nelson@avsdS.UUCP (06/18/83)
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
band@hao.UUCP (06/20/83)
Just a small correction to your comment that the Pleiades contain
about 500 stars. They, in fact, are some 3000 strong!! No little
open cluster the Pleiades!
Peter Bandurian
ucbvax!hplabs!hao!band
decvax!brl-bmd!hao!band
seismo!hao!band
PS: Reference: E. Hertzsprung, 'Catalogue of 3259 stars in the Pleiades',
Leiden Ann. 19 Part 1(1947).
The definition of membership was showing the appropriate proper motion.