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 GLOBULAR
CLUSTERS
A globular cluster is an object that consists
50 000 - 1 000 000 stars packed closely together in a sphere. They
are approximately several tens to about 200 light years in diameter.
The distribution of the globular clusters
in our Milky Way is concentrated around the galactic center in Saggitarius.
Of the 138 Milky Way globulars listed in the Sky Catalog 2000, Saggitarius-Scorpius-Ophiucus
region contains 71 globular clusters, or 51.4 percent of the NGC
globulars (some of them are in the SagDEG, Saggitarius elliptical
galaxy, dwarf discovered in 1994, among them M 54, the furthest
(Messier globular).
Of the 147 clusters listed in W. E. Harris
database, 134 (91 %) are concentrated in the hemisphere centered
on Saggitarius, while only 13 globulars (9 %)are on the opposite
side of us (like M 79 in Lepus). Radial velocity measurements have
revealed that most globulars are moving in highly excentric elliptical
orbits that take them far outside Milky Way; they form a halo of
roughly spherical shape which is higly concentrated to the Galactic
Center, but also reaches out to a distance of several 100 000 light
years, much more than the dimension of the galactical disk.
The H-R diagrams for globular clusters
(an example given here forM 5) have short main sequences and prominent
horizontal branches, this represens very old stars that have evolven
past giant of supergigant phases. They are very old, unlike their
cousins, open clusters, they are perhaps about 14 to 16 billion
years old. But, in the other local group galaxies, like Large and
Small Magellanic cloud, also young globulars exists.
These galaxies also contain extremly large
diffuse nebulae, like Tarantula Nebula (NGC 2070). Our galaxy has
a system of perhaps 200 - 300 globular clusters, but most other
galaxies contain several thousands of globulars ( M 87). Though
M 13, located in the constellation of Hercules, is probably the
most famous globular, there are many other beautiful and even brighter
globulars to be found. The greatest globular cluster in our galaxy
is Omega Centauri NGC 5139, situated low in the southern hemisphere.
The closest globular star clusters are M 4 and NGC 6397, also situated
in the southern hemisphere, at the distance
of about 7 000 light years. M-4 in Scorpius is the closest globular
cluster
 H-R diagram
for globular cluster M-5. at the distance of 6800 light years. Globular
clusters are situated in the gallactic halo.
 G1 (Mayall
II)
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