Guardian of the Galactic Dust

Space isn't empty. It may seem that way, when on average a cubic centimeter of the space between the stars -- a box the size of a six-sided die -- only has a single atom in it. Compare that to the Earth's atmosphere at sea level, which has 1019 atoms per cc! But empty it is not. That density is low, but space is big. If you have enough cubic centimeters, even those few particles start to add up. We call this material the interstellar medium, and it's made up of gas (mostly hydrogen, but with s...