Astronomy is the branch of physics which deals with heavenly bodies.

Solar system: Solar system is that , where the planets move round the sun.

The Star:The Star is that , which has its own light. Eg: The Sun.

The planet: Planets are those which have no own light but get light from the star. Eg: The earth, The Jupiter, etc. It moves round the star.

The satellite: The satellite moves round the planet and it has no own light but gets light of others.

 Gravitation & Gravity:

Gravitation: Attractive force between any two bodies in the universe.

Gravity: Attraction between two bodies which one is heavenly and one is normal body.