Globular clusters: The view from HST

Summary

Globular clusters represent only a small fraction of the total mass in their parent galaxy, but provide a vast array of tests for stellar physics, dynamics, and galaxy formation. This review discusses the prominent accomplishments of HST-based programs:

1. The definition of precise fiducial sequences in the HR diagram, extending down to the hydrogen-burning limit,

2. Discovery of the upper white dwarf cooling sequence in several clusters,

3. Discovery of a highly consistent IMF on the lower main sequence,

4. Definitive age measurements for the oldest clusters in the outermost halo of the Milky Way, the Magellanic Clouds, and the dwarf elliptical satellites of the Milky Way,

5. Elucidation of the innermost structure of M15 and other core-collapsed clusters,

6. Discovery of surprisingly large “anomalous” populations of stars within dense cluster cores: extended blue horizontal-branch stars, blue stragglers, and others,

7. The first reliable color-magnitude studies for globular clusters in M31, M33, and other outlying Local Group members,

8. Discovery of massive young clusters in starburst galaxies with ages as small as 1 Myr,

9. Measurement of metallicity distribution functions among globular cluster systems in many giant E galaxies—bimodality is common, but details differ strongly, and

10. Deep imaging of cluster luminosity distributions in gE galaxies in Virgo, Fornax, and other Abell clusters as distant as Coma.