# Big Bang and the Stars

1. Jul 2, 2009

### Pupil

So my first question is, how do we know how many stars are in our galaxy? And the second is, how do we know the big bang started with a singularity (as opposed to, say, the universe being a few light years wide and then start expanding 13.7 billion years ago until it reached our present size)?

The reason I ask is, I was watching this History channel video on the Big Bang called The Universe (http://www.youtube.com/watch?v=D6CXsvNGkhg&feature=PlayList&p=8AB46D948616D856&index=0), and though I knew a bit about the big bang and the number of stars we think there are, I have no idea why we think so. Kind of embarrassing.

2. Jul 2, 2009

### math_04

Of course we cannot know how many stars there are in our galaxy, that is like counting how many sand particles are there on the beach. If you go backwards in time, using general relativity, you get to a point of infinite density and temperature. That is what we call a singularity.

Let me guide you to a good, non technical website to learn about the early universe.

http://www.talkorigins.org/faqs/astronomy/bigbang.html#evidence

3. Jul 2, 2009

### negitron

No, but I think he means how did we arrive at the current estimate of around 100 billion stars. Obviously, we didn't count all those; instead, we took samples from various parts of the galaxy and extrapolated from those samples to arrive at a first-order approximation of the total number. We can do the same for the number of sand grains on a given beach or the number of ants on the island of Hawaii.

4. Jul 2, 2009

### Pupil

Thanks for the link. It's a lot of material, so I might be awhile getting through it. As for the counting of stars in our galaxy, I hear we have very good estimates (though not exact numbers). How is our estimate calculated?

EDIT: Looks like negitron beat me to my question and answered it. So we basically took a picture of a small area of the galaxy, counted the stars inside, and then assumed that number as a constant for all other parts of the galaxy and added the parts up?

5. Jul 2, 2009

### DaveC426913

Well, the galaxy is not homogenous of course, there are whole swaths almost devoid of stars and other areas where they're practically elbow-to-elbow, but yeah...

6. Jul 2, 2009

### negitron

Well, more than one, as I alluded to. As Dave says, the galaxy isn't homogenous: the core is densely-packed with stars, while the spiral arms are less so and the interstices between the arms are even more sparsely populated. But, you have the general idea about right.

7. Jul 3, 2009

### Chronos

Our sun resides almost edge on to the galactic plane, so we have a terrible view of the galaxy. Our sun is also very average in brightness. Most such stars would be difficult to detect at 10,000 light years, much less 100,000 light years. About half the stars in the galaxy are even fainter than our sun. We would be lucky to observe more than about 15% of stars in this galaxy under the best of circumstances.

8. Jul 3, 2009

### DaveC426913

Individually, no. But en mass, not so hard.