So, why do stars explode?
Well, to answer that question, it is prudent to first ask and answer the question:
What is the source of their light energy?
Once thought to have been divine light shining through an array of pinpricks on a dark shroud, the stars are now known to be mammoth gaseous spheres: all of them made self luminous by nuclear fusion reactions in their cores.
These fusion reactions convert light elements into heavier elements. During this conversion, some of the initial matter is converted directly into energy. It is this energy which powers stars.
The Sun, for instance, is currently fusing hydrogen into helium deep in its core. These fusion reactions generate enormous amounts of energy. (Each second, the Sun fuses 647 million tons of hydrogen into 643 million tons of helium. The remaining 4 million tons is turned into pure energy.)
Throughout most of their lives, stars remain in more or less a stable condition because the energy within their cores counteracts the gravitational contraction of their outer layers. When these two antagonistic forces are balanced, a star is said to be in a state of Hydrostatic Equilibirum. When a star exhausts its core reserves of hydrogen, this balance is disrupted.
A shell forms around the core in which fusion reactions continue, while the core itself contracts gravitationally. The heat energy generated by this core contraction, combined with that imparted into the core by the outer fusion shell, raises the core temperature high enough (100 million degrees) to initiate the next fusion reaction: helium into carbon.
During each transition – from the depletion of an element reserve to the commencement of a new core fusion process- the star alternately expands and contacts since the hydrostatic equilibirum is thrown off balance.
Stars as massive as the Sun will not be able to fuse carbon into heavier elements after its core helium is exhausted. They don’t have enough matter, and therefore cannot generate the pressures required to produce temperatures necessary for carbon fusion. In order to fuse carbon and even heavier elements, a highly massive star is needed. Only within the cores of these giant stars will there be temperatures heavy element fusion requires. The most massive stars will be able to fuse elements as heavy as iron. (That is a 3 billion degree temperature requirement, folks.)
However, iron in the core spells disaster for any star, no matter how massive. Iron fusion is an endothermic process, which means that the star would have to invest more energy to ignite and sustain the fusion than it would receive from it.
The balance between gravity pushing in and energy pushing out is disrupted catastrophically … The outer layers collapse rapidly … The energy of this gravitational collapse is converted into kinetic energy and the star explodes as a SUPERNOVA.
A supernova explosion causes the star to become as bright as a galaxy for a brief amount of time. The high energy release expels the remnants through space at high velocities. Also, some energy is expended to create all the elements heavier than iron.
(For instance, Gold, that precious metal that has caused so much turmoil and ecstasy in human history, is the product of an ancient supernova.)
Six billion years ago (or thereabouts) such a massive star exploded, sending remnants through space. Some of this debris became incorporated into a cold interstellar cloud of gas and dust. This impact enriched the cloud material with heavy elements and caused it to collapse. The collapse eventually formed the Sun and the planets that revolve about it.
All of our chemistry, biology, complex electronics, horticulture, and every facet of our everyday life exists as the interaction of complicated matter forged and dispersed by an ancient dying star.
So, we owe our existence in part to one of the most titanic explosions astrophysics has discovered: We are all stardust in some form …