by some quasars vary strongly over a period of a few months, indicate that quasars must be much smaller than ordinary galaxies. Because the size of a fluctuating radiation source cannot be much larger than the distance light would travel from one end of the object to the other during one fluctuation period, astronomers estimate that the variable quasars cannot be larger than one light year across, which is 100,000 times smaller than the Milky Way.

The only satisfying explanation for a mechanism that could produce such amounts of energy in a relatively small volume is the swallowing of large amounts of matter by a black hole. But some astronomers suspect that the red shifts in quasars are caused by some other mechanism than the Doppler effect, and that quasars are not really very distant. The American astronomer Halton C. Arp, for example, has found large differences between red shifts of the quasars and other galaxies that nevertheless appear to be physically linked. In many other apparent pairings of quasars and ordinary galaxies, however, the red shifts do correspond. One theory gaining wide acceptance is that quasars are the superluminous cores of galaxies and that they and radio galaxies may actually be equivalent objects seen from different angles.

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