Wednesday, June 3, 2015

cosmology and chemistry

can you estimate how many stars there are in the observable universe and give your answer in units of Avogadro's number?

4 comments:

  1. 100 billion stars per galaxy, 100 billion galaxies, that's 10^22. Allow for some we might have missed and it comes out pretty much exactly Avogadro's number.

    Next question: We understand nuclear matter, so, ignoring gravity, compress all of these stars to a ball with the density of a nucleus. How large is the ball? Guess first, then calculate.

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  2. thanks a lot for your reply, Phillip! my estimate would be very similar, by computing the mass in the Hubble volume $M=4\pi/3\rho_\mathrm{crit}\chi_H^3$ with the Hubble length $\chi_H$. that's roughy $10^{52}~$kg, resulting in $10^{22}$ stars of the mass of the Sun, which is $10^{30}~$kg. concerning your question: if I didn't make a mistake, the ball should be as large as the Sun, which would be surprising!

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    Replies
    1. It depends on the exact input numbers and details of the calculation. A back-of-the-envelope calculation resulted in a radius about the same as the orbit of Mars. I'm sure that even this is much, much smaller than most people would guess.

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  3. and I need to add that I'm very fond of the fact that there's roughly one mole of stars in the observable universe!

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