Supernovae, the Death Throws of Massive Stars

A supernova is an exploding star. When a star explodes it becomes very bright, outshining its host galaxies for several days. That is a lot of energy, similar to the amount energy the sun will put out over its entire lifetime. The light from the explosion quickly fades over the course of weeks. But the explosion can effect the subsequent evolution of its galaxy for billions of years. For instance supernovae drive winds that can sweep gas and dust out of a galaxy, or they can compress that gas to trigger star formation.

The energy of a supernova also drives nuclear fusion. Most of the heavy elements on the periodic table are produced inside supernovae. That means iron, gold, silver, uranium, basically everything heavier than oxygen. Without these heavy elements planets are not likely to form. So our own planet Earth is primarily star dust, material that was produced in supernovae explosions.

Will the sun explode as a Supernova?

No. Supernovae are produced by massive stars, stars that are more than ~10 times larger than the sun. These stars contain so much mass that the force of gravity creates very high pressures in their cores. The high pressure allows for very rapid fusion. As long as fusion continues, the energy produced can hold the weight of the star up against the force of gravity. However as the material in the core is fused to heavier elements, it becomes harder and harder to continue the fusion and support the great weight of the star. Eventually the star begins to collapse, very rapidly! As this happens the density of the core increases producing huge amounts of energy eventually causing the star to explode.

A second type of supernova.

A second type of supernova can occur in less massive stars, stars like the sun. At the end of the sun's life, after it has fused its hydrogen and helium into heavier elements, it will swell into a red giant, blowing off the outer layers of its atmosphere. What will be left behind is a solid core of carbon and oxygen, a white dwarf star. If the sun had a nearby companion star, and that star also began to swell into a red giant, it could pass material onto the white dwarf. As the white dwarf increased in mass, its core density would rise. Eventually the white dwarf would become so heavy that it can no longer support its weight against the force of gravity. When this happens it will begin to collapse, igniting fusion and producing a supernova.

Supernova reveal Dark Energy.

This second type of supernova has been very important for understanding the nature of the universe. Supernova of this type happen when a white dwarf exceeds 1.4 solar masses. Therefore, these explosions always produce roughly the same amount of energy. As a result they can be used to measure distances, and therefore study the expansion of the universe.

Currently all distant galaxies are moving away from our own galaxy, the Milky Way. This motion was set up in the initial moments of the universe by the Big Bang. As time goes on, gravity between the galaxies should slow this expansion. We can measure the distances to these galaxies through observations. If a supernova goes off in one of these distant galaxies we know how bright it should appear, based on our distance measurements. Observations of distant supernovae were made in the late 1990's and to everyone's surprise, these supernovae appeared fainter than they should have given their measured distance. The only way that this would be possible is if the separation between galaxies was accelerating. In other words the space between us and the supernova had grown in the time between when the supernova went off and when we observed it. In order for the separation to be accelerating, some force must be working against the force of gravity, an anti-gravitational force. Astronomers now call this force dark energy, primarily because we don't know anything about it. Dark energy is an astounding discovery, and fundamentally alters our understanding of the universe.