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To: thames_sider who wrote (25773)	4/23/2003 7:45:45 PM
From: lurqer	Read Replies (1) \| Respond to of 104191

More ramblin! Condensing out of some maternal cloud, stars can have a variety of masses. The more massive the star, the more rapidly it burns up its fuel and the hotter (bluer) its spectrum. In fact, the very bright O and early B stars burn through their fuel in only a few million years. Therefore, they never get very far from where they formed before they cease to be OB stars. Early photographic plates were preferentially blue sensitive. So the pictures of S and SB galaxies exhibiting the spiral arms were showing where OB stars (the ones that make up most of the blue light of a galaxy) were born. If today, photographic emulsions that are sensitive to other regions of the spectrum are used to photograph a S galaxy, you don't see any spiral structure. That's because green or yellow or orange or red stars move far away from their birth places before they change. So you can think of a S galaxy as having clouds of interstellar gas and dust. Passing through these clouds is a spiral shockwave that can initiate contraction that can yield stars of all masses. A snapshot in blue light will show very young stars near where they were born along a spiral pattern. As I'm sure you know, these patterns can be quite spectacular as some of the Hubble photos depict. So do stars in the spiral arms not tend to travel overall with similar velocity? No. The ones near the center are moving much faster than those further out - Keplerian motion. lurqer