Generally when a star is born with a mass of 8 times that of our sun, it will end its life with a type II supernova explosion.
It is at the core of such a star where the fusion of hydrogen to make
helium takes place, this relatively small region releases enough energy to stop the star collapsing under it's own gravity. Eventually the fuel runs out and gravity collapses the star. If the star is massive enough, gravity can compact the core to such an extent, that enough energy is available to continue the fusion process, fusing progressively heavier elements than hydrogen towards the core. (Shown below, outmost layers to innermost)
H => He
He => C,O
C => Ne, Mg
O => Si, S
Si, S => Fe
When sulphur fuses with silicon to make iron the process stops, as to fuse iron, you have to put more energy in than
you can get out. (Please see Wrinkly's write up 'Transferric element' for more.) Gradually a core of iron 'ash' will build up....
When the iron core exceeds 1.4 solar masses gravity overcomes electron degeneracy pressure and the core collapses from perhaps 5000 miles to perhaps only 10 miles, in seconds. Enough energy is released at this point to power 50 stars like our own for 20 billion
years, longer than the current age of the universe.
Most of this energy is carried away by neutrinos, only a small percentage of the energy is absorbed by the outer layers of the star, and this is what powers the actual supernova explosion. The enormous shockwave created slams into the outer layers of the star, ejecting the material at up to 10 million miles per hour, this provides enough energy to fuse together much heavier elements than iron. Within a day or two the shockwave has reached the outer surface of the star, and it's brightened by a billion times or more, sometimes outshining the galaxy the star is in.
The core, however has become a very compact massive object, a neutron star, or if massive enough a black hole
The envelope continues to expand, glowing brightly in x-ray light. The glowing cloud of gas left makes for some of the most beautiful astronomical photographs, indeed it's supernova that make the photograph possible; the silver and other chemicals used to make the negative, and photographer, can only be made in a supernova explosion.