Astronomers say there are more stars in the universe than there are grains of sand on all the beaches on earth. How do they know how many grains of sand there are in the whole world?

In: Mathematics

Follow up question. If there are seven quintillion stars, why is the night sky black and not white?

How do they know how many stars there are in the universe? It is an ever-changing number. As we see more distant galaxies we have to increase our estimation of stars.

According to astronomers, there are probably more than 170 billion galaxies in the observable Universe, stretching out into a region of space 13.8 billion light-years away from us in all directions.

And so, if you multiply the number of stars in our galaxy by the number of galaxies in the Universe, you get approximately 10^24 stars. This is a minimum value based on the observable universe.

The Universe could be much bigger. It’s even possible that the Universe is infinite. What defies belief though is that the game of chess still has more moves – 10^120, as shown by the [“Shannon Number”](https://en.wikipedia.org/wiki/Shannon_number)

Posters have offered a number of explanations and math. But what it really comes down to (just in my opinion) is… everybody’s guessing. They’re guessing about the sand and they’re guessing about the stars. Sure, they may be educated guesses. But you know what, these are really big numbers no matter how you do it, and it really doesn’t matter whether there’s more sand or more stars because it doesn’t matter and because we can’t really get our heads around those big numbers anyway.

In conclusion:

Grains of sand: Lots

Stars: Lots

We can estimate both the volume and mass of the Earth. Take a bucket of sand, weigh it, measure its volume, then count the number of grains of sand in it. You now have an estimate of the number of grains per kg or per litre of sand. You can use that to estimate the maximum number of grains. We could assume the whole world were nothing but sand, take the mass of the Earth and multiple it by the number of grains of sand in a kilogram, and you’d have the maximum possible number of grains. Obviously the whole Earth isn’t made of sand, so we could be a little more careful.

We could assume the top km of the Earths crust was made of sand, and multiple that volume by the number of grains of sand in a litre. This would be closer.

Keep repeating this process and you’d get a ballpark figure of the number of grains of sand on Earth. Not an exact number, but a reasonable maximum.