Globular Star Clusters
Click icon to view globular clusters of Messier’s catalog
>> Messier’s Globular Clusters;
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
(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
(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.
These galaxies contain also extremely large diffuse nebulae with masses of
the order of globular clusters, clear candidates for future young globulars
currently in formation, notably the
Tarantula Nebula NGC 2070 in the LMC and
NGC 604 in M33.
Messier‘s globular clusters:
M2,
M3,
M4,
M5,
M9,
M10,
M12,
M13,
M14,
M15,
M19,
M22,
M28,
M30,
M53,
M54,
M55,
M56,
M62,
M68,
M69,
M70,
M71,
M72,
M75,
M79,
M80,
M92,
M107.
Other early known globular clusters:
NGC 104 (47 Tucanae),
NGC 5139 (Omega Centauri),
Links
- Link to the most current
globular cluster data file, compiled by
of the Physics and Astronomy department of
We hold possibly older versions of the
data file and the
reference file at SEDS.
- Globular Cluster list
by Brian Skiff
- Globular Cluster Catalogs List
-
ARVAL Catalog of Bright Globular Clusters
- Virtual Tour of the Milky Way
Globular Cluster system
(Limber Observatory) - Globular clusters in other galaxies:
Catalog of Globular Cluster Systems of Other Galaxies
by W. E. Harris (we hold a possibly
Globular Clusters in Other Galaxies (Lick)
-
University of Leeds Globular Cluster page
- Look at Globular Clusters in Messier’s Catalog
- Also look at our collection of some
significant non-Messier globulars
Last Modification: 9 May 1999, 18:00 MET