stellar_explosions - Stellar Explosions (ch. 21) First, a...

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Stellar Explosions (ch. 21) First, a review of low-mass stellar evolution by means of an illustration shown below. You should be able to talk your way through this diagram and it should take at least half an hour. Remember that all stars less massive than about 8 Mo go through these phases (except not the helium flash above about 2 Mo). What is the major reason why the advanced evolution of higher-mass stars is so different?
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Before discussing high-mass stellar deaths, don’t forget the material in Sec. 21.1 : “nova explosions” by mass transfer from a RG to a WD companion. This is really a continuation of sec. 20.6 on binary star evolution. Read this for your interest, but it will not be on the exam. However, you will find it extremely useful throughout the rest of the course to understand the idea of an accretion disk that is introduced in this section. They will come up again and again. Death of a High-Mass Star In short: Envelope explodes as a core collapse supernova . The core implodes and ends up as a neutron star or (more massive) a black hole . Let’s see how this occurs. (Remember, this is all theoretical calculations, but later you’ll see that there is surprising observational confirmation for these calculations.) Core is layered like an onion, with heavier elements closer to center (since they are the ashes of a previous fuel): He, C, O, Ne, Mg, Si, Fe These are the main elements produced up to this phase because they are produced by adding helium (He, “alpha particles”) to heavier and heavier nuclei. (Iron Fe is in bold in this list because it is the “end of the line” for stellar nuclear fusion—see below.) (Question to see if you understand nuclear fusion: why is so much easier to burn He with, say, O, rather than C+O or O+O ? Hint: why do main sequence stars burn hydrogen rather than something heavier?) On the next page is a cutaway drawing of what the inside of a massive star might look like as it completes more and more burning stages. A good “cutaway” sketch of this structure is also given in your text, Fig. 21.5, p. 558.
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First way to get a supernova: core collapse . Massive stars burn nuclear fuels up to iron (Fe). But nuclear fusion of iron does not produce energy, it uses energy. (Fig. 21.6, p. 559) This leads to loss of pressure support core collapse Temp. is so large (~10 billion K) that the gamma ray photons (Wien’s law) have huge energies and photodisintegrate the iron into protons and neutrons. This absorbs even more thermal (heat) energy, so the core collapses even faster! Gravity is having its way… The protons combine with electrons to give neutrons
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This note was uploaded on 04/20/2008 for the course AST 301 taught by Professor Harvey during the Fall '07 term at University of Texas at Austin.

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stellar_explosions - Stellar Explosions (ch. 21) First, a...

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