Annu. Rev. Astron. Astrophys. 1989. 27: 139-59
Copyright © 1989 by . All rights reserved

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5.2 M33

Because relatively little has been published about any but its brightest individual stars, most information about populations in M33 has to come from studies of integrated light and of star clusters. It is obvious from photographs, which show large amounts of high-luminosity stars, H II regions, and dust (77), that M33 is currently experiencing a vigorous rate of star formation, but the lack of a clear bulge and halo leaves the distant past's star formation rate an open question. Until RR Lyrae stars are discovered and clear examples of old giants are identified, we are limited to what can be learned of old clusters. Recent work on 128 of the 250 known cluster candidates (23) indicates that there is a wide range in the ages of the clusters, from extremely young objects to those that are probably about 15 Gyr old. The age-abundance relation shows a mean that lies close to that of the Large Magellanic Cloud (LMC), intermediate between the MWG and the SMC. There is a spread in their derived values that, if real, would indicate about an order-of-magnitude spread in abundances for a given age. Although part of this spread is probably due to observational errors, some of it may also be intrinsic, which would thicken the figure for M33 in its population box. I have not plotted a figure for M33, however, because of the lack of sufficient information on the variation of the star formation rate with time.