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Gravitational collapse
Dec. 25th, 2018 06:02 pmToday I learned that supernova explosion is fueled not by fusion, but by gravitational collapse of star's core:
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http://faculty.wcas.northwestern.edu/~infocom/The%20Website/large.html
For a very short while the nuclear burning above it continues, but for a star as massive as this one there is not much time left before the burned-out iron "ashes" in the core grow into a ball 1.4 times as massive as the Sun. As predicted by Chandrasekhar in 1931, the degenerate iron is then as massive as a white dwarf can be.
It has reached Chandrasekhar's Limit.
In the blink of an eye, the entire iron core collapses from the size of the planet Mars to a sphere only 12 miles across. Under the fantastic pressure of the collapse, the iron nuclei are smashed so closely together that they are literally crushed out of existence and turn instead into a soup of swarming protons and neutrons. At such densities the rules of quantum mechanics force the electrons to fuse with the protons (which converts the protons into neutrons), and in a raging instant, neutrons are almost all that is left. The core of the red giant suddenly convulses into a bizarre, gargantuan "nucleus" with 1.4 solar masses of neutrons, very few protons, and a density of billions of tons per cubic inch.
The electromagnetic forces that had once held up the electron-degenerate matter in the white dwarf are gone, because there are no longer any electrons. As the neutrons are crushed down to the density of atomic nuclei, however, the strong nuclear force comes into play. The strong nuclear force doesn't like particles to get close together any more than the electromagnetic force does, and the strong nuclear force is, well, strong. When it finally exerts inself, the collapsing neutron matter ringingly slams to an almost instantaneous halt at a radius of perhaps six miles.
Meanwhile, behind the neutron matter, normal matter from the layers just above the core is plunging downward with a gravitational acceleration so phenomenal that in the few tenths of a second it takes to reach the center, it is already moving at 25,000 miles per second. A mass of sulfur, silicon, and oxygen that is a quarter-million times more massive than the Earth and moving at 15% the speed of light slams into the neutron core – and rebounds off it like a rubber ball hitting a solid steel bulk-head. An enormous shock wave begins to spread outward.
The collapse of the white dwarf core into a neutron mass has released far more gravitational energy in the span of one second than the star has released in the form of nuclear energy in its entire life, and we are talking about a very large star. (As I pointed out when discussing the helium flash of solar-type stars, it is flabbergasting how much energy there is in gravitational collapse, if the collapse is massive enough and deep enough.) Almost all of this gravitational energy has been transformed into heat in the neutron core, but it does not stay there. Almost as swiftly as it was created, the energy is radiated away by subatomic particles known as neutrinos.
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Only 0.3% of the energy released through neutrinos is used to blow up the supernova:
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About 99.7% of the neutrinos punch through the outer layers of the red giant as though they aren't there, and race into space at the speed of light. (Stopping a neutrino with ordinary matter is about like stopping a rifle bullet with a bowl of Jello – which is exactly why the neutrinos radiate away from the neutron core so readily.) The remaining 0.3% of the neutrino pulse is absorbed by the very dense matter in the shock wave retreating from the center. An absorption of 0.3% might not sound like much, but 0.3% of an unimaginable quantity is still unimaginable. The shock wave is instantly blasted into a super-heated maelstrom so hot, the resulting detonation literally blows away everything above the neutron core. At least five solar masses of gas, and possibly four times that much, are hurled away from the star at velocities of tens of thousands of kilometers per second. The energy of the ejected gas is so great, if it slams into a nearby interstellar cloud it can shock the entire cloud into a sudden collapse, thus creating scores of new stars at a single blow.
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========================
http://faculty.wcas.northwestern.edu/~infocom/The%20Website/large.html
For a very short while the nuclear burning above it continues, but for a star as massive as this one there is not much time left before the burned-out iron "ashes" in the core grow into a ball 1.4 times as massive as the Sun. As predicted by Chandrasekhar in 1931, the degenerate iron is then as massive as a white dwarf can be.
It has reached Chandrasekhar's Limit.
In the blink of an eye, the entire iron core collapses from the size of the planet Mars to a sphere only 12 miles across. Under the fantastic pressure of the collapse, the iron nuclei are smashed so closely together that they are literally crushed out of existence and turn instead into a soup of swarming protons and neutrons. At such densities the rules of quantum mechanics force the electrons to fuse with the protons (which converts the protons into neutrons), and in a raging instant, neutrons are almost all that is left. The core of the red giant suddenly convulses into a bizarre, gargantuan "nucleus" with 1.4 solar masses of neutrons, very few protons, and a density of billions of tons per cubic inch.
The electromagnetic forces that had once held up the electron-degenerate matter in the white dwarf are gone, because there are no longer any electrons. As the neutrons are crushed down to the density of atomic nuclei, however, the strong nuclear force comes into play. The strong nuclear force doesn't like particles to get close together any more than the electromagnetic force does, and the strong nuclear force is, well, strong. When it finally exerts inself, the collapsing neutron matter ringingly slams to an almost instantaneous halt at a radius of perhaps six miles.
Meanwhile, behind the neutron matter, normal matter from the layers just above the core is plunging downward with a gravitational acceleration so phenomenal that in the few tenths of a second it takes to reach the center, it is already moving at 25,000 miles per second. A mass of sulfur, silicon, and oxygen that is a quarter-million times more massive than the Earth and moving at 15% the speed of light slams into the neutron core – and rebounds off it like a rubber ball hitting a solid steel bulk-head. An enormous shock wave begins to spread outward.
The collapse of the white dwarf core into a neutron mass has released far more gravitational energy in the span of one second than the star has released in the form of nuclear energy in its entire life, and we are talking about a very large star. (As I pointed out when discussing the helium flash of solar-type stars, it is flabbergasting how much energy there is in gravitational collapse, if the collapse is massive enough and deep enough.) Almost all of this gravitational energy has been transformed into heat in the neutron core, but it does not stay there. Almost as swiftly as it was created, the energy is radiated away by subatomic particles known as neutrinos.
========================
Only 0.3% of the energy released through neutrinos is used to blow up the supernova:
----------
About 99.7% of the neutrinos punch through the outer layers of the red giant as though they aren't there, and race into space at the speed of light. (Stopping a neutrino with ordinary matter is about like stopping a rifle bullet with a bowl of Jello – which is exactly why the neutrinos radiate away from the neutron core so readily.) The remaining 0.3% of the neutrino pulse is absorbed by the very dense matter in the shock wave retreating from the center. An absorption of 0.3% might not sound like much, but 0.3% of an unimaginable quantity is still unimaginable. The shock wave is instantly blasted into a super-heated maelstrom so hot, the resulting detonation literally blows away everything above the neutron core. At least five solar masses of gas, and possibly four times that much, are hurled away from the star at velocities of tens of thousands of kilometers per second. The energy of the ejected gas is so great, if it slams into a nearby interstellar cloud it can shock the entire cloud into a sudden collapse, thus creating scores of new stars at a single blow.
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