SUPERNOVA

Supernovae are massive giant exploding stars. When the explosion occurs, the resulting illumination can be as bright as an entire galaxy! Supernova occurs at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space. The result of the collapse may be, in some cases, a rapidly rotating neutron star that can be observed many years later as a radio pulsar.

As a result of gravitational forces acting against the nuclear structure of the core of a fuel depleted star, tremendous shock waves are generated which cause the outside layers of the star to be blown away from the core.

One type of Supernova involves two stars, one of them being a white dwarf whose gravitational attraction is so intense that it is capable of siphoning off material from its companion. Unfortunately for the star (but fortunately for us at a long distance!), the white dwarf exceeds its Chandrasekhar limit of stability causing it to go into thermonuclear instability and produces one of the largest explosions known in the Universe.

Another type of supernova involves a collapse of the core of a fuel depleted star, tremendous shock waves are generated which cause the outside layers of the star to be blown away from the core. Gravitational forces condensing hydrogen gas raises the temperature at the center of the star to the point where nuclear fusion is initiated. Hydrogen is fused into helium and energy is given off in the process. As more helium accumulates at the center, the temperature rises due to compression until another nuclear fusion is initiated. This time helium is converted to carbon and oxygen and additional energy is given off during the nuclear fusion.

A similar process continues with carbon and oxygen fusing to neon, magnesium, and oxygen. These elements then undergo another fusion process as the temperature and pressure increase to produce silicon and sulfur. The latter two elements then fuse into iron. During each nuclear fusion, energy is given off. This takes two orders of magnitude less time to happen than on the previous fusion. However, nuclear fusion stops at iron because energy is no longer produced by fusion. The iron core collapses very quickly (within hours or less).

Since the iron core can collapse only so far and can no longer undergo fusion, it becomes extremely hot and now begins to expand rapidly. This occurs while the star's outer shells are rushing in to fill the void left by the collapsed iron core. The expanding iron and the collapsing outer gases collide with each other producing tremendous shock waves which blow the outer layers away from the core, thus causing the supernova's gigantic explosion.

What happens after the explosion depends on the type and mass of the progenitor stars. Mostly they produce a gas cloud called a supernova remnant which initially expands at a rate of about 10,000 km/s. Gradually the expansion rate slows down while dissipating into the interstellar medium, seeding the neighborhood with heavy elements and providing the necessary shock waves for new stellar formation.

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