# Dumbed-down explanation how scientists know the number of atoms in the universe?

It is often quoted that the number of atoms in the universe is 10$^{70}$ or 10$^{80}$.

How do scientists determine this number?

And how accurate is it (how strong is the supporting evidences for it)?

Is it more likely (logically >50% chance) that the numbers are right, or is it more likely that the numbers are wrong?

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 They don't. They estimate. – OmnipresentAbsence Feb 23 at 22:06

The observable universe contains about 100 billion galaxies, each containing on average close to a trillion stars. That is a total of about $10^{23}$ stars. A typical star is like our sun. Sun has a mass of about $2×10^{30}$ kg, which equates to $10^{57}$ atoms of hydrogen per star. A total of $10^{23}$ stars containing $10^{57}$ atoms each gives us a total number of atoms of $10^{80}$.
But how is it possible that "each containing on average close to a trillion stars" could be verified reasonably enough, for us to say with enough certainty and conviction that there's over 50% chance that it is correct? For all we know, out of this 100 billion galaxy there could be a single galaxy that already has over $10^{23}$ stars... just curious. – Pacerier Dec 30 '12 at 18:34
And if there's logically less than 50% chance that these numbers are right, isn't it more reasonable to say "the number of atoms in the universe is unknown" than to say "the number of atoms in the universe is ~$10^{70}$"? – Pacerier Dec 30 '12 at 18:38
@Pacerier - what makes you say these numbers are more likely than not correct? It occurs to me that the confidence interval for the numbers of atoms in the universe to be within say $10^{78}$ and $10^{82}$ certainly exceeds 50%. – Johannes Dec 30 '12 at 18:44