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From: rdc401@nmrdc1.nmrdc.nnmc.navy.mil (CDR Peter Kent)
Newsgroups: sci.electronics
Subject: Re: How to tell distance of Stars
Message-ID: <1991May15.114054.9013@nmrdc1.nmrdc.nnmc.navy.mil>
Date: 15 May 91 11:40:54 GMT
References: <8937@crash.cts.com> <17660159@hpfcdj.HP.COM>
Organization: Naval Medical Research & Development Command
Lines: 30

The Doppler red-shift method has been used for decades to determine the
distance of stars from earth, but can anyone out there explain how to 
correct for the following scenario?:

     Postulate 2 stars, each one in a galaxy that is roughly the same distance
from earth (the average magnitude of red-shift for all stars in that galaxy
is equal for both galaxies, although there will of course be differences in
measurements for any specific star within the galaxy, depending upon it's
location within the galaxy).  Now, postulate that one of the galaxies is
rotating around its own central axis (i.e. the stars that comprise that galaxy
are rotating around that galaxy's center) much much faster than is the
galaxy containing the second star.  Now, further postulate that in one case,
the star is rotating such that it is moving closer to the earth, even as it's
galaxy is moving away from our galaxy (the Milky Way), and the other star is
moving away from the earth as it rotates around it's own galaxy's center, 
even as it's galaxy is also moving away from us.    
     It seems to me that there would be a considerable difference the the
magnitude of red shift for these two stars even though we have postulated
that the two stars are actually the same distance away.  
     So, in summary, it seems to me that it is possible for two stars with
considerably different red shifts to really be the same distance from the
earth.  Can any one out there either explain why it would not be so, or
confirm my suspicion that red-shift is not accurate, but just better
than anything else astronomers have in their toolbox?  Peter  
    
-- 
CDR Peter Kent, MC, USN
Program Manager, Diving and Submarine Medicine
Naval Medical Research and Development Command
E-mail: rdc401@nmrdc1.nmrdc.nnmc.navy.mil