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rev.ex2.fall.10 - Astronomy Bizarre 309N Fall 2010(47700...

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Astronomy Bizarre Wheeler 309N Fall 2010 September 27, 2010 (47700) Review for Test #2 SUPERNOVAE - CONTINUED To burn a thermonuclear fuel, the star must get hotter to overcome the charge repulsion. This happens automatically in massive stars supported by the thermal pressure that regulates their burning. These stars produce shells of ever-heavier elements and finally a core of iron. Iron (with 26p and 30n) is endothermic, absorbing energy. This will reduce the pressure in the core and cause the collapse of the iron core to form a neutron star. The collapse of the core, a gravitational collapse, causes essentially all the protons to be converted to neutrons, releasing a flood of neutrinos and forming a neutron star. Massive star binaries - Explosions of massive stars in close binary systems are expected to occur in a bare thermal pressure-supported core from which the outer layers of hydrogen have been transferred to the companion star. The core, supported by the thermal pressure, will continue to evolve to iron, even in the absence of the hydrogen envelope. This is probably the origin of Types Ib and Ic. Repulsive nuclear force between compressed neutrons and neutron quantum pressure halt the collapse and allow the neutron star to form. Neutron star – mass of Sun, but size of a small city. Huge density, surface gravity. Maximum mass of about 2 solar masses. Forming a neutron star by core collapse produces about 100 times more energy than needed to create an explosion, but most of that energy is carried off by neutrinos. The core collapse explosion of the outer layers of the star may occur in one of three ways: 1. Prompt mechanism: The neutron star rebounds, driving a shock wave into the outer parts of the star. The bounce shock occurs, but is insufficient to cause an explosion. 2. Delayed mechanism: Neutrinos stirred out by the boiling neutron star deposit heat behind the standing shock and reinvigorate it. Not clear this is sufficient. 3. Jet mechanism: the collapsing rotating neutron star squeezes the magnetic field and sends a jet up the rotation axis. Naturally makes asymmetric explosion, but not yet clear sufficiently strong jets are produced. Standing shock – a strong pressure wave that forms due to neutron-star bounce, but which stalls a certain distance from the neutron star as outer material rains down on it.
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