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
----------------------------------------------
  basketball                 3
  
  baseball                
   
  soccer 

  cycling





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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 the Moon.



  Size of Earth:                about      10,000 km      yes, I know D = 12740 km 

  Distance from Earth to Moon:  about     400,000 km      yes, yes, 384,000 km

Therefore, the separation between planets-and-their-moons, in terms of the size of a planet, is something like


                      400,000 km
   planet-moon:     ---------------    =       40         fine, it's 30, you cheaters
                       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-moon                    40

  planet                               
  
  star
   
  galaxy 

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Take a look at my values

Okay, so what sports are would these celestial bodies play?



   Sport                 Separation        Played by 
--------------------------------------------------------
  basketball                 3
  
  baseball                
   
  soccer 

  cycling





                      5,000,000               stars




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