It's an estimate based on the average number density and volume.Originally posted by ShadowKnight
I was looking at some maps of the known universe (http://www.anzwers.org/free/universe/) and a question occured to me - how can astronomers say how many stars are in a certain galaxy? What method is being used to count them? Is it direct observation or is it some kind of formula?
Originally posted by chroot
It's an estimate based on the average number density and volume.
Originally posted by ShadowKnight
OK, that makes sense to me. So I wonder if astronomers take any dark matter into consideration when they make their estimates. I've heard compelling evidence that it exists (though unproven?) On one map of the galaxy (http://www.anzwers.org/free/universe/milkyway.html) it shows this data in a table:
Number of stars in the Galaxy: 200 billion
Mass of the Galaxy: 1 trillion solar masses
Ignoring any dark matter this would mean that they came up with an average of 5 solar masses per star. How do gas clouds and nebula and other non-stars in the galaxy get figured into this?