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COSMOLOGY : SUPERNOVA

Δ COSMOLOGY Δ

The catastrophic, explosive death of a star, accompanied by the sudden, transient brightening of the star to an optical luminosity comparable to that of an entire galaxy.

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A supernova shines typically for several weeks to several months with a luminosity between 2 × 10^8 and 5 × 10^9 times that of the Sun, then gradually fades away. Each explosion ejects from one to several tens of solar masses at speeds ranging from thousands to tens of thousands of kilometers per second. The total kinetic energy, 10^44 joules (2.5 × 10^28 megatons of high explosive), is about 100 times the total light output, making supernovae some of the highest-energy explosions in the universe. Unlike its fainter relative, the nova, a supernova does not recur for the same object.
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(fig.3 ATOMIC CLOCK:)Type Ia supernovae may be regarded as nature's largest thermonuclear bombs. They occur when an accreting white dwarf, composed of carbon and oxygen, grows to a mass 1.38 times that of the Sun, almost the critical mass that can be supported by electron degeneracy pressure, and ignites carbon fusion near its center. Ignition occurs when carbon fusion at the center releases energy faster than neutrinos can carry it away. Because the pressure is insensitive to the temperature, a nuclear runaway occurs. Fusion releases energy, which raises the temperature, which makes fusion go faster, but the gas cannot expand and cool. The nuclear runaway spreads in about 1 second through the star. The energy released by this nuclear burning is more than enough to completely blow the white dwarf apart with high velocity. Nothing remains—no neutron star, no black hole, and no burst of neutrino emission.

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dn

(fig.4 POLLUTION & INDUSTRIES:) A typical type II supernova results from a star somewhat over 8 solar masses, on the main sequence, that spends its last years as a red supergiant burning progressively heavier fuels in its center. The radius of the star, after hydrogen has burned and the star is part way through helium burning, is roughly 500 solar radii, and its luminosity is already about 100,000 times that of the Sun. Each burning stage is shorter than the previous one. The last stage turns silicon and sulfur into a ball of roughly 1.4 solar masses of iron. Once iron has been produced, no more nuclear energy is available.A combination of instabilities now leads to the implosion of the iron core to a neutron star. When the density at the center reaches several times that of the atomic nucleus, the collapse halts and briefly springs back owing to the short-range repulsive component of the nuclear force. But the energy of this bounce is soon dissipated, and a hot young neutron star remains which, over the next few seconds, radiates away its heat and binding energy as neutrinos. See also Neutrino; Strong nuclear interactions.

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