# 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 m^{3}, 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/cm
^{3}
- 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 in
^{2}, or a footprint
of 5" x 8". No problem.

Steven W. McDougall /
resume /
swmcd@world.std.com /
1999 August 13