🚀 go-pugleaf

In article <22594@galaxy.ucr.edu> baez@ucrmath.ucr.edu (john baez) writes:


   In article <29SEP199220244986@zeus.tamu.edu> dwr2560@zeus.tamu.edu (RING, DAVID WAYNE) writes:
   >This makes sense. What I don't understand is why a region of high density
   >homogeneity does not recollapse in a time appropriate to that density. I.e.
   >if the universe were filled with stars, in the density of a galaxy, the
   >universe would have recollapsed long ago. The region of space which is
   >inside a galaxy satisfies the same local einstein equation as the above
   >recollapsing example, so why do galaxies live so long?

   'Cause they're smaller.  I'm sure one can work out the math, but
   it's bound to amount to just that.  For example, one can have
   a fist-sized ball of iron, which will essentially never collapse
   (quantum fluctuations aside), but a galaxy-sized ball of iron
   would collapse mighty fast.

   Galaxies are not all *that* dense and they're spinning around (at least
   lots of them).  So they will take a while to collapse.  It's become
   widely accepted that many galaxies have black holes at their centers.
   These black holes are eating stars at a decent rate and should only
   be billions and billions of years before they suck up most of the
   galaxy, or at least the dense inner part.

   Does anyone have estimates on how long this munching process should
   take and how it will proceed?

See Binney & Tremaine and Blandford, Begelman and Rees.
Some characteristic time scales are the relaxation time
scale and the gravitational radiation time scales
for typical galaxy cores. They vary from 10**9 years
for some systems to 10**13 +++ for more ordinary
systems.
Your typical galaxy will stick around for longer than
the evolution time scale of the individual stars.

*  Steinn Sigurdsson   			Lick Observatory      	*
*  steinly@lick.ucsc.edu		"standard disclaimer"  	*
*  The laws of gravity are very,very strict			*
*  And you're just bending them for your own benefit - B.B. 1988*