From: phfrom@nyx.uni-konstanz.de (Hartmut Frommert)
Subject: Re: Age of the galaxy (was: stars & carbon)
Organization: Dept. of Physics, University of Constance
Date: Tue, 28 Jul 1992 16:05:33 GMT

sichase@csa2.lbl.gov (SCOTT I CHASE) writes:

>In article <92204.124953DOCTORJ@SLACVM.SLAC.STANFORD.EDU>, DOCTORJ@SLACVM.SLAC.STANFORD.EDU (Jon J Thaler) writes...
>> 
>>This brings to my mind the question, how old is our galaxy?  I suppose that
>>it is older than the Sun, but how much older?  I realize that I ought to be
>>looking in an astronomy text, but posting is so much easier, as well as more
>>fun.  Are all galaxies remnants of the primordial density fluctuations, or
  ^^^
>>are there second and third generation ones?

Especially since that are most interesting questions.

>The Universe is 7-15 billion years old, depending on who you ask.  [..]

If you asked me (or, I suppose, many others), I would first point to the
globular clusters, which are at least 13 to 15 GYs (== billion years) old,
according to their color-magnitude diagrams and the standard theory of 
stellar evolution (oldest Main Sequence stars...).

The values you (Scott) give are calculated on the basis of Einstein's
General Relativity with **neglectable** cosmological constant Lambda.
Assuming this, the Hubble constant *H* gives an upper bound of 
$$
   t_H = 1/H
$$ 
for the age of the universe. The "critical" (spatially flat) case between
open and closed universes occurs at 2/3 this value, which roughly co-incides
with those given by Scott, since

H = 100 km/(s*Mpc) --> t_H =  9.7 GY
H =  50     "      --> t_H = 19.5 GY

With a significant value for the cosmological constant Lambda, you can
circumnavigate this upper bound to arbitrary large ages. So a Hubble constant
near H=100 could also point to a significant contribution of Lambda to the
cosmological expansion. 

Priester and co-workers have recently proposed a cosmological model with an 
age of approx. 30 GY with a closed universe, which they claim to match best
the Lyman Alpha absorption forrest of quasars.

>Recent HST measurements of Cepheid variables in a distant galaxy (can't
>remember which) turn the tables once again in favor of the upper end of the
>scale.  

Up to what distance can HST view Delta Cephei variables (now and after the
proposed optical correction)? Which distance is managed by Keck ? Can they
be resolved in the Virgo Cluster ? 

(This would also give a better understanding of the structure of our
galactic neighborhood, the Local Supercluster.)

[..]

>It's pretty clear that we know the answer within roughly a factor of two,
[..]

That applies only for the Hubble constant. The mean density, e.g., is about
a factor 10 unsure.

>If there are any experts out there - my question is how precise will 
>the current generation of measurements allow us to get and what does the
>ultimate limit of resolution depend upon?

I'll forward this interesting question...
--
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