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Just to think about this is mind boggling. But how do scientist get these numbers? What technology/system/theory do they they use?

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I've always heard "100 billion stars in each of 100 billion galaxies", rather than 3 billion stars. –  Moriarty Dec 6 '13 at 22:31
    
Actually that was supposed to be 300 billion, I will edit. If you type "how many stars in a galaxy", in Google search, it says 300 billion. But whether its 100 billion or 300 billion, it still is mind boggling!! –  cosmic Dec 6 '13 at 23:01
    
We really only know that number to an order of magnitude anyway, so 300 vs 100 is no real difference. I don't know the scientific basis for these guesses, though I would be interested to find out. –  Moriarty Dec 6 '13 at 23:08
    
I have always wondered about this question. Thanks for asking. –  Unicorn Dec 7 '13 at 9:32
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They have graduate students that count all of them. –  Mark Adler Dec 9 '13 at 7:41

1 Answer 1

It's a matter of statistics.

Scientists take a small amount of the space (let's say 1 second of arc). They look at it carefully with strong telescopes, and count all the stars and galaxies they see. Then, they extrapolate that number at the total visible space.

Of course they can compute several spots of the space and make an average count.

Since the number is extrapolated, that's why it doesn't really matter if it is 100 billion or 300 billion of stars. The goal is to have an order of magnitude as pointed by Moriarty.

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I would be interested to know how they extrapolate, the exact "statistical" technique. I do not think, however, counting number is statistics. But I do not know what name I would give to counting numbers. –  cosmic Dec 12 '13 at 4:20
    
@cosmic Statistics is the "study of the collection, organization, analysis, interpretation and presentation of data." Of course counting things is part of statistics. Taking a sample population and inductively inferring from samples the parameters of a larger population is a well-used statistical technique. –  ghoppe Jan 22 at 4:46

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