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by: Carlotta Dare DVM


Carlotta Dare DVM
GPA 3.68


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Class Notes
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This 10 page Class Notes was uploaded by Carlotta Dare DVM on Saturday September 12, 2015. The Class Notes belongs to Physics 20 at University of California - Irvine taught by Staff in Fall. Since its upload, it has received 25 views. For similar materials see /class/201921/physics-20-university-of-california-irvine in Physics 2 at University of California - Irvine.




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Date Created: 09/12/15
Another summary of fate of stars Hydrostatic balance in a star is maintained by stable equilibrium between gas pressure and gravity Gas pressure depends on the temperature of the core and the fusion reaction it supports A star39s energy source is nuclear fusion which builds up heavier elements out of the lighter ones Nuclear burning depends on 1 Temperature of the region Where fusion takes place 2 Chemical composition of the star 3 For any fusion chain it depends on the density of the constituents in the core The next slides show two main sources of energy which maintain the high temperature and pressure needed to balance gravity which depends on stellar Mass and its composition Star is made up of mostly hydrogen and helium when it is formed out of prestellar molecular cloud The percentage of elements heavier than helium is called the metallicity of the star This fraction is determined by the composition of the pre stellar gas Nuclear Energy Sources in stars The pp Chain 3 gtlt106 ltTm lt2 gtlt107 deg K Reaction Energy released Time scale MeV pp gtdev 14 133X1010yrs dp gtHe3y 549 5sec He3He3 gtHe4pp 1286 106years The dp reaction must take place twice to get two Helium threes Total energy released per Helium atom formed is 262 MeV or in more conventional units per helium nucleus formed energy supplied by the fUSiOD is 412gtlt1012 Joules Helium p p cycle stops when 10 of Hydrogen has been burned that in the core of a star Knowing the energy available from Ech2 we can calculate how long the burning has been going on as we know luminosity of the sun MW2gtlt 1030kg Mp 16x 10 27 kg N 1057 protons P MP Four protons are needed to make 1 Helium hence Total energy available for fusion is 2 energy per He formed X number of He formed Hence Total Energy E 1045 Joules Knowling the sun39s luminosity L 4 X 1026 watts J s 45 Sun has been burning hydrogen for 1010 years 1026 If the temperature in the core is higher 30 million deg K then the fusion chain Which comes into operation is the CN cycle CN fusion Cycle The core temperature range 2 to 4X107 deg K This also converts 4 p into 1 He CEH1 gtNi3y 194MeV Ni3 gtC3enu 222MeV C 3Hi gtNi4y 755 MeV 5 Ni4Hi gt0 y 729MeV 05 gtN5ev 276 MeV Ni5H1 gtC2He 496MeV Energy released into particles other than neutrinos 251 MeV If core temperature reaches 100 million degrees at the end of CN cycle 3 Helium nuclei can fuse to make a Carbon nucles called the 3 alpha reaction and leads to the Helium ash 3 x Reactions T my 108 degrees K The sequence 4 4 8 HeZHeZ gt Be 4 8 4 12 Be 4HeZ gt C 6 Releases 73 MeV per Carbon formed What happens after H and He are burnt out It depends on the Mass of the object after the time on MS Case aMflt124ltMsun 1 A p0ssible stable c0nfig is WD with density range p 104 to 1010 kg m3 2 Electran degeneracy pressure sapparts star against collapse 3 R 3000 km 4 Surface Temp 104t0 2 gtlt104 K 5 Faint Blue Stars 2 M G 104kmsec 6 Escape velacily Case b Stars with masses in the range 12 MSWltMlt25 MS 7 1 Cl ld up as neutron stars 1 Neutron degeneracy supports against collapse 2 High densities p 1016 cgIn3 3 Star radius is very small R 10 km 4 Energy released if mass m fell onto a neutron star Via accretion disk a great deal of gravitational energy is released About 10 of its mass 01 mc2 Case 0 If star mass M gt 25 M m a black hole is formed 1 Size of BH is defined in terms of its event horizon or Schwarzchild radius GM C 3 Schwarzchild radius depends on mass RS Elemental synthesis needs higher and higher temperatures as the nuclear charge increases Ci2C2 gtN620He 12 12 23 C 6 C6 gtNa p In radioactive decays and other weak processes Which are important neutrinos and antineutrinos Which are chargeless and interact weakly produced Until very high densities are reached they escape from the star cooling it down or provide refrigeration If the temperature and densities are high enough these protons can be converted into neutrons by electrons and neutrons can be converted to protons by positrons en gtpV e39p gtnv At such temperatures the star emits more neutrinos than photons


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