• The energy-producing process encountered in stars as well as in hydrogen bombs, and involving the production of heavier nuclei from lighter ones. The small difference in mass between the starting and final nuclei is converted to energy, which makes the stars shine. Most stellar energy production takes place by the conversion of hydrogen to helium, and these elements account for the bulk of the mass of most stars. This fusion process involves converting just 0.7 per cent of the material involved into energy, but this is enough to keep stars shining for billions of years. The realisation in 1920 that fusion powers the stars ended millennia of speculation about how the stars keep shining, in which every idea was pursued from making them out of burning coal to powering them by kinetic energy released from matter falling into the stars from space. Many of these ideas would have worked but none would allow the stars to shine for millions of years, as the Sun is known to have done from the evidence of geology. Kinetic energy, however, is still regarded as important in the early life of stars, while enough material builds up to produce the high temperatures and pressures needed for fusion.

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