White Dwarfs and Supernovae Part II
From Cassiopedia
Supernovae (the plural of supernova) are extremely important for understanding our Galaxy. They heat up the interstellar medium, distribute heavy elements throughout the Galaxy, and accelerate cosmic rays. But just what is a supernova? And is there more than one type?
Supernovae are massive giant exploding stars. When the explosion occurs, the resulting illumination can be as bright as an entire galaxy. Supernova occurs at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. 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.
In massive stars the core is more complicated since it is under a gravitational field pull that is greater then those in less massive stars. In less massive stars (stars with a mass < 8 Msun --See Part I) the helium core eventually fuses to form a carbon core (with the liberation of more energy) and the carbon core further contracts and gets hotter to form a core of more complicated atoms---a core now composed of oxygen and neon.
But in more massive stars, due to the even greater gravitational force, the core is even hotter and the fusion reactions can continue past the oxygen-neon stage of smaller stars. The star is becoming like an onion, where the different concentric layers correspond to different fusion reactions. The outermost layer is burning hydrogen (H) to form helium (He), next, it's helium which is changing into carbon (C), then oxygen (O) is forming, and when we go deeper to the core, we find more and more heavy elements. The neon can now combine further to magnesium, which then can combine further to form silicon, and finally to IRON. At a late stage in it's life the central temperature of the star may have reached 3 to 4 billion degrees and there may be more then half a dozen concentric rings formed around the star, each of which a different fuel is being consumed. Once the star's core has reached the iron stage then it has reached a dead end since it is at this point that the iron atoms represent the point of maximum stability and minimum energy content (because of iron's nuclear structure, it does not permit its atoms to fuse into heavier elements). Beyond this stage, only an additional input of energy could alter the iron atoms in the direction of more or less complex atoms.
The end of a massive star is a very fast process: if the fusion of hydrogen, as long as the star is on the main sequence, can last billions of years, all of the carbon is transformed in 10,000 years, all of the neon and the oxygen in one year, and the final transformation of silicon to iron requires only one day.
As central temperatures within the iron core increase with age, radiation pressure will increases in proportion to the fourth power of the temperature. When the temperature doubles the radiation pressure increases sixteenfold and the delicate balance between gravitation and radiation pressure now becomes even more and more delicate.
Due to this delicate balance between gravitation and radiation pressure, and since nuclear fusion stops at iron because energy can no longer be produced by fusion, this additional energy is generated from the gravitational force when the iron core begins to collapse under its own weight. Within a very short (less then an hour) time after the iron core has formed, the star begins the final phase of gravitational collapse. The core temperature now rises to over 100 billion degrees as the iron atoms are crushed together and the star shrinks drastically to a tiny fraction of its former volume.
The repulsive force between the nuclei overcomes the force of gravity. So the core compresses, but then recoils. The energy of the recoil is transferred to the envelope of the star, which then explodes and produces a shock wave. As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. The shock then propels the matter out into space. The material that is exploded away from the star is now known as a supernova remnant.
All that remains of the original star is a small, super-dense core composed almost entirely of neutrons -- a neutron star. Or, if the original star was very massive indeed (say 15 or more times the mass of our Sun), even the neutrons cannot survive the core collapse...and a black hole forms. During the supernova explosion and the stars final collapse into a neutron star, all of its binding energy in radiated in the form of neutrinos, most of which have energies in the range 10-30 MeV. These neutrinos come in all flavors, and are emitted over a timescale of several tens of seconds. The neutrino luminosity of a gravitational collapse-driven supernova is typically 100 times its optical luminosity. The neutrino signal emerges from the core of a star promptly after core collapse, whereas the photon signal may take hours or days to emerge from the stellar envelope. The neutrino signal can therefore give information about the very early stages of core collapse, which is inaccessible to other kinds of astronomy. In fact, an optical supernova display may never be seen at all for a given core collapse: some collapsing stars may never blow up into supernovae, or the star may live in an obscured region of the galaxy.
See Black Hole