Q&A: General Astronomy and Space Science

Q:
I just watched a program on space which quoted that for every grain of sand on the Earth there are a million stars in space. Is this true, and if so, how do we know this?

A:
We can assume that the program was referring to the number of stars in the observable universe -- not to be confused with the number of observable stars, since astronomers can observe many distant galaxies that contain billions of stars, but can't resolve the individual stars. From the brightness of the galaxy, and knowing the brightness of an average star at the distance of the galaxy, astronomers can estimate the number of stars in a galaxy. The number of galaxies can be estimated from deep surveys of selected regions of the sky. When these estimates are combined, the number of stars in the universe out to about 13 billion light years, assuming that the average star is about half as massive as the Sun, is in the range of a few hundred million trillion: say 300,000,000,000,000,000,000.

A medium-sized grain of sand has a diameter of about 0.3 millimeters, so allowing for a little air space, a sand dune 20 kilometers square and 200 meters deep would contain about 300 million trillion sand grains, or about the same number of stars in the observable universe.

"The Earth teaches us more about ourselves than all the books in the world, because it is resistant to us." Antoine de Saint-Exupery Wind, Sand and Stars, Penguin Books 1995