There are two kinds of supernova, Type I and Type II. Both are caused by a sudden collapse of an old star into a neutron star or a black hole. Type II is caused by a superlarge red giant star that has spent most of it nuclear fusionable material. Type I is caused by medium sized stars that have spent all of their nuclear fuel and have become what is called a white dwarf star.

When a star shine normally, there are nuclear reactions taking place inside the star that give off a flow of high energy particles towards the outer layers of the star. These high energy particles speeding in all directions form a pressure on the outer layers of the star keeping it from contracting further under the influence of gravity. But when the nuclear material is used up, then there is nothing from keeping the star from collapsing under its own weight.

As you might recall, atoms are made of negatively charged electrons circling positively charge atomic nuclei. Opposite charges attract each other. Whereas like charges repel each other. So each positively charged atomic nuclei repels other positively charged nuclei. But when the force of gravity is so strong that it forces the like charged nuclei to become close enough, then each nuclei begins to feel the attractive strong nuclear force of the other nuclei.

The strong nuclear force is always attractive and much stronger than the force of an electric field. But its influence is only felt when particles are very close together. The strong nuclear force is what keeps the two positively charged protons of a helium atom bound together even though they both have the same electric charge.

The falling together of two particles under an attractive force always releases energy. Click here for some examples. When an electron falls towards a proton, this reaction gives off energy in the form of photons of light. When two particles fall together under the attractive strong nuclear force, this reaction gives off energy in the form of light, neutrinos, and other particles. So, when a star collapses under gravity until each particle falls together from the strong nuclear force, all these reactions giving off energy all at once is what is seen in a supernova.

The only way the force of gravity among particles can overcome the electric force so as to feel the strong nuclear force is that there be a dense enough gravitational field attracting all the particles. This occurs in very large stars that have spent all their fuel or in white dwarf stars which are what remains of smaller stars when they have spent their fuel. White dwarf stars are a solid ball of very hot iron which do not undergo anymore nuclear processes. Iron is the heaviest element that gives off energy in the process of making it. Heavier elements actually require energy to make. And in order to sustain the chain reaction in a star, the elements it makes must give off energy so as to force other particles together and produce still more reactions. So when a smaller star has reached the point of producing a core of iron, any further reactions in the outer layers have a tendency of blowing off the outer layers until only the core of iron remains to cool over a very long period of time.

If these white dwarf stars collect even more material, then the gravitational forces can be enough to still cause it to collapse under the strong nuclear force to form a neutron star or black hole. The result is a Type I supernova.

When a normal star becomes a white dwarf, it is possible that what remains may be very close to having enough mass to produce the gravitational field needed to collapse it and cause a supernova. It would only require a small amount of additional mass to send it over the edge. This additional mass might be collected by interstellar gas. However, a white dwarf can collect material from a neighboring star if they are orbiting each other very closely. Such systems are called binary systems because there are two stars orbiting each other.

White dwarfs are very hot at first and cool over a long period of time. Those that have cooled enough cannot be seen. Larger stars spend their fuel more quickly and their remaining white dwarfs are more massive and need less material to go supernova. So, if it is a darker white dwarf that will supernova, then it must have been cooling and gathering material over a longer period of time. And if it has been cooling for a long period of time, then it must have formed early in the life of the universe and spent its fuel quickly. And if it spent its fuel quickly, then it must be massive and close to the edge. So if it is dark by now, then it is more likely that it has collected enough material to go supernova.

There is some debate as to how close a supernova must be to cause damage to the earth. Most agree that anything closer than 50 light years would be felt. Anything about 1 light year away would probably incinerate the earth completely.

Are there any nearby stars that might go supernova?