Globular Clusters
Globular clusters are large, spherical clusters of stars (usually 104 to 106 stars, typically) which are usually very old objects -- often the oldest stellar populations in galaxies. As such they are very useful tracers for studies of galactic halos (where GCs reside), and have been successfully used for such diverse aspects of galaxy studies such as galactic evolution, galactic merger histories, chemical evolution, and ages. GCs often exist in large numbers around galaxies : our Milky Way has about 160 clusters (including M15, left), but some giant elliptical galaxies (such as M87) have well over 10 000! As such, globular cluster systems in other galaxies have been used as distance indicators.
I have worked on colour-magnitude diagrams (CMDs) for 2 of GCs in our own Milky Way; the luminous cluster M15 (Durrell & Harris 1993) and the lower-luminosity, remote cluster Palomar 10 (Harris et al. 1997). Below is the CMD for M15, in which colours have been added to illustrate positions of blue and red stars in the cluster. In this diagram, a stars colour (related to its temperature) is plotted along the bottom axis (where stars towards the right are redder, or cooler, stars), while the magnitude (or brightness) of stars are plotted on the vertical axis -- with the brighter stars located towards the top of the diagram.
The stars in the different parts of the CMD denotestars in different evolutionary states, including main sequence stars (bottom), the red giant phase (top right) and the more evolved `horizontal branch' stars in the left half of the CMD.
Such diagrams allow astronomers to derive ages for the stars in these clusters via comparison with theoretical models of stellar evolution. From the above CMD, the age of the cluster M15 was derived using model isochrones from Bergbusch & VandenBerg (1991; ApJS 81, 163) -- about 15 Gyr (although this value may be an overstimate if recent changes to the distance scale from the Hipparcos satellite are correct). More recent estimates of GC ages are about 11-14 Gyr. As these objects are among the oldest objects we know of, they provide us with a lower limit for the age of the universe.
Globular Clusters in Dwarf Galaxies
My Ph.D. thesis (completed in July 1996 -- supervised by Dr. Bill Harris) was centred on seaching for and studying globular cluster systems around 12 dE (and dE,N) galaxies. 11 of the galaxies are in the Virgo Cluster, the other galaxy is the bright (for a dwarf!) companion to the S0 galaxy NGC 3115 (dubbed NGC 3115 DW1). Significant detections of a GCS was found in 10 of the galaxies -- not an easy task as dwarf galaxies typically have very few (if any!) globular clusters surrounding them. The image to the right is a false R-band color image of the luminous dwarf VCC 1254 (located near the giant elliptical M49), and a clear excess of point sources clearly surround the bright nucleus of the galaxy. 
The specific frequency SN (GC number/unit luminosity) of GCs in dE galaxies is about 4, thus similar to that found in the (structurally dissimilar) giant elliptical galaxies, and not similar to that found in spiral galaxies (often with SN less than 1). The mass spectrum (a manifestation of the more familar GC luminosity function) of the GC system around dwarfs is also similar to that in all other giant galaxies, suggesting that GCs formed via the same mechanism in all galaxy types. In turn, the slopes of these mass spectra are consistent with the recent model of Harris & Pudritz 1994 (ApJ 429, 177), in which globular clusters form via a similar mechanism as (lower mass) open clusters form today -- in molecular clouds. However, the GCs form in much larger versions of `normal' molecular clouds -- `they may form in the cores of supergiant molecular clouds' (SGMC's).
As observed in other galaxies, the mean metallicity (metal-content of the constituent stars) of GCs in dwarfs is less than that of the halo stars in the host galaxy (by about 0.5 dex). This difference is not simply due to metals expelled into the galaxy proper from the globular clusters -- other sites of star formation are required.
These results were published in Durrell et al. 1996 a, b. (ApJ 463, 543 + AJ 112, 972)
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