Image Credits: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT
Massive stars shine steadily until the hydrogen has fused to form helium ( it takes billions of years in a small star, but only millions in a massive star), when it becomes a red supergiant and starts off with a helium core surrounded by a shell of cooling, expanding gas. Over the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. When the core collapses in less than a second, it causes an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. If the core -between 1.5 - 3 solar masses- survives, it contracts to become a neutron star. If the core is much greater than 3 solar masses, the core contracts to become a black hole.