What do stars and hydrogen bombs have in common? They're both powered by nuclear fusion.
Nuclear fusion occurs when the nuclei of two or more light atoms, like hydrogen, combine to create one big nucleus, like that of a helium atom. This process also results in the conversion of mass into energy.
In order for nuclear fusion to occur, protons and neutrons must be exposed to temperatures approaching 100 million degrees Celsius (180 million degrees Fahrenheit) which, to put things in perspective, is even hotter than the sun.
Under normal conditions, a positively charged atomic nucleus keeps other positively charged nuclei at bay. But during nuclear fusion, nuclei are brought very close together, which allows the attractive forces holding them together to override the repulsive forces keeping them apart. 

When nuclei fuse together, part of their mass is converted into energy. The sun and other stars convert this energy into light. And the same fusion process gives hydrogen bombs their destructive energy. Scientists are continually looking for ways to harness the energy produced during nuclear fusion into safe and practical uses.
This is a very different process from nuclear fission, a reaction in which a large nucleus breaks apart into two smaller nuclei, releasing a tremendous amount of energy.
Researchers believe the first instance of nuclear fusion on this planet occurred just minutes after the Big Bang, in a process known as Big Bang nucleosynthesis.