We find the distance modulus, m — M, m — M = 5 log10 (d/10 pc)

Since the apparent magnitude is 13, the absolute magnitude is —26. For comparison the absolute visual magnitude of the Sun is +5. A difference of 31 magnitudes corresponds to a brightness ratio of a little more than 1012. This means that 3C273 gives off 1012 times as much visible light as does the Sun. What makes this even more remarkable is that 3C273 gives off more energy in the radio part of the spectrum than in the visible!

The proposal that the spectrum of 3C273 could be explained with a large redshift was made in 1963 by the Caltech astronomer Maarten Schmidt. At that time, the existence of a similar object, 3C48, was known. It had been noted in 1960 that there is a possible correspondence between the radio source and a 16th magnitude star. The spectrum of this star showed an even greater redshift than 3C273, corresponding to a 37% shift. These objects were given the name quasi-stellar radio sources, because of the starlike appearance on short-exposure photographs. The name was shortened to QSR or quasar. Further studies have revealed a class of objects that are like QSRs in optical photographs, and also have large redshifts, but don't have any radio emission. These are called quasi-stellar objects, or QSOs. We now loosely call both types of objects quasars.

19.4.2 Properties of quasars

The spectra of many quasars show a large ultraviolet excess. This means that they are brighter in the ultraviolet than one would expect from just knowing their visual brightness. This provides us with a way of searching for quasars. We cannot take spectra of all stars to see if they have large redshifts. Since quasars are so faint, it takes a long time to observe a spectrum. We can study radio sources, but not all quasars are radio sources. However, we can compare visible, blue and (now with space observations) ultraviolet images of large fields to find objects with a large ultraviolet excess. Spectra of these objects can then be taken to see if they have large redshifts.

Some quasars are quite variable in their energy output (Fig. 19.19). We have good records of the visible and radio variability of quasars for the past 30 years as a result of specific studies. However, we have optical records going back even farther, since observatories save photographic plates. A quasar may be on a plate exposed for an entirely different purpose. Once the quasar is discovered, an astronomer can go back through plate archives and find its image as far back as 100 years ago.

An important feature of this variability is that it allows us to place an upper limit on the size of the emitting region. We discussed this idea in our study of masers, in Chapter 15. If a significant fraction of the total power varies on a time scale t, then the emitting region can be no larger than ct (as long as motions close to the speed of light are not involved). In the case of quasars, variations on a time scale of a few months limit the size of the emitting region to about 1012 km. (This is only about 104 AU.)

The spectra of quasars, such as that in Fig. 19.20, show both emission lines and absorption lines. Generally, all of the emission lines can be explained by a single redshift z, but a few groups of absorption lines appear with different red-shifts, always less than or equal to that of the emission lines. Often the spectrum is dominated by the Lyman-alpha line at various redshifts. (Remember, Lya is the lowest transition and is often the most easily excited.) We sometimes refer to this as the Lyman-alpha forest. Over the years, extensive absorption line surveys have been carried out, providing good statistical information on absorption line properties. The absorption lines are generally narrow, less than 300 km/s

0 0

Post a comment