10^24 (a trillion trillion) = number of stars in the universe = number of atoms in a cubic foot of air

First, go look at this picture of a tiny patch of the sky at night. Be amazed: those aren't stars – those are galaxies.

Consider that there are estimated to be a trillion (10¹²) galaxies in the universe:
1000000000000

And that each galaxy contains about a trillion (10¹²) stars:
1000000000000

So how many stars are there? About a trillion trillion (10²⁴):
1000000000000000000000000

This is hard to imagine. In our everyday experience, do we ever come across a trillion trillion of anything? Actually, yes – there are about a trillion trillion molecules in a cubic foot of air:
1000000000000000000000000

Another way to remember this is that it is roughly the same as Avogadro's number (about 6x10²³), or the number of atoms in a gram of hydrogen atoms.

(Pondering immense quantities like these can make an atheist or a theist out of a person, the latter particularly if you accept Aquinas’s proofs that God is, among other things, existence itself.)