Figure: Supernova 1994D in Galaxy NGC 4526

The supernova 1994D in the NGC 4526 galaxy is the bright spot on the lower left.

A supernova (plural supernovae) is a stellar explosion which produces an extremely bright object that fades to invisibility over weeks or months. A supernova may briefly out-shine its entire host galaxy. It would take 10 billion years for the Sun to produce the energy output of an ordinary Type II supernova.

Type II Supernova

Stars far more massive than the sun evolve in much more complex fashions. In the core of the sun, hydrogen is fused into helium, releasing energy which heats the sun's core, and providing pressure which supports the sun's layers against collapse. The helium produced in the core accumulates there since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted, fusion begins to slow down and gravity begins to cause the core to contract. This contraction raises the temperature high enough to initiate a shorter phase of helium fusion, which accounts for less than 10% of the star's total lifetime. In stars with less than eight solar masses, the carbon produced by helium fusion does not fuse, and the star gradually cools to become a white dwarf. White dwarf stars, if they have a near companion, may then become Type Ia supernovae.

A much larger star, however, is massive enough to create temperatures and pressures needed to cause the carbon in the core to begin to fuse once the star contracts at the end of the helium-burning stage. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, surrounding a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon, surrounding layers that fuse to progressively heavier elements. As a star this massive evolves, it undergoes repeated stages where fusion in the core stops, and the core collapses until the pressure and temperature is sufficient to begin the next stage of fusion, reigniting to halt collapse.

Core collapse

The factor limiting this process is the amount of energy that is released through fusion, which is dependent on the binding energy of these atomic nuclei. Each additional step produces progressively heavier nuclei, which release progressively less energy when fusing, until iron is produced. As iron and nickel have the highest binding energy per nucleon of all the elements, iron cannot produce energy when fused, and an iron core grows. This iron core is under huge gravitational pressure. As there is no fusion to further raise the star's temperature to support it against collapse, it is supported only by degeneracy pressure of electrons. When the core's size exceeds the Chandrasekhar limit, degeneracy pressure can no longer support it, and catastrophic collapse ensues.

The outer part of the core reaches velocities of up to 70,000 km/s (0.23c) as it collapses toward the center of the star. The rapidly shrinking core heats up, producing high energy gamma rays which decompose iron nuclei into helium nuclei and free neutrons (via photodissociation). As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and neutrinos. The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star. Some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion.

For Type II supernovae, the collapse is eventually halted by short-range repulsive neutron-neutron interactions mediated by the strong force, as well as by degeneracy pressure of neutrons, at a density comparable to that of an atomic nucleus. Once collapse stops, the infalling matter rebounds, producing a shock wave that propagates outward. The energy from this shock dissociates heavy elements within the core. This reduces the energy of the shock, which can stall the explosion within the outer core.

The core collapse phase is known to be so dense and energetic that only neutrinos are able to escape. Most of the gravitational potential energy of the collapse gets converted to a ten second neutrino burst, releasing about 10⁴⁶ joules . Of this energy, about 10⁴⁴ Joules is reabsorbed by the star producing an explosion. This energy revives the stalled shock, which blows off the rest of the star's material. The neutrinos produced by a supernova have been actually observed in the case of Supernova 1987A leading astronomers to conclude that the core collapse picture is basically correct.

When the progenitor star is below about 20 solar masses (depending on the strength of the explosion and the amount of material that falls back), the degenerate remnant of a core collapse is a neutron star. Above this mass the remnant collapses to form a black hole. The theoretical limiting mass for this type of core collapse scenario is about 40-50 solar masses. Above that mass, a star is believed to collapse directly into a black hole without forming a supernova explosion.

For more explanations about the SN phenomenon, see http://en.wikipedia.org/wiki/Supernova.

Credit: NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team

http://hubblesite.org/newscenter/archive/releases/1999/19/image/i