Fusion is the process that powers the sun and the stars. In the sun, it is the reaction in which four hydrogen nuclei combine together (in several intermediate stages), or fuse, to form a helium nucleus. The mass of the helium nucleus is less than the four hydrogen nuclei, and in the process this mass difference gets converted into energy (E = mc2). Fusing 1 kg of hygrogen into helium releases 650 trillion Joules of energy.

The picture at left shows the successive burning of the star which is 18 times the solar mass, shown at the hydrogen burning stage (topmost). It emits energy continuosly which comes at the expense of its mass. Once most of the hydrogen has been fused into helium, the star has gotten hotter and helium buring starts to produce carbon. Then carbon buning (fusion) starts. At each stage the temperature rises, mass of the star is transformed into energy, and the next stage of fusion (of the next heavier element) proceeds. The very last stage, if the star is massive enough, is iron (Fe). After this stage, the fusion reactions do not produce energy at an expense of mass. Once the star is depleted of its fusion-sustaining fuel, death comes to the star, large or small.

The death of a star is as full of splendor as is its life. Particularly spectacular are the deaths of the supergiant stars. Once the fusion fuel is pretty much exhausted, the lack of burning removes the radiation pressure generated by the fusion in the star's core, which was holding the outer layers, wanting to crush into the core because of gravity, at bay. So, now this causes a giant collapse. In the process a huge shock is generated, and the whole outer layers are blown off in a gigantic explosion, which can spread over several gallaxies. This is what we know as a Supernova.

The supernova observed in 1054 AD (Crab Nebula) 6000 light years away.

The remnant of the supernova observed by Tycho Brahe in 1572 (satellite photo taken in 2000).