(Abdul Ahad) wrote in message . com...

I know bright galaxies and quasars produce ample quantities of light
for spectroscopy, but surely the multiple stellar make-up of these
objects produces meaningless 'noise' at that level...

Not at all! First of all, most of the light from quasars is *not*
from multiple stars, but from the active core itself -- a single
source. Second, it doesn't matter if the light of a galaxy comes
from multiple stars, as long as those stars are all doing the same
thing. When you are measuring red shift, you are looking for certain
spectral lines. Assuming no radial motion to or away from Earth,
all stars in the universe will put those lines in exactly the
same place, so the spectrum of a galaxy would look more or less
like the spectrum of a single star multiplied by a few billion.

Now in fact, galaxies rotate, so that even if the galaxy as a whole
is at rest w.r.t. the Earth, the spectral lines are "smeared" due
to the fact that the stars at one edge are moving towards Earth
and the stars at the other edge away. But when you are looking
at cosmologically significant distances, galaxies as a whole are
moving away from Earth at a large fraction of the speed of light,
which is *far* greater than the rotational speed of a galaxy.

As for other people's comments, barring the ability to measure the
energy of individual photons -- which is *not* currently possible
in the visible spectrum -- of course you need more light to do
spectroscopy than to do simple photography. How much more depends
on how finely you want to resolve those spectral lines.

- Tony Flannders