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What Is A Supernova?
While there are many types of supernova, there are a few unifying factors between them. All supernovae are some sort of stellar explosion, which may or may not consume the entire star, and typically involves either the abrupt beginning or ending of nuclear fusion within the star.
Supernovae are extremely luminous, often outshining the entire galaxy in which they are hosted, or emitting more light than they did over their lifetime. Afterwards, there is some sort of supernova remnant left behind, the smoking gun.
Supernovae are technically classified by their spectra (plot of energy emitted by a star versus frequency or wavelength), which are attached to various models to explain both the elements revealed and the shape of the light curve:
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Type 1a Supernovae
This is the most common—and arguably most useful—type of supernova. Essentially, they happen when a white dwarf star reaches the Chandrasekhar Limit at 1.4 solar masses. The white dwarf will typically accrete this mass from a companion star in a binary system, such as from the outer atmosphere of a red giant.
Upon reaching the Chandrasekhar Limit, the force pushing outwards due to the nuclear fusion at the core can no longer counter the force pressing inwards from the star's own mass, so the star begins to collapse inwards. This releases gravitational potential energy, resulting in some pretty amazing fireworks as the star destroys itself from the inside out.
But how can a supernova be useful? Well, all type 1a supernovae have the same absolute peak luminosity because they are fueled by precisely the same amount of mass, as defined by the Chandrasekhar Limit. Using the distance modulus for astronomy, one can then find an accurate distance to the explosion and thus to the host galaxy—a powerful tool for astronomers.
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Type II Supernovae
Type II supernovae are even more powerful than type 1a—partly because they are derived from more massive stars, meaning more fuel for their explosions. Think nine times the mass of the Sun as a minimum.
As with type 1a supernovae, these explosions come about from the loss of balance, or hydrostatic equilibrium within the core of the star, from the thermal energy of the nuclear fusion pushing outwards and gravity pressing inwards. When all hydrogen is exhausted in the core, one side of the equation is gone, and the core collapses. Unlike with type 1a supernovae, this is enough to ignite the next densest fuel in the core, which then burns along merrily until it too is consumed and so on. During all this, the star remains relatively stable. This process is also occurring at various levels within the star, with progressively less dense fuels being burned as you move outwards from the core. Think onion thoughts.
This can happen multiple times until, at some point, the force from gravity is not enough to ignite the next stage of fusion, usually at the nickel-iron stage. At this point, the star suffers a critical core collapse, one with some pretty impressive displays of energy (and pretty complex thermodynamics.) Depending on the mass of the star, this can create either a neutron star or a black hole, the line between the two being set at twenty solar masses.
So, as you might guess, type II supernovae can't be used in the same manner as type 1a supernovae to calculate distance because the beginning mass (and thus the luminosity of the ending explosion) may vary so much.
There are few kinks left in this basic model that scientists haven't fully puzzled out yet. Keep a weather eye on the horizon for further developments as the required neutrino physics receives more research.
Check out the next page for the different types of type II supernovae, example pictures, information on type 1b and 1c, and more.
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Types Of Type II
There are two main types of type II supernovae, distinguished by their light curves, IIP and IIL, the letters standing for plateau and linear respectively. These are representative of the pattern of the light curves. With IIP, a plateau is formed after the maximum luminosity because the outer layer becomes opaque; this does not occur with IIL
There are also a few other types of type II supernovae. IIn, for instance, is a subtype that may involve interaction of the supernova output with other material around the star. IIb are theoretically formed from the same massive stars that have had their outer atmospheres stripped away by a companion star.
Hypernovae are also a type of supernova, albeit a theoretical one. These would consist of the brightest of the bright, supernovae that develop from the core collapse of hypergiants, stars whose mass are 100 to 150 times that of our own Sun.
What's holding this theory down is a lack of observational evidence: while there are some candidates, from stars that may develop into them to remnants of hypernovae gone by, not enough have accumulated to definitively nail it down as fact.
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Type 1b & 1c
Wait, why weren't these included earlier? Well, 1b and 1c are more similar to type II than to type 1a supernovae. They involve the same massive stars, except that the outer atmosphere has been stripped prior to the supernova, hydrogen in the case of 1b and both hydrogen and helium in the case of 1c.
These are believed to be more or less similar to type 1a supernova in cause, except that much of their outer atmosphere would have been stripped prior to the explosion, hence the term stripped core collapse supernova.
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Exceptions & Variations
Things aren't always quite so simple, however. There are always the occasional supernova explosion that seems to defy the classification system and mystify astronomers.
Asymmetry in supernova explosions is one of the greatest oddities. Peering into the supernova remnants will often reveal grossly asymmetric formations, but our current models have difficulty accounting for this behavior. Related to this is the sometimes-observed rapid movement of the remaining core after the explosion.
If the star in question is rapidly spinning, this may complicate the equation somewhat. The centrifugal force will contribute to the total force pushing outwards, which can change the amount of mass required to trigger a supernova.