Chapter 20 - Physics1902 TheLifeofaStar Lecture W8.a...

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1 Physics 1902 The Life of a Star Lecture W8.a Chapter 20
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2 Life on the Main Sequence For most of its life a star will consume hydrogen producing helium Basic (pp) chain dominates in our sun P+p->d + e + + ν D + p -> 3 He 3 He + 3 He -> 4 He + p + p
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3 Nuclear Reactions in the Sun e +
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4 What controls the rate? For 2 protons to react they must come close together (range of strong force ~ 1 fm (10 -15 m) The are each positively charge so Coulomb repulsion holds them apart Can only overcome the Coulomb force if they collide at high speed To get atoms at high speed we need high temperatures
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5 Pp chain The reaction p+p -> d + e + + ν requires the weak force. This greatly reduces the rate
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6 CNO Cycle 12 C + p -> 13 N 13 N -> 13 C + e + + ν 13 C + p -> 14 N 14 N + P -> 15 O 15 O -> 15 N + e + + ν 15 N + p -> 12 C + 4 He Weak processes occur twice but in decays Higher z implies higher Coulomb barriers Important process in large stars with higher core temperatures
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7
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8 Stellar Stability Star is balanced between gravitational attraction and expansion due to pressure The greater the temperature the greater the pressure If there is a slight contraction the reaction rate will rise producing more heating This will cause an increase in pressure and an expansion Thus the system is stable
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9 Initial stellar abundance
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10 Abundance in our Sun
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11 After 10 billion years
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12 Life on the main sequence
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Chapter 20 - Physics1902 TheLifeofaStar Lecture W8.a...

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