A Fermi-question: Are there more stars than grains of sand?

Question

This claim is pretty popular in the web and obviously cannot be answered by counting grains or stars. Thats what physicists call a Fermi-question. It deals with problems/questions where we dont have at all/enough measurement data to calculate a exact result, but can make a approximation based on plausibility, general knowledge and common sense.

When trying to answer this question give out a approximated number of grains of sand on earth and stars in the universe, the assumptions the approximation is based and the rough error of your approximation so everybody can compare the quality & plausibility of different answers.

_{PS: the more upvotes this questions gets the more detailed i expect the answers. Remember, we have a nice LaTeX formula renderer here ;)}

Answer 1

There is a great answer (with references) to this at http://answers.google.com/answers/threadview/id/539329.html which I'll summarize as follows:

From http://www.hawaii.edu/suremath/jsand.html the estimate for the grains of sand is 7.5 x 10^18 or 7.5 billion billion.

From http://www.faqs.org/faqs/astronomy/faq/part8/section-3.html the estimate for the number of stars in our own galaxy is between 2 x 10^11 and 6 x 10^11 stars.

From http://www.faqs.org/faqs/astronomy/faq/part8/section-4.html the lower end estimate for the number of galaxies in the visible universe is 8 x 10^10 galaxies.

So:
Sand grains: ~7.5e18
Stars (low estimate): 2e11 * 8e10 = 16e21

That gives ~2000 grains of sand per star for the low estimate of the number of stars. Although, isn't the Milky Way on the medium to big size as far as galaxies go? If so then multiplying the number of stars in our galaxy by the number of galaxies isn't correct. However, the number used, 2e11, was on the low end so and even using something like 1e10 as an estimate of the average number of stars per galaxy still gives ~100 stars per grain of sand.

"Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." -- Douglas Adams