Frequently Asked Questions About Stars Frequently Asked Questions About Stars Compiled by Dr. John Simonetti of the Department of Physics at Virginia Tech . Fission has nothing to do with it.
Fusion does, however.
Fusion is the joining of nuclei to make larger nuclei, with the release of energy. A ball of contracting interstellar gas becomes a star like the Sun when fusion reactions start in its center. No specific gases are actually needed. It is hydrogen that is fused at the center of the gas sphere when the gas sphere becomes a star. Just about any gas will do.