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Although the exact mechanism that causes type I supernovae is still not entirely understood, for type Ia supernovae, it is generally accepted to occur most often in binary star systems where one of the binary stars is a white dwarf (typically a [[carbon]]-[[oxygen]] dwarf) that is drawing mass from its high mass companion.  As the mass from from the companion accumulates on the surface of the [[white dwarf]] it compresses and heats up.  Often, the hydrogen in the accumulated mass will heat enough to initiate hydrogen fusion, causing a [[nova (star)|nova]].  If however the white dwarf accumulated enough matter from its companion to achieve a mass that surpasses its [[Chandrasekhar limit]], the white dwarf will no longer be able to support itself via electron degeneracy pressure against the increase in [[gravity]] from the additional mass.<ref>http://adsabs.harvard.edu/abs/1987ApJ...323..140L</ref> A significant portion of the white dwarf will then undergo nuclear fusion within seconds, releasing enough energy to cause the star to explode as a supernova.<ref>http://www.sciencemag.org/cgi/content/abstract/315/5813/825</ref>

Although the exact mechanism that causes type I supernovae is still not entirely understood, for type Ia supernovae, it is generally accepted to occur most often in binary star systems where one of the binary stars is a white dwarf (typically a [[carbon]]-[[oxygen]] dwarf) that is drawing mass from its high mass companion.  As the mass from from the companion accumulates on the surface of the [[white dwarf]] it compresses and heats up.  Often, the hydrogen in the accumulated mass will heat enough to initiate hydrogen fusion, causing a [[nova (star)|nova]].  If however the white dwarf accumulated enough matter from its companion to achieve a mass that surpasses its [[Chandrasekhar limit]], the white dwarf will no longer be able to support itself via electron degeneracy pressure against the increase in [[gravity]] from the additional mass.<ref>http://adsabs.harvard.edu/abs/1987ApJ...323..140L</ref> A significant portion of the white dwarf will then undergo nuclear fusion within seconds, releasing enough energy to cause the star to explode as a supernova.<ref>http://www.sciencemag.org/cgi/content/abstract/315/5813/825</ref>

Type Ia supernovae are important to [[astronomy]] because they act as standard candles for measuring extra-galactic distances, especially pertaining to other galaxies.  This is due to such supernovae explosions having the same luminosity, an absolute magnitude of -19.3 (some five billion times the sun's brightness).  The source of this brightness is the generated by radioactive decay of nickel-56.  The [[nickel]]-56 is formed during the explosion event itself and decays through [[cobalt]]-56 to [[iron]]-56.<ref>http://adsabs.harvard.edu/abs/2000ARA&A..38..191H</ref>.  

Type Ia supernovae are important to [[astronomy]] because they act as standard candles for measuring extra-galactic distances, especially pertaining to other galaxies.  This is due to such supernovae explosions having the same luminosity, an absolute magnitude of -19.3 (some five billion times the sun's brightness).  The source of this brightness is the generated by radioactive decay of nickel-56.  The [[nickel]]-56 is formed during the explosion event itself and decays through [[cobalt]]-56 to [[iron]]-56.<ref>http://adsabs.harvard.edu/abs/2000ARA&A..38..191H</ref>   

In 1998, observations of the apparent magnitude of several type 1a supernovae in other galaxies, resulted in the surprising finding that the universe is expanding at an accelerated rate, due to [[Dark energy|Dark Energy]].<ref>http://arxiv.org/abs/astro-ph/9805201</ref>

In 1998, observations of the apparent magnitude of several type 1a supernovae in other galaxies, resulted in the surprising finding that the universe is expanding at an accelerated rate, due to [[Dark energy|Dark Energy]].<ref>http://arxiv.org/abs/astro-ph/9805201</ref>

===Type 1b and 1c===

===Type 1b and 1c===

Type lb and Ic supernovae, like Type II supernovae, are from massive stars that are running out of core fuel for fusion.  The stars for type Ib explosions had their outer shell of [[hydrogen]] lost or striped away. The stars for type Ic explosions have lost most of both their hydrogen and subsequent [[helium]] shell. This loss is either due to intense stellar winds, or from interaction with a nearby binary companion of sufficient mass.<ref>http://adsabs.harvard.edu/abs/1997rdbs.conf..153P</ref>  It is believed that type 1b supernovae are from exploding [[Wolf-Rayet star|Wolf-Rayet stars]].  There is some evidence that a small percentage of type Ic supernovae are the source of [[Gamma-Ray Bursts|Gamma Ray Bursts]].<ref>http://adsabs.harvard.edu/abs/2004MNRAS.349.1093R</ref>

Type lb and Ic supernovae, like Type II supernovae, are from massive stars that are running out of core fuel for fusion.  The stars for type Ib explosions had their outer shell of [[hydrogen]] lost or striped away. The stars for type Ic explosions have lost most of both their hydrogen and subsequent [[helium]] shell. This loss is either due to intense stellar winds, or from interaction with a nearby binary companion of sufficient mass.<ref>http://adsabs.harvard.edu/abs/1997rdbs.conf..153P</ref>  It is believed that type 1b supernovae are from exploding [[Wolf-Rayet star]]s.  There is some evidence that a small percentage of type Ic supernovae are the source of [[Gamma-Ray Bursts|Gamma Ray Bursts]].<ref>http://adsabs.harvard.edu/abs/2004MNRAS.349.1093R</ref>

==Type II==

==Type II==

[[Image:Sn94d_hiz_big.jpg|right|thumb|Supernova 1994D, visible as the bright star in the lower left, occurred in disk galaxy NGC 4526]]

[[Image:Sn94d_hiz_big.jpg|right|thumb|Supernova 1994D, visible as the bright star in the lower left, occurred in disk galaxy NGC 4526]]

A type II supernova is a violent stellar event involving a massive [[star]] of at least eight solar masses at the end of its life, when it's unable to maintain itself anymore through nuclear [[fusion]], causing a chain reaction that leads to the collapse of the star's core and subsequent explosion. <ref>http://www.sciencemag.org/cgi/content/summary/304/5679/1915</ref>

A type II supernova is a violent stellar event involving a massive [[star]] of at least eight solar masses at the end of its life, when it's unable to maintain itself anymore through nuclear [[fusion]], causing a chain reaction that leads to the collapse of the star's core and subsequent explosion.<ref>http://www.sciencemag.org/cgi/content/summary/304/5679/1915</ref>

When a star converts most of its core hydrogen fuel to helium through thermonuclear [[fusion]], it evolves off of the main sequence.  As the [[hydrogen]] fuel is exhausted, [[helium]] accumulates and the core begins to contract.  As the core contracts, it heats up and the helium begins to fuse into [[carbon]] and [[oxygen]].  For stars with masses eight times or less than that of our [[Sun]], there is not enough mass to compress and heat up the core sufficiently to fuse elements heavier then helium, once all the helium fuel is used up.  Such stars simply lose their outer atmospheres as [[planetary nebula]] and end their lives as [[white dwarf|white dwarfs]] that gradually cool over time.

When a star converts most of its core hydrogen fuel to helium through thermonuclear [[fusion]], it evolves off of the main sequence.  As the [[hydrogen]] fuel is exhausted, [[helium]] accumulates and the core begins to contract.  As the core contracts, it heats up and the helium begins to fuse into [[carbon]] and [[oxygen]].  For stars with masses eight times or less than that of our [[Sun]], there is not enough mass to compress and heat up the core sufficiently to fuse elements heavier then helium, once all the helium fuel is used up.  Such stars simply lose their outer atmospheres as [[planetary nebula]] and end their lives as [[white dwarf]]s that gradually cool over time.

   

   

For more massive stars, once the core again contracts after the helium fuel is used up, there is sufficient mass and energy to fuse the carbon and oxygen at the core into heavier [[element|elements]].  This process is repeated over and over when the nuclear fuel is exhausted, and the fusion at the core again stops until the core contracts and heats up enough to start the next phase of fusion of these heavier elements.  At each phase, more and more energy is required to fuse the increasingly heavy atomic nuclei.  The result are "onion layers" of subsequently heavier elements in the interior of these massive stars.   

For more massive stars, once the core again contracts after the helium fuel is used up, there is sufficient mass and energy to fuse the carbon and oxygen at the core into heavier [[element]]s.  This process is repeated over and over when the nuclear fuel is exhausted, and the fusion at the core again stops until the core contracts and heats up enough to start the next phase of fusion of these heavier elements.  At each phase, more and more energy is required to fuse the increasingly heavy atomic nuclei.  The result are "onion layers" of subsequently heavier elements in the interior of these massive stars.   

[[Image:Evolved_star_fusion_shells.png|right|thumb|Artist rendition of a massive star just before supernova collapse.]]

[[Image:Evolved_star_fusion_shells.png|right|thumb|Artist rendition of a massive star just before supernova collapse.]]

==External links==

==External links==

[http://www.nasa.gov/worldbook/supernova_worldbook.html NASA - Supernova]

* [http://www.nasa.gov/worldbook/supernova_worldbook.html NASA - Supernova]

[[Category:Astronomy]]

[[Category:Astronomy]]