If I had a long stick that was millions of light years long, and with a width of about the Sun, and I then swiped it through the center of the milky way galaxy, what would the probability be of me hitting a star?


  • Ignore the speed of light. Assume it is possible to just swipe it through instantaneiously.
  • Assume that the stars in the galaxy are equally distributed. Obviously if you were to swipe right through the middle you would hit the galactic center.
  • The purpose of this thought experiment is just to help visualise the density vs size of the galaxy.
  • $\begingroup$ my own gut feeling would be about 1%!!! $\endgroup$ – Oliver Watkins Feb 17 '16 at 8:53
  • $\begingroup$ Googling average distance stars galaxy points you to e.g. Exploring the interstellar medium stating the average distance between stars is over four light-years. You can do your calculations form there (Sorry, I don't have the time for that). $\endgroup$ – user1569 Feb 17 '16 at 10:55

For your experiment, you can take it 2D, so let's ingnore the thickness of Milky Way.

So, we have 400 billions stars(the real number is between 100B-400B) in a circle with 100k ly(the real number is between 100k-160k).

The area of the circle is pi*50k*50k ~= 7800M sqly = 7.8 billions sqly.

We have then ~51 randomly stars per square ly.

But you do not sting the Milky Way with your stick, but cut through it.

If you swipe through the middle, you have 51*100k stars in a width of 1 ly.

So, you have a chance to hit a star per 1/5.100.000 ly which means 1.855.000km.

But the diameter of sun(of your stick) is 1.395.000km. Taking diameter of sun as medium diameter of a star(this is not true), you have a chance of 1.4/1.8 = 77% to hit a star. If you go with the "low density" numbers, 100B stars and 160k ly diameter, you'll get ~31%.

My conclusion is you often hit a star.

  • 2
    $\begingroup$ I concur. Nice calculation. $\endgroup$ – ProfRob Feb 17 '16 at 14:23

Well Urmmmmm... its a weird question but to put it one way, you wouldn't hit any as our lovely black hole would consume this stick before you could hit any. Apart from that the chance is actually high, there are so many stars in our galaxy(and even more near the centre) that you could hit on. But lets say if you to count the black hole, but it didn't get consumed, it would take so long (because of the black holes gravity) that you would die of old age before you managed to swing down through.

Edit anyone who can improve on my theory!

  • $\begingroup$ I added some edits to my question! $\endgroup$ – Oliver Watkins Feb 17 '16 at 8:52
  • 2
    $\begingroup$ There are so many stars in our solar system? I don't believe you! $\endgroup$ – Featherball Feb 17 '16 at 9:11

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