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From: nelson@avsdS.UUCP
Newsgroups: net.physics
Subject: re: Clusters of stars
Message-ID: <499@avsdS.UUCP>
Date: Fri, 17-Jun-83 19:47:41 EDT
Article-I.D.: avsdS.499
Posted: Fri Jun 17 19:47:41 1983
Date-Received: Sun, 19-Jun-83 15:17:23 EDT
Lines: 36

It is quite natural for stars that are born in clusters to drift apart.
In our galaxy there are 2 basic types of clusters - "open" clusters
and "globular" clusters.

Globular clusters occur mostly closer to the galactic center than these
boondocks of the galactic arms that we live in. Average stellar densities
(as I recall) are 1 per cubic light year. The number of stars in a globular
cluster ranges from 10's of thousands up to ~10^6. There is ample gravity
for them to stay together. Last I heard globular clusters were thought
to swing in and out of the galactic center like a rock dropped in a
hole that goes through the center of the earth. The most famous cluster
is M13 in Hercules - can be seen with the naked eye, eaasy with binocs.
Numerous clusters can be seen near the galactic center by looking in
Sagittarius with binocs.

Open clusters usually contain no more than a few thousand stars,
typically 500-1000. They are "loosely bound", average densities of
1 per few light years (still greater than near Sol). They will stay
near each other as they evolve through young stages, but external
forces and presumably internal turbulence and pressures will eventually
lead to a dissolution of the cluster. The spiral arms are populated with
these clusters. Random variations in motion lead to a drift that is great
enough to separate us from our sibling stars by an entire galaxy as
Sagan claimed. After all, the galaxy rotates every 250 million years,
so we've been around it 20 times! A quick look at stars near us will
show you that there are significant variations in velocity.
The most prominent open cluster is the Pleiades, a cluster of about
500 stars laced with glowing gases. The Pleiades is most easily seen
when not looking directly at it! It shows as a prominent glowing patch
in your peripheral (more sensitive) vision. When you look at it the glow
disappears and you see 6 (7 with binocs) rather unremarkable stars
in a tiny dipper pattern.

I hope this has assuaged some doubts and informed you.

	Glenn Nelson, Ampex, Redwood City