Lecture18OriginsofElements-2_001

Lecture18OriginsofElements-2_001 - L Lecture 18 Origin of...

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L H Lecture 18 Hertzsp 8: Origin o rung Ru of the Elem ussell D ments 2 iagram
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Temperature, size and luminosity We know that for objects that are approximately blackbodies Hotter things are brighter. o Energy radiated per unit time per unit area is proportional to T 4 , o so bigger T means more energy radiated Bigger things are brighter. o Energy radiated per unit time per unit area is proportional to T 4 , o so bigger surface area means more energy radiated. Putting this together, we have [Taken from http://zebu.uoregon.edu/~soper/Stars/hrdiagram.html ]
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C. Explosive Nucleosynthesis – Nova Explosions – Massive Stars 1. Carbon and Oxygen Burning: C/O core a . C o n d i t i o n s ~ 5 10 5 g/cm 3 T ~ 5 10 8 K b. Heavy-ion Reactions: 12 C + 12 C 20 Ne + 4 He 12 C + 16 O 24 Mg + 4 He 16 O + 16 O 28 Si + 4 He c. Reaction Rates now rapid – all strong, two-body interactions. Nuclide mixture becomes richer ; secondary reactions from odd-Z nuclei – less probable since must be made from even-Z nuclei. e . g . , 12 C( 16 O, n) 27 Si   27 Al 24 Mg(n, ) 25 Mg 2. Silicon Burning: e-process [e for equilibrium] a. Conditions: ~ 10 6 g/cm 3 T ~ 5 10 9 K b. Reactions: ( , ) and ( , ) in "equilibrium" (i.e., 4n nuclei) 28 14 28 4 16 32 4 40 Si Mg: Si He S + He Ar Ca (,)  24 18 36 28 56 N i All reactions exothermic: Q = + c. Reactions are in "equilibrium'', but material is processed t o w a r d s 28 56 N i d. Net Product:
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Lecture18OriginsofElements-2_001 - L Lecture 18 Origin of...

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