# What kind of sports would galaxies play?

Yes, I'm serious. If galaxies wanted to play sports, what sort of sports would suit them best? And stars -- what sort of sports would they play? What about planets?

These may sound like crazy questions, but, if we look at them in one particular way, we may actually learn something important about the nature of stars, galaxies and planets. Just follow along and see.

#### One way to classify sports: the separation of its players

There are plenty of sports in the world. We could divide them based on the number of players on each team, or the condition for winning, or the type of equipment used; but instead, let's choose this way to characterize a sport. We'll measure the typical separation between players, in terms of the player's size.

So, for example, look at this basketball game:

I'll use the player in blue to measure the height of a typical player:

Now, how far apart are two teammates? Let's choose player number 3 on the white team, who is about to catch the ball, and the player on the white team who is waiting for a pass.

In this case, the separation between the two players is about 2.5 times each player's height. We could therefore describe basketball as a game in which

```
basketball: separation = 2.5 times player size
```

What about other sports? Can you estimate the typical separation for players of these activities?

```

Sport                 Separation
----------------------------------------------

baseball

soccer

table tennis

rock-paper-scissors

marathon running

-----------------------------------------------
```

Okay, so what does this have to do with galaxies? Well, it turns out that there are many differences between the types of objects which astronomers study in space. Consider the following classes of object:

• Planets in our solar system, like Saturn or Mars

• Stars, like the Sun or Sirius

• Galaxies, like the Milky Way or M31 in Andromeda

Image courtesy of Rob Gendler

One way that we can describe these objects is by finding the typical ratio between members of each class, in terms of the typical size of member. So, for example, we could pick the Earth and Mars.

```

Size of Earth:                     about 10,000 km

Distance from Earth to Mars:  about 100,000,000 km

```

Therefore, the separation between planets, in terms of the size of a planet, is something like

```
100,000,000 km
Planets:      ----------------    =    10,000
10,000 km

```

Can you fill in the rest of this table? You might need to look up some of the values ....

```

Object                Separation
----------------------------------------------
planet                     10,000

star

galaxy

-----------------------------------------------
```

Take a look at my values