Why does the fusion of hydrogen in stars release energy?

22nd July 2009

Asked by: Frank Davies, Doncaster

Our Sun churns out energy at a rate equivalent to a billion billion nuclear power stations. The fact that it’s done this for five billion years, and is good for at least as many again, is the result of a powerful combination: the Sun’s vast reserves of hydrogen fuel, plus Einstein’s famous equation E = mc², relating energy and mass. The sheer mass of the Sun crushes the atoms of hydrogen within it under gravity, forcing the pressure and temperature up to the point where the nuclei of hydrogen atoms fuse together. A chain of nuclear reactions then takes place, creating nuclei of the next heaviest chemical element, helium. And this is where Einstein’s equation comes in. The mass of this helium ‘ash’ is slightly less than that of the original hydrogen nuclei, some of the mass having been released as energy. The difference in mass is very small – amounting to just 0.7 per cent of the mass of the original hydrogen atoms. But, because the ‘conversion factor’, c², is the square of the speed of light and thus a huge quantity, even this small mass difference is enough to release huge amounts of energy per hydrogen atom.