It is the basic reaction which drives the Sun. Lighter elements fuse and form heavier elements. When a nucleus reaches mass sixty, no more fusion occurs in a star because it is energetically unfavorable to produce higher masses. Once a star has converted a large fraction of its core's mass to iron, it has almost reached the end of its life. The fusion chain cannot continue so its fuel is reduced.
Some stars keep shrinking until they become a cooling ember made up of iron.
However, if a star is sufficiently massive, a tremendous, violent, brilliant explosion can happen. A star will suddenly expand and produce, in a very short time, more energy then our Sun will produce in a lifetime. When this happens, we say that a star has become a supernova. While a star is in the supernova phase, many important reactions occur. The nuclei are accelerated t o much higher velocities then can occur in a fusing star.