The Galaxy

It's not so big

Our galaxy contains ~100 billion stars. 100 billion is one of those numbers that is so big that it is hard to think about in meaningful terms. Here's a way to cut it into manageable pieces.

The galaxy is a flat disk. Let's square it up and say that it is a pizza box. If the stars are uniformly distributed, then it is 1000 stars high, 10,000 stars long, and 10,000 stars wide:

1000 x 10,000 x 10,000 = 100 billion

and numbers like 1000 and 10,000 are almost small enough to think about.

The stars, like dust - Isaac Asimov

Now let's make it big again.

Suppose each star is a grain of sand. Sand grains range in size from 1/16 to 2 mm. For discussion, we'll say they are 0.5 mm. If we put 10,000 of these side by side, we get a line 5m long, and our pizza box is

5m x 5m x 0.5m
That's a big pizza box. It has a volume of 12 m3, and a mass of 30 metric tons. It is enough sand to fill a dump truck.


Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.
--The Hitchhiker's Guide to the Galaxy
The sun is 1 million miles in diameter, the earth orbits at a distance of 100 million miles, the nearest star is 4 light-years away, and the entire galaxy is 100,000 light-years across.

A light year is 6 trillion (6 million million) miles. If we scale the sun down to 0.5 mm, then a light-year is 3 million mm, or 3 Km, and the speed of light is 0.1 mm/second.

The earth then orbits at a distance of 5 cm, the nearest grain of sand is 12 Km away, and the galaxy is 300,000 Km across. 300,000 Km is most of the distance from the earth to the moon.

A truck load of sand is a lot if you have to sweep it off of your front step, but scattered from here to the moon, it is but the merest dust in the emptiness of space.


uniformly distributed
they aren't
1/16 to 2 mm
Grolier Encyclopedia, vol 16, p. 279
30 metric tons
taking the density of sand as 2.5 g/cm3
dump truck
Reality Check. 30 metric tons is 60,000 lbs. A truck has 16 wheels, so we have 4,000 lbs/wheel. Truck tires run at 100 psi, so each tire needs a contact area of 40 in2, or a footprint of 5" x 8". No problem.

Steven W. McDougall / resume / / 1999 August 13