Answer:
1. The Sun is mostly made up of hydrogen. The planets are not
2. The Sun (and all other stars) under go a process called "nuclear fusion". Which is what produces a star's energy. Planets do not do this.
3. Stars go through drastically different phases in their life. Starting as a Nebula that condenses into a star, that star then turns into a Red Giant. It explodes in a Supernova then becomes a White Dwarf or Black Hole depending on it's size.
Step-by-step explanation: