The Local Group spirals have the most complex and varied star formation histories of all Local Group galaxies. Different subpopulations can be distinguished by their ages, metallicities, and kinematics. The oldest populations are found in the halos and thick disk components. Extremely metal-poor ([Fe/H] < -3 dex) halo stars are tracers of the earliest star formation events (Ryan et al. 1996), but it is difficult to derive ages for them.
The earliest significant star formation episodes in the Galactic thick disk appear to have occurred 13 Gyr ago, while the thin disk began to experience multiple bursts of star formation ~ 9 Gyr ago (Rocha-Pinto et al. 2000). The metallicity in the thin disk depends more strongly on Galactocentric radius than on age and shows a large spread at any position and age (Edvardsson et al. 1993).
While halos may have largely formed through accretion of metal-poor Searle & Zinn (1978) fragments, bulges also host metal-rich old populations (mean metallicity of the Galactic bulge: -0.25 dex; Minniti et al. 1995), indicating that they experienced early and fast enrichment. M31 appears to have undergone rapid enrichment as a whole, whereas M33 shows a pronounced radial abundance gradient. The mean metallicity of M31's halo is -1 to -1.2 dex, more metal-rich than the halo of the Milky Way (~ -1.4 dex) and of M33 (~ -1.6 dex). While M31's bulge emits ~ 30% of the visible light of this galaxy, M33 lacks a bulge.
M31's total number of globular clusters may be as high as ~ 600. The Milky Way contains ~ 160 globulars, and in the smaller M33 54 globulars are currently known (see Grebel 2000b for a review of star clusters in the Local Group). Main-sequence photometry of Galactic globular clusters suggests a range of ages spanning more than 3 Gyr. We lack such detailed information for M31's and M33's globulars, but the blue horizontal branch (HB) morphology observed in some of them may suggest similar ages as for the Milky Way globulars. On the other hand, the red HBs of M33's globulars may indicate that star formation was delayed by a few Gyr (Sarajedini et al. 1998).
The spiral arms in all three galaxies contain numerous OB associations and young star clusters. The UV line strengths of massive OB stars suggest that the young population of M31 is comparable to that of the Milky Way, whereas M33 resembles the Large Magellanic Clouds (Bianchi, Hutchings, & Massey 1996). Present-day star-forming regions in the Milky Way range from very extended associations to compact starburst clusters such as the central cluster of NGC 3603 and the clusters Quintuplet and Arches near the Galactic center. M31's current star-forming activity is low. The increase in cluster formation in M33 over the past 10 - 100 Myr may be correlated with gas inflow into M33's center (Chandar, Bianchi, & Ford 1999).
Warps in the stellar and H I disks of the Milky Way and M31 may have been caused by tidal interactions with the Magellanic Clouds and M32, respectively. The Milky Way disk may also have been significantly distorted by interacting with the currently merging Sagittarius dwarf galaxy (Ibata & Razoumov 1998). M33's stellar and H I disks are tilted with respect to each other, but no nearby companion is known that might be responsible.