Supernovae as tools for distance measurement
Index
 
HyperPhysics***** Astrophysics R Nave
Go Back





Supernovae

A supernova is an explosion of a massive supergiant star. It may shine with the brightness of 10 billion suns! The total energy output may be 10^44 joules, as much as the total output of the sun during its 10 billion year lifetime. The likely scenario is that fusion proceeds to build up a core of iron. The "iron group" of elements around mass number A=60 are the most tightly bound nuclei, so no more energy can be gotten from nuclear fusion.

In fact, either the fission or fusion of iron group elements will absorb a dramatic amount of energy - like the film of a nuclear explosion run in reverse. If the temperature increase from gravitational collapse rises high enough to fuse iron, the almost instantaneous absorption of energy will cause a rapid collapse to reheat and restart the process. Out of control, the process can apparently occur on the order of seconds after a star lifetime of millions of years. Electrons and protons fuse into neutrons, sending out huge numbers of neutrinos. The outer layers will be opaque to neutrinos, so the neutrino shock wave will carry matter with it in a cataclysmic explosion.

Supernovae are classified as Type I or Type II depending upon the shape of their light curves.

The synthesis of the heavy elements is thought to occur in supernovae, that being the only mechanism which presents itself to explain the observed abundances of heavy elements.

Index

Supernova concepts
 
HyperPhysics***** Astrophysics R Nave
Go Back





Type I and Type II Supernovae

Supernovae are classified as Type I if their light curves exhibit sharp maxima and then die away smoothly and gradually. The model for the initiation of a Type I supernova is the detonation of a carbon white dwarf when it collapses under the pressure of electron degeneracy. It is assumed that the white dwarf accretes enough mass to exceed the Chandrasekhar limit of 1.4 solar masses for a white dwarf. The fact that the spectra of Type I supernovae are hydrogen poor is consistent with this model, since the white dwarf has almost no hydrogen. The smooth decay of the light is also consistent with this model since most of the energy output would be from the radioactive decay of the unstable heavy elements produced in the explosion.

Type II supernovae are modeled as implosion-explosion events of a massive star. They show a characteristic plateau in their light curves a few months after initiation. This plateau is reproduced by computer models which assume that the energy comes from the expansion and cooling of the star's outer envelope as it is blown away into space. This model is corroborated by the observation of strong hydrogen and helium spectra for the Type II supernovae, in contrast to the Type I. There should be a lot of these gases in the extreme outer regions of the massive star involved.

Type II supernovae are not observed to occur in elliptical galaxies, and are thought to occur in Population I type stars in the spiral arms of galaxies. Type I supernovae occur typically in elliptical galaxies, so they are probably Population II stars.

Index

Supernova concepts

Reference
Chaisson & McMillan
Ch. 21
 
HyperPhysics***** Astrophysics R Nave
Go Back





Type I and Type II Supernovae

Supernovae are classified as Type I if their light curves exhibit sharp maxima and then die away gradually. The maxima may be about 10 billion solar luminosities. Type II supernovae have less sharp peaks at maxima and peak at about 1 billion solar luminosities. They die away more sharply than the Type I. Type II supernovae are not observed to occur in elliptical galaxies, and are thought to occur in Population I type stars in the spiral arms of galaxies. Type I supernovae occur typically in elliptical galaxies, so they are probably Population II stars.

Index

Supernova concepts
 
HyperPhysics***** Astrophysics R Nave
Go Back