They are all the same type of galaxy. They all have the same number of stars. They are all attracted to each other by gravity. They can all be seen without the aid of a telescope.
I gave my students this multiple choice question: The Milky Way is a part of 30 or more galaxies called the Local Group. Which would be the most likely reason these galaxies are considered to be part of the Local Group?

Only one answer is correct among these 4 choices:
- They are all the same type of galaxy.
- They all have the same number of stars.
- They are all attracted to each other by gravity.
- They can all be seen without the aid of a telescope.

Could you tell me which one is correct?

They are all attracted to each other by gravity.