What are stars made of? how do they expand? how do they explode?

Stars are made of very hot gas. This gas is mostly hydrogen and helium, which are the two lightest elements,  star is in a delicate and fragile balance between gravity, which would collapse the star to a point, and its internal pressure caused by the energy of the nuclear fusion reaction in the core. When a star's fuel has been spent, it explodes into a giant fireball known as a supernova.. I really hope this helps

The Stars are made up of large amounts or Hydrogen 
Hydrogen Fusion 
The star runs out of Nuclear Fuel and the core gets so heave that it cant stand its own gravitational force and explodes 

The particles of the gas inside the ball heats up, bouncing off each other and spreading farther apart.hope my answer has you : )

It maybe because uranus is made of gas but uranus has a solid core in the middle