- A galaxy is held together by gravity
- A galaxy is made of stars
- Therefore, a galaxy is made of stars that is held together by gravity

- A galaxy is made of stars that is held together by gravity
- A galaxy is a kind of very large system of stars
- So, a galaxy is a large system of stars that is held together by gravity

Therefore, a galaxy is a large system of stars that is held together by gravity
What is a galaxy?
- A galaxy is held together by gravity 
- A galaxy is made of stars 
- A galaxy is a kind of very large system of stars