a moon that orbits a planet a planet that orbits the Sun a very large system of stars that is held together by gravity a very large system of planets that is held together by the Sun
I gave my students this multiple choice question: What is a galaxy?

Only one answer is correct among these 4 choices:
- a moon that orbits a planet
- a planet that orbits the Sun
- a very large system of stars that is held together by gravity
- a very large system of planets that is held together by the Sun

Could you tell me which one is correct?

a very large system of stars that is held together by gravity