Q&A: General Astronomy and Space Science
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?
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