Introduction to Quasars
In 1960, the position of a strong radio source was defined with great accuracy, and its angular diameter was found to be less than a second of arc. Comparing the position of this radio source with various visible objects in its vicinity, this “radio star” was found to be a faint blue star in photographs. There was something especially odd about this star: The astronomers J. L. Greenstein and A. Sandage could not identify the absorption lines in its spectrum. It did not take them long to find the problem. The red shift in the spectrum of this cosmic energy source is so great that the lines are greatly altered, suggesting that the object is receding from us at a sizable fraction of the speed of light.
Soon after the discovery of this “radio star,” several other similar objects were found, and they also had very large red shifts in their spectral lines. The objects, because of their visual resemblance to stars and because of their strong radio emissions, were called quasi-stellar radio sources . This name has since been shortened to the more palatable term quasar .
After the first few quasars were found, many others were discovered and observed. Some quasars had been photographed previously, but in the photographs they had been dismissed as ordinary stars. In one case, when several photographs having been taken over a period of decades were examined, it was found that large changes in brightness had occurred within periods of a few months. This implied that the quasar is a fraction of a light-year in diameter. However, if its red shift is a correct indicator of its distance, its energy output is many times that of a normal galaxy! Quasars are concentrated, as well as intense, sources of energy.
When observed with radio telescopes having high resolution—less than 1 second of arc in some cases—some quasars still appear as point sources. This is also true of the nuclei of certain radio galaxies. Optically, many of the quasars look like point sources of light, and therefore, they resemble stars, until the extreme magnification and resolving power of the Hubble Space Telescope (HST) is put to work on them. Then some quasars show evidence of glowing matter around a central, intense core.
Some quasars can be resolved into components by radio telescopes, but this requires the use of multiple antennas and a baseline of hundreds or even thousands of kilometers. Antennas in diverse locations on Earth are linked by satellite communications systems, and their outputs are combined by computer programs in order to accomplish this. This provides the equivalent resolving power of an antenna much larger than any single structure that could be constructed. The angular resolution goes down to less than 0.001 second of arc. With such sophisticated apparatus, radio galaxies and quasars have been probed in detail.
There is another, quite different way to estimate the angular diameter of an object that emits energy at radio wavelengths: observing and measuring changes in intensity, called scintillations , that occur as the radio waves pass through turbulent ionized clouds of particles in space.
Everyone has noticed the twinkling of the stars, while the planets appear to shine almost without blinking. The reason for this difference is that the planets have a much greater angular diameter than any star. Small telescopes show the planets as disks, but even the nearest stars resolve only as points of light, even at high magnification. Turbulence in the air, such as that produced on summer evenings as the warm land heats the atmosphere and causes convection currents, make a point of light seem to twinkle because the light rays are refracted more and then less, and then more again by parcels of air having variable density. The charged subatomic particles of the solar wind , as they stream outward from the Sun, have a similar effect on radio waves coming from far away in space. Other stars produce “winds” too, making interstellar space a turbulent sea of charged subatomic particles. A source of radio waves with a small angular diameter therefore scintillates.
By observing quasars with a single radio telescope antenna to avoid diversity effect (averaging out of the strengths of radio signals as received at different locations), and by carefully recording the intensity of the waves reaching the antenna, it is possible to get an accurate idea of the angular size of a radio object. Quasars always appear as small sources of radio energy, at most a few light-years in diameter. Galaxies, in contrast, are many thousands of light-years across. Other observed properties of the quasars, such as curvature in the spectral lines, have led astronomers to believe that they are small compared with galaxies, even though they emit fantastic amounts of energy.
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