Quasars Help (page 2)
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.
The sizes of the quasars, as well as estimates of their energy output, have been determined according to the Hubble relation between red shifts and distances. All quasars show significant red shifts in the absorption lines of their spectra. This has led most astronomers to surmise that they are billions of light-years away from us.
Suppose, however, that the red shifts are being misinterpreted? Are quasars actually local objects of modest size that are thrust outward from the nucleus of our galaxy at tremendous speeds? This is an interesting theory, but it is not widely accepted. If quasars are being ejected from our galaxy, then it is reasonable to suppose that they are ejected from other galaxies too. In such a case, some of the quasars ejected from other galaxies should be observed as approaching us. This would give such objects a pronounced blue Doppler shift. However, no quasar has ever been found that exhibits a blue shift in its spectral lines.
Another attempt has been made to prove that quasars are “local.” Albert Einstein showed, in the formulation of his general theory of relativity, that a powerful gravitational field can produce a red shift in the spectrum of the light coming from the source of the gravitation. This effect has been observed and measured, so scientists know that Einstein’s theory is correct. Can the red shifts in the spectral lines of quasars be explained in terms of the relativistic effect of gravitation? A super dense object with extreme gravitation near its surface could produce a large red shift. This remains an open question. Still, an affirmative answer would not constitute conclusive proof that quasars are “local.”
Recent observations of quasars using the HST have begun to resolve the riddle. Evidence is accumulating to support the theory that quasars are among the most distant objects we can see in the Cosmos and that they therefore present a picture of the Universe as it was when it was much younger than it is now. This gives astronomers a way to look back in time to whatever extent they want simply by observing galaxies and quasar objects at various distances as indicated by their red shifts.
A severe blow was dealt to the local quasar theory when scientists calculated that the first quasar that was discovered, called 3C4S , would have to have a mass the same as the Sun, be only 10 km (6 mi) in diameter, and be in the Earth’s atmosphere in order to account for the radiation intensity it possesses. Even if 3C4S has thousands of times the mass of the Sun, calculations show that it still must reside within our Solar System, and this obviously is not the case. The derivations in these terms for other quasars give similar results.
The determination of the distances to quasars represents a good example of the devil’s-advocate method of lending support to a theory by discrediting all its plausible refutations. The quasars, even after attack by the devil’s-advocate scientific method of inquiry, appear to be distant and energetic cosmic phenomena.
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