Supernovae
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In a sense, stars are like people: they are born, they live and they die. A star "lives" by fusing lighter elements into heavier ones in its central regions. The pressure generated by this "combustion" holds the star up against the enormous gravitational force that its outer layers extert on the stellar core. The supply of elements that the star can fuse is limited, and when this runs out the star "dies": its properties change rapidly and violently, and a new astronomical object is created. Supernovae represent the most catastrophic (and picturesque!) of these stellar deaths.
Anatomy of a Supernova

Stars of all masses spend the majority of their lives fusing hydrogen nuclei into helium nuclei: we call this stage the main sequence. When all of the hydrogen in the central regions of a star is converted into helium, the star will begin to "burn" helium into carbon. However, the helium in the stellar core will eventually run out as well; so in order to survive, a star must be hot enough to fuse progressively heavier elements, as the lighter ones become exhausted one by one. Stars heavier than about 5 times the mass of the Sun can do this with no problem: they burn hydrogen, and then helium, and then carbon, oxygen, silicon, and so on... until they attempt to fuse iron. Iron is special in that it is the lightest element in the periodic table that doesn't release energy when you attempt to fuse it together. In fact, instead of giving you energy, you end up with less energy than you started with! This means that instead of generating additional pressure to hold up the now extended outer layers of the aging star, the iron fusion actually takes thermal energy from the stellar core. Thus, there is nothing left to combat the ever-present force of gravity from these outer layers. The result: collapse! The lack of radiation pressure generated by the iron-fusing core causes the outer layers to fall towards the centre of the star. This implosion happens very, very quickly: it takes about 15 seconds to complete. During the collapse, the nuclei in the outer parts of the star are pushed very close together, so close that elements heavier than iron are formed.

What happens next depends on the mass of the star. Stars with masses between about 5 and 8 times the mass of our Sun form neutron stars during the implosion: the nuclei in the central regions are pushed close enough together to form a very dense neutron core. The outer layers bounce off this core, and a catastrophic explosion ensues: this is the visible part of the supernova. Stars with masses greater than about 10 times the mass of the Sun meet a very different fate. The collapse of the outer regions of the star is so forceful that not even a neutron star can support itself against the pressure of the infalling material. In fact, no physical force is strong enough to counter the collapse: the supernova creates a black hole, or a region of spacetime that is so small and so dense that not even light can escape from its clutches. In this case, the details of how the ensuing explosion actually occurs have still to be worked out. Observationally, supernovae are found by patiently observing the sky and looking for bright objects where there were none before. At its peak luminosity, the supernova resulting from a single star may be bright enough to outshine an entire galaxy.
A Cosmic Cycle...

Supernovae play a fundamental role in a great cosmic recycling program. We believe that almost all of the elements in the Universe that are heavier than hydrogen and helium are created either in the centres of stars during their lifetimes or in the supernova explosions that mark the demise of larger stars. Supernovae then disperse this newly synthesized material in the interstellar neighbourhood. From this material a new, enriched generation of stars will form, and the cycle begins anew. This is how we think that the heavy elements in the Sun came to be. Since the planets in the solar system formed from leftover material in a disk around the proto-Sun, all of the heavy elements in the Earth (including those in humans!) must have come from the same source. This means that in the most literal sense, we are stardust!
The Ask an Astronomer team's favorite links about Supernovae:

* Introduction to supernovae: More introductory stuff about supernovae
* Supernova pages on the web: A set of links to everything you could possibly want to know about supernovae!
* International supernova network: Up-to-date information about the supernovae discovered by amateur and professional astronomers

Previously asked questions about Supernovae:

* What is a nova? (Beginner)
* Can supernovae hinder the formation of life in galaxies? (Beginner)
* How many stars are born and die each day? (Beginner)
* How long does the supernova stage of a star last? (Intermediate)
* How are elements heavier than iron formed? (Intermediate)
* What supernova created the Crab nebula? (Intermediate)
* Was the Sun made in a supernova? (Intermediate)
* How close does a supernova need to be to damage the Earth's environment? (Intermediate)
* What happens when you change the mass of a White Dwarf or Neutron Star? (Intermediate)
* Are planetary nebulae the result of supernovae? (Intermediate)
* How are supernovae discovered? (Intermediate)
* Why do supernova remnants look like rings rather than spheres? (Intermediate)
* How long do supernova remnants last? (Intermediate)
* Where is the supernova remnant that led to our solar system? (Intermediate)
* How does a star take mass from another star? (Intermediate)
* What causes gamma ray bursts? (Intermediate)
* Could a planetary system survive if its star merged with another, or if its star went supernova? (Intermediate)
* Why do the explosions of Type Ia supernovae have a more predictable spectrum than those of regular Type II supernovae? (Advanced)
* How are light and heavy elements formed? (Advanced)
* Can superheavy elements (such as Z=116 or 118) be formed in a supernova? Can we observe them? (Advanced)
* How do supernovae show us that the Universe's expansion is accelerating? (Advanced)