(image of Kepler's Supernova Remnant above is compliments of en.wikipedia.org)
When a large star reaches the end of it's life it explodes in what is called a Supernova. This immense explosion causes the star to become brighter than an entire galaxy for a short period of time when viewing the star through a telescope.
But why does the star explode when it runs out of fuel? What causes the immense explosion that releases such incredible energy that it can be seen millons of light years away?
It all starts with nuclear fusion, the basic process that begins when large volumes gas come together and are compressed combining lighter atoms into heavier ones. Each time fusion occurs large amounts of energy are released in the form of heat and light.
A young star fuses hydrogen into helium. Hydrogen is the most abundant element in the universe and most stars have so much hydrogen, that they spend most of their lives performing this form of nuclear fusion. Our Sun has spent half of its lifetime (5 billion years) so far in this stage. It is estimated that our Sun will continue to fuse hydrogen to helium for another 5 billion years before reaching the end of its hydrogen fuel.
However when most of the hydrogen has run out, the star begins fusing other heavier elements. It starts to fuse helium into carbon releasing energy in the process of fusion. (By the way, energy released through fusion is several times greater than the energy released through nuclear fission-splitting the atom).
When most of the helium is used up, the star begins to fuse carbon into oxygen. When most of the carbon is used up, the star begins fusing oxygen into neon, then into magnesium, then into silicon. When most of these elements are fused, the star begins fusing silicon into iron and this is where the process of nuclear fusion stops.
When a star develops a solid iron core, it has left-over layers of remnant elements that it has fused. Below is a cross-section of what an old star might look like just before it dies. You can see the layers of elements (some still burning) around the central iron core.
(image compliments of imagine.gsfc.nasa.gov)
Eventually, the amount of fusion taking place becomes insufficient to counteract the immense gravity of the star, and the iron core implodes, together with all the outer layers of lighter elements. When layers of lighter elements implode toward the central core, they essentially bounce off the core, sending out an immense shock wave explosion. The blast is a supernova explosion that drives off all the lighter elements into the universe.
The above process describes a Type II Supernova, involving hydrogen and an imploding star.
(above is the Crab Nebula viewed from the Hubble Space telescope-This is a remnant of a Supernova 1st detected by Chinese astronomers in the year 1054)-(image compliments of NASA and STScI)
What happens next depends on the size of the original star. In a star about 3 times the size of our sun, implosion of the iron core fuses protons and electrons creating a core composed almost entirely of neutrons, a neutron star, the densest objects in the Universe. Supernovas of stars that are 3-10 times the size of our Sun can create massive singularities of gravity called Black Holes. Supernovas of stars that are greater than 10 times the size of our Sun can create such immense singularities of gravity (Super Massive Black Holes) that entire galaxies revolve around them.
Here is a good overall video on supernovas.
So you see, supernova explosions are essentially result in the transformation of a dying star into another form and disperse elements into the universe to be collected again to form other stars and even planets.
Isn't our universe amazing!
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