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==Type II==

==Type II==

A type II supernova is a stellar explosion involving a [[star]] at the end of its life, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. This is one of the most powerful events in the universe. It often causes stars that are not normally visible to the naked eye to become apparent in the night sky due to the extreme amount of light given off in the explosion. If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space. The result of the collapse may be, in some cases, a rapidly rotating [[neutron star]] that can be observed many years later as a radio [[pulsar]].  

 

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.

 

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>

 

As the core collapses, [[protons]] and [[electrons]] are forced together to produce [[neutrons]] and [[neutrinos]].  Neutrinos don't normally interact with matter very easily, so they escape the core, carrying with them a tremendous amount of outward energy and pressure.  Meanwhile, the core continues to collapse until the neutrons become too tightly packed together and the collapse is halted by ''neutron degeneracy''. This causes the inbound matter to rebound in a tremendous shock wave.  The result of these twin effects of a rebounding shockwave and neutrino outburst causes the rest of the star outside the core to be blasted away in a tremendous supernova explosion.

 

Only around 1% of the energy produced is released in the form of heat and kinetic energy from the ejected outer gas layers. The other 99% being released as neutrinos during the supernova explosion.  Despite this, there is still enough visible energy for the supernova to exceed the visual luminosity of the sun by a factor of a billion or more. The source of most of the [[metallicity|metals]] (all elements heavier then hydrogen and helium) in interstellar space that leads to the formation of new star systems are from these supernova explosions. These "metals" are either produced through fusion during the end of the star's life, liberated during the explosion, or in the case of elements heavier then iron-56, are formed via [[supernova nucleosynthesis]] through the r-process during the explosion itself.<ref>http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html</ref><ref>http://adsabs.harvard.edu/abs/1998ApJ...494..285Q</ref>

 

The remaining core of the supernova will collapse into a degenerate [[neutron star]], if the original star core is under 2 to 3 solar masses.  For a core of greater mass, the core will continue to collapse into a [[black hole]], as neutron degeneracy will not be able to hold back the force of gravity.<ref>http://map.gsfc.nasa.gov/universe/rel_stars.html</ref>

==Observed Supernovas==

==Observed Supernovas==