What element is the fuel for main-sequence stars? What will be ...

[NClaborn]

What element is the fuel for main-sequence stars?
 
  What will be an ideal response?

Question 2

Describe how main-sequence stars become giants.
 
  What will be an ideal response?

[owenfalvey]

Answer to Question 1

Hydrogen.

Answer to Question 2

Once the usable hydrogen fuel is all burned up, there remains a helium-rich core. Once fusion is finished in the star's core, there is insufficient pressure to prevent collapse due to gravity, and the contraction generates a huge increase in internal temperature, triggering a higher level of hydrogen fusion in the star's shell. The outer shell expands from the heat of accelerated fusion, but this expansion simultaneously cools the star's surface, yielding a large, cool star.