Globular Star Clusters

Click icon to view globular clusters of Messier’s catalog

>> Messier’s Globular Clusters;

Links

The icon shows 47 Tucanae (NGC 104).

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 as had been thought

previously (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 (here shown 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.

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 (including

28 of the 29 Messier globulars, all but above mentioned

M54).

Most other galaxies have globular cluster systems as well, in some cases