What makes the sun so hot?

Question: What makes the sun so hot?
- Keywords:
  heat,
  sun
Asked by stellamaris123 to Ed, Hayley, Jason, Nathan, Sophie on 13 Mar 2013. This question was also asked by iluvnibbley456.
- Hayley Evers-King answered on 13 Mar 2013:
  
  Nuclear fusion! The sun fuses hydrogen nuclei together to form helium, this releases a lot of energy (heat!).

Question: What makes the sun so hot?