Expert: Philip Stahl - 11/21/2007

Question
If a galaxy is rotating (say around black hole) don't stars one edge of the galactic plane  have blue shift and stars on the opposite end have red shift And with good measurements we can compute  rotational velocity.Ignoring rotation of star on its own axis

Answer
Hello,

I think two different versions of red and blue shift are being confused here.  Yes, IF one is confining observation to stars in the disk - say of the Milky Way - and these are seen in rotation about a central mass (whatever it arises from) then stars approaching will be see to exhibit a blue shift while those at the opposite side end will be seen to be receding - hence their departing waves will get longer and longer. Provided the data is good, and assuming one has an edge on view, yes, one can perhaps estimate the rotation velocity of a galaxy.

But this doesn't mean all galaxies will "exhibit" such data, mainly because galaxies visible to us will occur in a variety of orientations and no more than a small fraction will be seen edge on - hence, allow red and blue shifts of their member stars to be found.

Also, bear in mind most galaxies occur in galaxy clusters. For example, the Milky Way occurs in the Local group of 20-22 galaxies. One should not confuse apparent motions of member galaxies -toward or away from us- with the example you gave of STARS INTERIOR to galaxies exhibiting blue and red shifts.

Neither should these be confused with the fact that all galaxy clusters themselves exhibit only RED shifts, in receding away from us - which is how we deduce their distances from the Hubble law (e.g. v = H d) where v is the recessional velocity obtained from the red shift, H is the hubble constant, and d is the distance)

This is all by way of clarification since your subject header didn't seem very clear.