question: How and why does supernova's happen?
comment: In a star, there's lots of nuclear fusion going on which turns hydrogen to helium and then to berillium I believe. Nuclear fusion is what let's the star burn bright and give out energy to live but when it's run all out of hydrogen and all that's left is the heaviest element it can possibly make (iron), the star collapses on itself because it can't withstand its own gravitational force. Imagine an balloon and you squeezed it harder and harder until it just popped. This is what the gravitational force does on this iron ball now. When it 'pops' this is the supernova, the death of a star.
passages: Section::::Observation history.
   Section::::Current models.:Core collapse.:Type II.
   Section::::Current models.:Thermal runaway.:Normal Type Ia.
   Section::::Current models.:Core collapse.:Type Ib and Ic.
   Supernovae type codes, as described above, are "taxonomic": the type number describes the light observed from the supernova, not necessarily its cause. For example, Type Ia supernovae are produced by runaway fusion ignited on degenerate white dwarf progenitors, while the spectrally similar Type Ib/c are produced from massive Wolf–Rayet progenitors by core collapse. The following summarizes what is currently believed to be the most plausible explanations for supernovae.
  
  Section::::Current models.:Thermal runaway.
   A supernova is a violent explosion of a star that occurs under two principal scenarios. The first is that a white dwarf star, which is the remnant of a low-mass star that has exhausted its nuclear fuel, undergoes a thermonuclear explosion after its mass is increased beyond its Chandrasekhar limit by accreting nuclear-fuel mass from a more diffuse companion star (usually a red giant) with which it is in binary orbit. The resulting runaway nucleosynthesis completely destroys the star and ejects its mass into space. The second, and about threefold more common, scenario occurs when a massive star (12–35 times more massive than the sun), usually a supergiant at the critical time, reaches nickel-56 in its core nuclear fusion (or burning) processes. Without exothermic energy from fusion, the core of the pre-supernova massive star loses heat needed for pressure support, and collapses owing to the strong gravitational pull. The energy transfer from the core collapse causes the supernova display.
   The model for the formation of this category of supernova is a close binary star system. The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant. The two stars now share a common envelope, causing their mutual orbit to shrink. The giant star then sheds most of its envelope, losing mass until it can no longer continue nuclear fusion. At this point, it becomes a white dwarf star, composed primarily of carbon and oxygen. Eventually, the secondary star also evolves off the main sequence to form a red giant. Matter from the giant is accreted by the white dwarf, causing the latter to increase in mass. Despite widespread acceptance of the basic model, the exact details of initiation and of the heavy elements produced in the catastrophic event are still unclear.
   Supernova
  
  A supernova ( plural: supernovae or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. At its peak brightness, the optical luminosity of a supernova can be comparable to that of an entire galaxy, before fading over several weeks or months. A supernova is a transient astronomical event, occurring during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original star, called the "progenitor", either collapses to a neutron star or black hole, or it is completely destroyed.
   Section::::Current models.:Progenitor.
  
  The supernova classification type is closely tied to the type of star at the time of the collapse. The occurrence of each type of supernova depends dramatically on the metallicity, and hence the age of the host galaxy.
   Section::::History.
   Section::::Other impacts.
  
  Section::::Other impacts.:Source of heavy elements.
   Within a few seconds, a substantial fraction of the matter in the white dwarf undergoes nuclear fusion, releasing enough energy (1–) to unbind the star in a supernova. An outwardly expanding shock wave is generated, with matter reaching velocities on the order of 5,000–20,000 km/s, or roughly 3% of the speed of light. There is also a significant increase in luminosity, reaching an absolute magnitude of −19.3 (or 5 billion times brighter than the Sun), with little variation.
   Stars with an initial mass up to about 90 times the sun, or a little less at high metallicity, result in a Type II-P supernova, which is the most commonly observed type. At moderate to high metallicity, stars near the upper end of that mass range will have lost most of their hydrogen when core collapse occurs and the result will be a Type II-L supernova. At very low metallicity, stars of around will reach core collapse by pair instability while they still have a hydrogen atmosphere and an oxygen core and the result will be a supernova with Type II characteristics but a very large mass of ejected Ni and high luminosity.
   A few percent of the Type Ic supernovae are associated with gamma-ray bursts (GRB), though it is also believed that any hydrogen-stripped Type Ib or Ic supernova could produce a GRB, depending on the circumstances of the geometry. The mechanism for producing this type of GRB is the jets produced by the magnetic field of the rapidly spinning magnetar formed at the collapsing core of the star. The jets would also transfer energy into the expanding outer shell, producing a super-luminous supernova.
   The visual light curves of the different supernova types all depend at late times on radioactive heating, but they vary in shape and amplitude because of the underlying mechanisms, the way that visible radiation is produced, the epoch of its observation, and the transparency of the ejected material. The light curves can be significantly different at other wavelengths. For example, at ultraviolet wavelengths there is an early extremely luminous peak lasting only a few hours corresponding to the breakout of the shock launched by the initial event, but that breakout is hardly detectable optically.
   Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting it. Possible causes are accumulation of sufficient material from a binary companion through accretion, or a merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics have been established and accepted by most astronomers for some time.
   Section::::Formation.
  
  The most massive stars, with masses ranging from eight to roughly one hundred solar masses and spectral types of O and early B, are usually found in groups called OB associations. Massive O stars have strong stellar winds, and most of these stars explode as supernovae at the end of their lives.
  
  The strongest stellar winds release kinetic energy of 10 ergs (10 J) over the lifetime of a star, which is equivalent to a supernova explosion. These winds can form stellar wind bubbles dozens of light years across.
   Section::::Silicon burning.
   Remnants of many supernovae consist of a compact object and a rapidly expanding shock wave of material. This cloud of material sweeps up the surrounding interstellar medium during a free expansion phase, which can last for up to two centuries. The wave then gradually undergoes a period of adiabatic expansion, and will slowly cool and mix with the surrounding interstellar medium over a period of about 10,000 years.
   Type Ia supernovae are produced from white dwarf stars in binary systems and occur in all galaxy types. Core collapse supernovae are only found in galaxies undergoing current or very recent star formation, since they result from short-lived massive stars. They are most commonly found in Type Sc spirals, but also in the arms of other spiral galaxies and in irregular galaxies, especially starburst galaxies.
   List of stellar explosion types
  
  Stellar explosion can refer to
  
  BULLET::::- nova
  
  BULLET::::- supernova
  
  BULLET::::- type Ia supernova
  
  BULLET::::- Type Ib and Ic supernovae
  
  BULLET::::- Type II supernova
  
  BULLET::::- Superluminous supernova
  
  BULLET::::- Pair-instability supernova
  
  BULLET::::- Supernova impostor, stellar explosions that appear similar to supernova, but do not destroy their progenitor stars
  
  BULLET::::- failed supernova
  
  BULLET::::- Luminous red nova, an explosion thought to be caused by stellar collision
  
  BULLET::::- solar flares are a minor type of stellar explosion
  
  BULLET::::- Tidal disruption event, the pulling apart of a star by tidal forces
   Type Ia supernovae derive their energy from a runaway nuclear fusion of a carbon-oxygen white dwarf. The details of the energetics are still not fully understood, but the end result is the ejection of the entire mass of the original star at high kinetic energy. Around half a solar mass of that mass is Ni generated from silicon burning. Ni is radioactive and decays into Co by beta plus decay (with a half life of six days) and gamma rays. Co itself decays by the beta plus (positron) path with a half life of 77 days into stable Fe. These two processes are responsible for the electromagnetic radiation from Type Ia supernovae. In combination with the changing transparency of the ejected material, they produce the rapidly declining light curve.
   Section::::The "r"-process.
   Section::::Origin and lifetime.
  
  The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20 K) and dense cloud consisting mostly of molecular hydrogen. GMCs can exist in a stable state for long periods of time, but shock waves due to supernovae, collisions between clouds, and magnetic interactions can trigger its collapse. When this happens, via a process of collapse and fragmentation of the cloud, stars are born (see stellar evolution for a lengthier description).
   In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about 100 billion kelvin, 6000 times the temperature of the sun's core. At this temperature, neutrino-antineutrino pairs of all flavors are efficiently formed by thermal emission. These thermal neutrinos are several times more abundant than the electron-capture neutrinos. About 10 joules, approximately 10% of the star's rest mass, is converted into a ten-second burst of neutrinos which is the main output of the event. The suddenly halted core collapse rebounds and produces a shock wave that stalls within milliseconds in the outer core as energy is lost through the dissociation of heavy elements. A process that is is necessary to allow the outer layers of the core to reabsorb around 10 joules (1 foe) from the neutrino pulse, producing the visible brightness, although there are also other theories on how to power the explosion.
  