THE STELLAR POPULATIONS OF M-33

A review is given of present ideas on the evolution and stellar content of the Triangulum nebula = M 33 = NGC 598. A distance modulus of (m - M)0 = 24.5 +/- 0.2 (D = 795 +/- 75 kpc) and a Galactic foreground reddening E(B-V) = 0.07, from which M(V) = -18.87, are adopted throughout this paper. The disk of M 33 is embedded in a halo of globular clusters, metal-poor red giants, and RR Lyrae stars. Its nuclear bulge component is weak (M(V) > -14). This suggests that the halos of galaxies are not extensions of their bulges to large radii. The ages of M 33 clusters do not appear to exhibit a hiatus in their star-forming history like that which is observed in the Large Magellanic Cloud. Young and intermediate-age clusters with luminosities rivaling the populous clusters in the LMC are rare in M 33. The integrated light of the semistellar nucleus of M 33, which contains the strongest X-ray source in the Local Group, is dominated by a young metal-rich population. At optical wavelengths the disk scale length of M 33 is 9'.6, which is similar to the 9'.9 scale length of OB associations. The ratio of the nova rate in M 33 to that in M 31 is approximately equal to the ratio of their luminosities. This suggests that the nova rate in a galaxy is not determined entirely by the integrated luminosity of old bulge stars. The gas-depletion time scale in the central region of M 33 is found to be approximately 1.7 x 10(9) yr, which is significantly shorter than a Hubble time. Available data do not yet allow an unambiguous choice between the density wave and self-propagating star-formation models for the two main spiral arms of M 33.