|Annu. Rev. Astron. Astrophys. 1989. 27:
Copyright © 1989 by . All rights reserved
Galaxies of the Local Group are all near enough that rather reliable methods can be used to diagnose what kinds of stellar populations each contains. There are still many uncertainties in detail, but experience has shown that the problem is at least tractable for the local sample, whereas it becomes extremely difficult and uncertain for more distant galaxies, where only integrated properties can be measured [see the excellent discussions in Norman et al. (94)]. There is usually no difficulty in ascertaining whether or not a galaxy has any very young stars or uncondensed interstellar material, though even this question becomes tricky when very small amounts are being sought. The problem becomes more difficult as older components are looked for because of the faintness of older stars and because of the ambiguity in age dating them. (It is difficult to distinguish a low-mass old star from a low-mass young star.) Even in the Magellanic Clouds, where we can measure stars down to luminosities fainter than the Sun's, one cannot be certain about how to interpret a given field star color-magnitude diagram (CMD) because of the unknown and probably variable star formation rate (SFR), and, possibly, a variable initial-mass function (IMF).
One method that at least provides fairly reliable age data is the use of star clusters as probes of the galaxy. From good charge-coupled device color-magnitude diagrams it is possible to determine the ages and chemical compositions of star clusters in the Magellanic Clouds and to obtain less accurate data for clusters in the more distant Local Group members, such as M31 and M33, where integrated colors, and spectra can be used (with calibration from the MWG and the Magellanic Clouds). But there is no good way to compensate for the fact that star clusters disintegrate in time at a rate that probably depends on both the properties of the clusters and those of the host galaxies. Thus, though it is relatively simple to determine the present star formation rate for stars in clusters in a galaxy, it is not possible to trace the rate back in history farther than a time that is about the mean lifetime of the clusters, which is about 108 yr for the MWG (131). Attempts to do this have been made for the Magellanic Clouds (38, 62) and for the dwarfs NGC 6822 and IC 1613 (65). These tracings go back only a fraction of the lifetimes of the galaxies, however, and so we are forced to reconstruct the oldest times from other evidence.