# 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.

## Mind-Boggling

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.

# Notes

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 / swmcd@world.std.com / 1999 August 13