Supernovae
Background image: The supernova remnant SNR 0519 is located over 150,000 light-years from Earth in the southern constellation of Dorado (The Dolphinfish). It is the remnant of an exploding white dwarf some 600 years ago.
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The life of a star is sustained by the interplay between gravitational energy and nuclear energy. When this equilibrium is broken the star enters a runaway process that leads to a final explosion called supernova where the light emitted outshines the total light of the host galaxy. There are two classes of supernovae: Type Ia (SNIa) and Type II (SNII). The populations of both types are statistically equal.
SNIa are dominated by heavy elements (Oxygen to Iron) and a little contribution of Hydrogen. They can form in all types of galaxies and anywhere in a galaxy. They are not known to leave a remnant like a neutron star. Their light curve (that is the variation of their luminosity with time) has a peak luminosity that lasts about 50 days and decays exponentially. It is this characteristic that makes them a good standard candle for cosmology. They occur as a result of an accreting white dwarf within a binary system. The gravity of the white dwarf being very strong the matter accreted from the companion star (Hydrogen, Helium) is converted into Carbon and Oxygen, triggering Carbon burning and hence detonation. Most of the star burns into Ni and it is the radioactive decay Ni56- Co56- Fe56 that feeds the light curve of SNI.
Other elements, from Si to Ca, are produced in the outer layer of the exploding SNIa and ejected into the interstellar medium. A neutrino burst is produced as well. Note that the supernova class SNIb is actually SNII with a binary star companion.
SNII are dominated by Hydrogen and have few heavy elements. They are concentrated in the spiral arms of galaxies. Depending on the mass of the progenitor star SNII can leave behind remnants such as a neutron star or a black hole. The neutron star can be detected as a pulsar if its magnetic field is strong enough (eg. Crab Nebula). The progenitors of SNII are single stars with a mass larger than 8 solar masses. These stars spend their lifetime –about 10 million years- synthesising elements through Hydrogen, Helium, Carbon and Silicon burning, releasing huge amounts of nuclear energy and building up layers of chemical elements until the phase of production of Iron in the core. When the core reaches the Chandrasekhar limit (about 1.4 solar masses) it can no longer support itself against gravity and runs into gravitational collapse, dragging with it all the outer layers which bounce back into a massive explosion. All chemical elements of the layers and others produced during this explosive phase are ejected into the interstellar medium in an event that lapses fractions of a second. Core collapse is also accompanied by the release of a powerful neutrino flux as detected during the explosion of Supernova 1987A in the Large Magellanic Cloud.
Observations of supernovae date back to the beginning of the first millennium of our era. Their understanding as explosive stars is associated to the emergence of nucleosynthesis as a new discipline.
Our solar system is the archaeological site of supernovae events that occurred a few billion years ago during the evolution of the proto-solar system.
Links
Harvard: The High Z SN Search >
NASA’s Imagine the Universe: Supernovae >
Multimedia
Supernova Cosmology Project: What we can ‘see’ in a supernova’ >
