lecture21_08 - Lecture 21 The Death of Stars: Novae...

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Lecture 21 The Death of Stars: Novae Supernovae Remnants, Neutron stars Black Holes
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Term Test 2 Results (no make-up results yet available). A Multiple choice marks are on line!
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Novae 1. A binary system of a white dwarf and a newly formed red giant will result in the formation of an accretion disk around the white dwarf. The material in the disk comes from the red giant and is mostly hydrogen. 2. An accretion disk is a rotating disk of gas orbiting a star, formed by material falling toward the star.
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Novae (cont.) 3. The hydrogen builds up on the surface of the white dwarf, becomes denser and hotter and can ignite in an explosive fusion reaction when the temperature reaches 10 million K. 4. This surface explosion blows off the outer layers of the white dwarf. Though this shell contains a tiny amount of mass (0.0001 M / ) it can cause the white dwarf to brighten by 10 to 20 magnitudes (10,000 to 100 million times brighter) in a few days.
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Nova Cygni 1975, before and after outburst.
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Nova Cygni 1975: This nova was also observed spectroscopically at the David Dunlap Observatory by then-graduate student Bruce Campbell using an early solid-state photodioce array detector.
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Novae (cont.) 5. A nova is a star that suddenly and temporarily brightens, thought to be due to new material being deposited on the surface of a white dwarf. 6. Because so little mass is blown off during a nova, the explosion does not disrupt the binary system. Ignition of the infalling hydrogen can recur again with periods ranging from months to thousands of years.
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Nova explosion: paintings by Pat Rawlings , NASA/STScI Expanding Nebula Thermonuclear Runaway
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A nova explosion (thermo-nuclear runaway) throws out about a millionth of a solar mass into the interstellar medium.
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Type I Supernovae 1. If accretion brings the mass of a white dwarf above the Chandrasekhar limit, electron degeneracy can no longer support the star, and it collapses. The collapse raises the core temperature and runaway carbon fusion begins, which ultimately leads to the star exploding
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lecture21_08 - Lecture 21 The Death of Stars: Novae...

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