A Supernova (plural supernovae) is a huge explosion of a star which shines brightly for a time, usually 10,000 times more brightly than a nova., The Supernovae seen in other galaxies are often so bright that they outshine their whole galaxy for a number of weeks. The most recent supernova seen in our galaxy was in 1640, but remnants of supernovae in 1680 and 1860 have been found that were apparently hidden by dust so that they were not seen from earth.  Supernovae are thought to occur at the rate of 1 or 2 per century in our galaxy.
Studies of novae and supernovae depend on a concept known as the Chandrasekhar limit. The Indian astronomer Chandrasekhar calculated that if a star has a mass greater than about 1.4 solar masses, its gravity can shrink it to the point that it even the atomic nuclei that make up the star cannot stand up. White dwarfs are found to be always smaller than the Chandrasekhar limit, and so presumably can maintain their size because their atomic nuclei hold up. A white dwarf almost certainly has no hydrogen left, because the hydrogen would immediately fuse into helium at the pressures of the white dwarf's core.
A nova is believed to occur when one of two binary stars is a white dwarf. When hydrogen starts to transfer from the normal star to the surface of the white dwarf, eventually enough hydrogen accumulates that it suddenly is pressured into thermonuclear fusion by the extreme gravity of the very dense white dwarf. This neatly explains repeating nova stars, because it takes a certain amount of time for the hydrogen to accumulate and then it burns into helium in a spectacular flare of light. After burning all the hydrogen, the nova seems to die down while more hydrogen accumulates.
A Supernova seems to depend on a star's mass being greater than the Chandrasekhar limit. Some stars are much larger than our sun, and accumulate iron at their core. Iron is the waste material of fusion since it cannot fuse and give off energy, so as hydrogen fuses to helium, and helium fuses to carbon and heavier atoms, eventually iron is produced which sinks to the center of the star. When the iron core is so large that it passes the Chandrasekhar limit, there is a sudden collapse of its atoms and either a neutron star is formed or maybe a black hole. This collapse generates so much energy that the outer layers of the star are blown away, and the light produced outshines billions of stars for a short time. If a supernova were to happen within 100 light years of the earth, the radiation would be so strong that it would affect the earth's atmosphere. If a star receives enough hydrogen from a binary partner, supernovae can happen.
Supernovae are divided into two types, known as type I and type II that differ in their spectra (type I tends to have little hydrogen, while type II usually has bright hydrogen lines in its spectrum) and how long they last.
Type I Supernovae
Type I supernovae are thought to happen when a white dwarf in a binary star accumulates so much helium from fusing the hydrogen it steals from its partner star that it is suddenly larger than Chandrasekhar's limit. The white dwarf was already in a state where the atoms were barely able to support the gravitational field that the white dwarf generates with its extremely dense material. With too much more mass, it breaks down even its atomic nuclei and explodes with an enormous amount of energy. Usually it does not leave behind a remnant.
These supernovae have been used to calibrate distances to galaxies and star clusters because their light output is thought to be very consistent. Over time they were divided into type Ia and Type Ib supernovae. They are still the subject of many studies because the recent observation of the accelerating expansion of the universe depends on them.
Type II Supernovae
A Type II supernova is presumed to be the result of a large, massive star accumulating a core of spent nuclear fuel (probably iron) that becomes larger than the Chandrasekhar limit. The explosion of the star leaves an expanding ball of gas that can be seen by telescopes for centuries after and is called a supernova remnant. These are sometimes referred to as "Core Collapse" supernovae as well as type II supernovae. They seem to happen somewhat more frequently than type I.
A large star became a supernova recently, and astronomers which followed its development report that it was 'immature' since it had not reached the point that models recognize as being ready for a supernova. Some feel that the models will need to be revised. 
- A Review of Stellar Remnants:Physics, Evolution, and Interpretation by Danny Faulkner, CRSQ Vol 44:2 pp 76-84, Fall 2007
- Youngest Stellar Explosion in Our Galaxy Discovered, accessed 29 March, 2009
- Near Earth Supernova Wikipedia, accessed 29 March, 2009
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