Nuclear energy is released during fission and fusion.

WSJ: The New Nuclear Revolution

Acting alone, gravity would cause stars to collapse completely, but as long as energy is produced at their centers by nuclear fusion (the joining of atomic nuclei to form new elements, as in a hydrogen bomb), the star is heated and puffed up.

CNN: Why gamma-ray burst shocked scientists

The good news is that the big names in nuclear energy -- like Areva, Hitachi, General Electric and Toshiba -- have recently been joined by a bevy of high-tech start-ups seeking to develop advanced nuclear-reactor designs for both fission and fusion energy production.

WSJ: The New Nuclear Revolution