What exactly happens to a star about to go supernova?

The above Hubble Space Telescope image is of Eta Carina, a very massive star ( 100 times the sun) which seems to be very near the end and about to become a supernova in perhaps the next few 100,000 years or so. There have been several recorded minor outbursts that have ejected huge clouds of gas since the 1800's. The above image shows a tilted hour-glass shape of outrushing bubbles of gas.

Supernovae are the end stage of the evolution of stars more than about 6 times the mass of the Sun. Betelgeuse is such a star which is probably a few million years away from the end of its life. Many of the stars in Orion and the Pleiades are also more massive than this. Here's what we expect is going on inside a star that is moments away from detonation.

Once a significant amount of iron 'ash' has accumulated in the core, the core begins to collapse and heat up, but by this time the temperature is just over 1 billion degrees. At these temperatures, neutrinos are produced in such huge quantities that they transport energy out of the interior of the star into space. For a time, most of the energy emitted by the star is in the form of neutrinos not light, and some astronomers call this the 'neutrino star' phase.

At the same time, the gamma ray photons in the core carry so much energy that each time they slam into an iron nucleus, the nucleus shatters into 13 helium nuclei and the gamma ray photon is lost from the core. This means that the core looses pressure support from light pressure as the gamma rays are absorbed by the iron nuclei. meanwhile, the iron nuclei collide buy are prevented from fusing into heavier nuclei because of an insurmountable energy barrier. The net result is that the core continues to collapse, produce more neutrinos, and loose still more internal pressure. This process runs away with itself until the density of the core, in a matter of hours, becomes so large that the neutrinos can no longer escape. In virtually an instant, the neutrinos dump most of their energy in the outer layers of the collapsing core and trigger an explosion. A shock wave propagates from the outer core, out through the star's extended atmosphere ( it is a red super giant star by this time), and when this breaks through we see the huge pulse of light that signals the supernova explosion. Because for every reaction there is an equal and opposite one, the expanding shock wave also spawns an inward-traveling shock wave that implodes the core of the star either into a neutron star cinder, or a black hole. The details depend on the mass of the core when the supernova detonates.

The latest example of a nearby supernova shown above was seen in 1987 in the Large Magellanic Cloud (called SN1987A by astronomers). It made all the newspapers when it was discovered in March of that year. It could be seen with the naked eye. By 2016 its expanding nebula shows some remarkable details never before seen by astronomers!