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1.5 Quasars

The term quasar is a short form for the full name ``quasi-stellar radio source''. Quasars were first discovered in 1963 as a result of the optical identification programme. The radio position of the quasar 3C 273 (see section 1.7 for the meaning of these catalogue numbers) was accurately determined by lunar occultation. If the Moon happens to cross the line of sight to a source, the source is said to be occulted. The drop in the intensity of a radio source as it is blocked by the Moon and the rise when the Moon has moved out of its way give accurate indication of when it is occulted and hence where it is located on the sky. The optical identification of this object (see Figure 1.16) and of another radio source, 3C 48, revealed starlike objects with emission lines, and it was originally assumed that these were radio stars in the galaxy. When their spectra were carefully examined, however, it became clear that the wavelengths were strongly redshifted.

Figure 1.16

Fig. 1.16. The quasar 3C 273. (Courtesy of Kitt Peak National Observatory.)

If the wavelength of an emission line in the laboratory is lambda0 and if the observed wavelength is lambda > lambda0, then the line is said to be redshifted by a fraction z given by

Equation 1.2 (1.2)

It is usual to call z the redshift of the object. For 3C 273, z = 0.158, while for 3C 48, z = 0.367. (The word redshift is used to indicate a shift to the red end of the visual spectrum.)

These were high values of z for stars in the galaxy, which have values < 10-3. What were these objects? In 1964 Terrell suggested that they were high-velocity stars ejected from the galaxy. The more popular interpretation, however, has been that the redshifts arise from the expansion of the universe, a concept we will discuss in section 1.8.

If this latter interpretation is correct, it implies that quasars are very distant objects, and since from such large distances they look bright enough to be mistaken for stars, they must be intrinsically very powerful. Many quasars show rapid variation in their light and radio output. This fact places a limit on their physical size; for if an object shows variability on a characteristic time scale T its size must be limited by cT, where c = the speed of light. This limitation, arising from the special relativistic result that no physical disturbance can propagate with a speed > c, makes quasars very compact indeed. We saw in section 1.2 how big our galaxy is. A quasar by comparison may emit a comparable amount of energy per unit time from a volume whose linear extent may be only a few light-hours!

By now more than 5000 quasars are known. Only a few percent of the total quasar population emit radio waves. Thus the early qualification ``radio source'' is not applicable to the bulk of the quasar population, and although the term ``quasar'' is used today also for radio quiet objects, the purist may prefer the term ``quasi-stellar object'' (QSO). More recently, the X-ray astronomy satellite ``Einstein Observatory'' has revealed that X-ray emission is also a common feature among quasars, indeed is much more common than radio emission.

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