•Crossing times: Tcross ~
R/V
•Relaxation time then varies as
•Trelax ~ 0.2(N/logN) Tcross
where N is the number of stars in the system
•For galaxies, crossing times ~1/100 the age of the universe (100 million years), and the relaxation
times are on order of 1017 years
– so close interactions between
stars essentially never happen.
“Collisionless” is a good approximation. This is because N is large, and the collision must be very close for another
star’s potential
to overwhelm the ensemble potential.
•Note: this argument is NOT true for Globular
Clusters! Also not true for galaxy clusters!
•The upshot of this is that you can treat galaxies in terms of motions in a global potential.
–