Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter.

The distribution of the globular clusters in our Milky Way galaxy is concentrated around the galactic center in the Sagittarius -- Scorpius -- Ophiuchus region: Of the 138 Milky Way globulars listed in the Sky Catalog 2000, these constellations contain 29, 18, and 24 globulars, respectively, so a total of 71 clusters, or 51.4 percent (though one must admit that of the 29 clusters in Sagittarius, probably four are members of the Sagittarius Dwarf Elliptical Galaxy discovered 1994, and not really of the Milky Way, among them M54). Of the 147 clusters listed in W.E. Harris' database (also see below), 134 (91 percent) are concentrated in the hemisphere centered on Sagittarius, while only 13 globulars (9 percent) are on the opposite side of us (among them M79). This pronounced anisotropy in the distribution of globular clusters was of historic importance when Harlow Shapley, in 1917, derived from it that the center of our galaxy is lying at a considerable distance in the direction of Sagittarius and not close to our solar system (however, he significantly overestimated the size of the Milky Way as a whole, as well as the size of the globular cluster system and our distance from the galactic center)

Radial velocity measurements have revealed that most globulars are moving in highly excentric elliptical orbits that take them far outside the Milky Way; they form a halo of roughly spherical shape which is highly concentrated to the Galactic Center, but reaches out to a distance of several 100,000 light years, much more than the dimension of the Galaxy's disk. As they don't participate in the Galaxy's disk rotation, they can have high relative velocities of several 100 km/sec with respect to our solar system; this is what shows up in the radial velocity measurements.

Spectroscopic study of globular clusters shows that they are much lower in heavy element abundance than stars such as the Sun that form in the disks of galaxies. Thus, globular clusters are believed to be very old and consisted from an earlier generation of stars (Population II), which have formed from the more primordial matter present in the young galaxy just after (or even before) its formation. The disk stars, by contrast, have evolved through many cycles of starbirth and supernovae, which enrich the heavy element concentration in star-forming clouds and may also trigger their collapse.

The H-R diagrams for globular clusters (shown below this paragrapph for M5) typically have short main sequences and prominent horizontal branches, this again represents very old stars that have evolved past giant or supergiant phases. Comparison of the measured HRD of each globular cluster with theoretical model HRDs derived from the theory of stellar evolution provides the possibility to derive, or estimate, the age of that particular cluster. It is perhaps a bit surprising that all the globular clusters seem to be of about the same age; there seems to be a physical reason that they all formed in a short period of time in the history of the universe, and this period was apparently long ago when the galaxies were young. Semi-recent estimates yield an age of 12 to 20 billion years; the best value for observation is perhaps 14 to 16 billion (see e.g. the discussion at M92). As their age is crucial as a lower limit for the age of our universe, it was subject to vivid and continuous discussion since decades. In early 1997, the discussion of the age of the globular clusters got revived because of the general modifications of the distance scale of the universe, implied by results of ESA's astrometrical satellite Hipparcos: These results suggest that galaxies and many galactic objects, including the globular clusters, may be at a 10 per cent larger distance; therefore, the intrinsical brightness of all their stars must be about 20 % higher. Considering the various relations which are important for understanding stellar structure and evolution, they should also be roughly 15 % younger, in a preliminary off-hand estimate.

Plotting a relation between visual magnitude (V) and color index (B-V) for individual stars of the cluster M 5, one obtains this picture of an old, distorted apple-tree with only two branches. It is, nevertheless, the Tree of Knowledge for astrophysicists. (A): Main Sequence. (B): Red Giant Branch. (C): Helium flash occurs here (D): Horizontal Branch (HB) (E): Schwarzschild space in the HB (F): White Dwarfs (below the arrow)

As globular clusters follow their orbits around the Milky Way's Galactic center through the billion years, they are subject to a variety of disturbations:

• some of their stars escape as they get randomly accelerated in mutual encounters,

• tidal forces from the parent galaxy acts on them, particularly heavy in that part of their orbit which is closest to the galactic center (near the periapsis),

• each passing through the galactic plane, as well as close encounters with greater masses like (any type of) clusters or big nebulous clouds contributes to disturbation,

• stellar evolutionary effects and loss of gas also contribute to increasing the rate of mass loss (and thus deflation) of the clusters

Although significantly slower compared to the less densely packed and less populated open clusters, these disturbations are tending to disrupt the clusters. The currently existing globulars are just the survivers of a perhaps significantly larger population, the rest of which has been disrupted and spread their stars throughout the Galactic halo. The process of destruction still works, and it was estimated that about half of the Milky Way globulars will cease to exist within the next 10 billion years.

Our galaxy has a system of perhaps about 200 globular clusters. Most other galaxies have globular cluster systems as well, in some cases (e.g., for M87) containing several thousands of globulars!

While all the globulars in our Milky Way, and our big companion, the Andromeda Galaxy M31, are old, other Local Group galaxies as the Large and the Small Magellanic Cloud as well as the Triangulum Galaxy M33 also contain considerably younger globular star clusters, which can be concluded with certainty from spectroscopic investigations.