All stars turn hydrogen into helium...

• A cloud of gas becomes a star when nuclear reactions (changing hydrogen into helium) start in the core of the star.

• A star's core has to be pretty hot in order to start these reaction, but these reactions create even MORE heat.

• This energy heats up the gas, and the gas wants to expand. But, gravity is pushing it back.

• A star's life is ruled by the battle between gravity pushing in towards the center of the star and gas pressure pushing out.

What happens when a star runs out of hydrogen?