How Do Astronomers Accurately Count Stars in the Universe?

[ShadowKnight]

I was looking at some maps of the known universe (http://www.anzwers.org/free/universe/) and a question occurred 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?

 

[chroot]

Originally posted by ShadowKnight
I was looking at some maps of the known universe (http://www.anzwers.org/free/universe/) and a question occurred 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?

It's an estimate based on the average number density and volume.

- Warren

 

[ShadowKnight]

So for example, in some far off area where astronomers are estimating there are 1 million stars in a cluster, it very well could be a few massive stars?

 

[marcus]

Originally posted by chroot
It's an estimate based on the average number density and volume.

- Warren

this is a pretty interesting question

the MASS of a galaxy can be estimated by its speed of rotation
which can be told by doppler
at various distances from center

there is a kind of rotation speed curve that plots speed of rotation compared with distance out

the mass can be told from this rotation curve

and the mass is related to the number of stars
but the relationship is not a completely simple one because of the
existence of dark matter

a few years back an astronomer told me that it was believed that the average mass of star in Milkyway was 1/2 solar mass. That may be true or it may not be----estimates change.

If Milky mass is 1 trillion solar (told from rotation speeds) and if there is no dark matter (untrue!) and if the average star is 1/2 solar-----then Milky has 2 trillion stars.

But those numbers are just "for example let's say" type numbers.

I think its interesting and could be basis of some more discussion.

Warren (chroot) teaches astro, I believe, so he may have quite a bit more to say about this.

 

[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?

 

[marcus]

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?

tho not an expert I will tell you how I understand those numbers

I feel very sure that astronmers have seen enough stars they have come to the conclusion the average one is 1/2 solar mass

so the guy with the numbers is telling you that Milky is LOADED with dark matter

he measures it at 1 trillion solar (which is in line with many estimates I have seen over the past 10 years, it is not a bad figure based on rotation speeds which can be measured by radioastronomy of neutral hydrogen even in parts of Milky which we cannot see optically)

But he says that of the 1 trillion solar masses only about 100 billion solar masses belongs to the (estim. 200 billion) stars!

He is telling you that he thinks only a TENTH of the mass of Milky is stars.