Expert: Tom Whiting - 10/31/2007

Question
What keeps star clusters gravitationally stable?  Star clusters don't appear to rotate and they are denser toward the center. It would appear that gravity would cause them to collapse in on themselves over the billions of year they have existed.

Answer
Hi Johann,
You really mean "revolve" don't you....not "rotate"...see
the PS at the very end of this message......

Well, what type of star cluster?  Old globular star clusters that surround the hub of the Milky Way, or younger open star clusters that inhabit the Milky Way itself?
Probably the globulars, as they are the ones billions of years
old, whereas most open star clusters are only a few hundreds
of millions of years old....youngsters, by comparison.

But, no matter which one, I guess the answer is still  the same.....FOR the same reason that the 4.6 billion year old Solar System hasn't "collapsed" in on itself either , (or the 10 billion year old Milky Way Galaxy too!)  even though the center...the sun, contains 99.86% of the mass (read that gravity) of the entire Solar System.  The planets orbit the sun at high speeds so the centrificial force of revolution around the center keeps the entire system stable.  (We, the Earth,
are traveling some 18.5 miles per second around the sun).

While the stars in a globular star cluster don't "appear" to revolve much in our short lifetimes, they actually are revolving around a common center....  (there is no such thing as absolute rest- everything in the Universe is moving, relative to everything else)....And the spectroscope can even measure their speed of approach and speed of recession using the doppler effect, or spectral shift.   In fact, the massive black hole in globular star cluster M-15 in Pegasus (Google it)  was determined by the very high speed of revolution of the core stars close into the center of  the cluster.    AND, if all this wasn't true above, then yes indeed, they would have all collapsed on themselves by now.....BUT they haven't, so there you are....the proof is obvious by their current existence,  so Q.E.D....the answer is the centrificial
force of revolution around the center of mass.
Home all this helps,
Clear skies,
Tom Whiting
Erie, PA

PS... astronomers use the words revolution and rotation very specifically...we have to.  Revolution is motion about another body....the Earth revolves around the sun.
Whereas rotation is motion around an axis...also called spin.
For example, the Earth rotates once daily.
NEVER  use one for the other in astronomy....they are, and have to be, very specific terms...never interchange them in astronomy....your trivia for the day.  ;-)