Changes

===Type 1a===

===Type 1a===

 

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]].  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]].  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 II==

==Type II==

 

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|white dwarfs]] 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|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.   

The repeating collapse and fusion process of core elements will continue until [[nickel]]-56 is produced through fusion, which decays into [[iron]]-56.  The star is unable to fuse this iron-nickel core due to their extremely high binding energy (highest of all the [[elements]]), as it would take more energy then what would be released.<ref>http://adsabs.harvard.edu/abs/1995AmJPh..63..653F</ref>  This results in the iron-nickel core growing more and more massive over time (due to continued fusing of lighter elements into nickel-56).  Eventually the core will reach the [[Chandrasekhar limit]] (1.4 times the mass of our Sun) where electron degeneracy pressure is no longer able to hold the core up against [[gravity]] and the core catastrophically collapses.<ref>http://adsabs.harvard.edu/abs/1987ApJ...323..140L</ref>

The repeating collapse and fusion process of core elements will continue until [[nickel]]-56 is produced through fusion, which decays into [[iron]]-56.  The star is unable to fuse this iron-nickel core due to their extremely high binding energy (highest of all the [[elements]]), as it would take more energy then what would be released.<ref>http://adsabs.harvard.edu/abs/1995AmJPh..63..653F</ref>  This results in the iron-nickel core growing more and more massive over time (due to continued fusing of lighter elements into nickel-56).  Eventually the core will reach the [[Chandrasekhar limit]] (1.4 times the mass of our Sun) where electron degeneracy pressure is no longer able to hold the core up against [[gravity]] and the core catastrophically collapses.<ref>http://adsabs.harvard.edu/abs/1987ApJ...323..140L</ref>