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Considerations
Supernovae are difficult to predict, and happen too quickly and too far away to be observed in detail. But by studying supernova remnants and using mathematical and computer models, scientists have arrived at the current theory that supernovae happen in one of two ways. The types of supernovae are classified accordingly as Type I or Type II, both having several subtypes. Types 1a, 1b and II are most useful for illustrating the basic differences between the types.
Type 1a Supernova
A Type 1a supernova is thought to occur when a white dwarf star, as one of a pair of stars in a binary system, acquires mass from its companion through the phenomenon of accretion. Under normal circumstances, a white dwarf star is about two-thirds the size of our sun. But when the white dwarf star reaches a critical mass of about 1.4 times the mass of the sun, by process of accretion, it becomes chemically unstable and the surface of the star ignites. The result is a massive thermonuclear explosion that consumes the entire star.
Type II Supernova
A Type II supernova is thought to occur when a star about the same size as our sun, but approximately 10 times the sun's mass, reaches the end its life. As the star ages, it produces less energy from nuclear fusion. When energy production reaches a critical low, the star can no longer support its own mass. Gravitational forces cause it to implode into a black hole or neutron star, depending on the mass of the star's core. The implosion releases potential gravitational energy that forces the matter in the star's surface layers outward with explosive force.
Classification
We distinguish between the two types of supernovas by examining the characteristics of their optical spectra. Stars and other celestial bodies look white when observed by the naked eye, but actually each star has a spectrum that reveals what elements the star and its atmosphere are made of. The spectrum of light emitted by a Type 1a supernova indicates that the star had no hydrogen atmosphere and was therefore a white dwarf. The spectrum of a Type II will show that the star originally had a hydrogen atmosphere, and therefore was a massive singular star.
Type 1b Supernova
An exception to the classification rule is the Type 1b supernova, which is now understood to actually be subtype of Type II supernova. A Type 1b supernova occurs as the result of the explosive destruction of a massive singular star at the end of its life, just like a Type II supernova. The only difference is that in the Type 1b, the star's hydrogen atmosphere has already been blown away by the star's stellar wind, due to the star's great mass. The lack of hydrogen accounts for its initial misclassification.