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?


  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.

  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!

    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.

  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!