Nuclear fusion powers the sun by combining hydrogen atoms to form helium in a process called thermonuclear fusion. At the high temperatures and pressures found at the sun's core, hydrogen atoms collide with enough energy to overcome their electrostatic repulsion and fuse together to form helium. This process releases a tremendous amount of energy in the form of light and heat, which is what makes the sun shine. The energy released from nuclear fusion also provides the pressure that counteracts the force of gravity and keeps the sun from collapsing in on itself.