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lec-18-1 - Lecture 18 Stellar Evolution(II The evolution of...

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Lecture 18: Stellar Evolution (II) The evolution of high-mass stars is discussed in Chapter 15 of Carroll & Ostlie. I. Evolution of High-Mass Stars ( M 8 M ) Like low-mass stars, high-mass stars go through stages of core H burning, shell H burning, core He burning, and shell He burning. What makes high-mass stars different is they don’t stop there. The temperature is high enough ( T 10 8 K) that carbon and oxygen can burn by reacting with helium: 12 6 C + 4 2 He 16 8 O + γ 16 8 O + 4 2 He 20 10 Ne + γ As the temperature increases still further, carbon and oxygen can begin to react with each other, producing a whole range of byproducts. Above 6 × 10 8 K we can have the carbon reactions: 12 6 C + 12 6 C 16 8 O + 2 4 2 He 20 10 Ne + 4 2 He 23 11 Na + p 23 12 Mg + n 24 12 Mg + γ Above 10 9 K we have the oxygen burning reactions: 16 8 O + 16 8 O 24 12 Mg + 2 4 2 He 28 14 Si + 4 2 He 31 15 P + p 31 16 S + n 32 16 S + γ Above 3 × 10 9 K the heavier elements can burn through reactions such as: 28 14 Si + 4 2 He 32 16 S + γ 32 16 S + 4 2 He 36 18 Ar + γ . . . 52 24 Cr + 4 2 He 56 28 Ni + γ 1
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These reactions are collectively referred to as silicon burning . We see that the star works its way up the periodic table. It develops an onion-like structure as shown in Figure 1. Figure 1: Onion-like structure of a massive star that has evolved through silicon core burning. (From Carroll & Ostlie Fig. 15.10.) The process of moving up the period table is called nucleosynthesis . All the elements in the universe heavier than hydrogen, helium, and lithium are created by nucleosynthesis in dying stars. As nucleosynthesis creates heavier and heavier nuclei, fusion releases less and less energy, because the available binding energy gets smaller and smaller. The burning must go faster and faster in order to supply the energy the star needs to avoid collapsing. As a result, the stages are progressively more rapid. For a 20 M star, here are the durations: core hydrogen burning 10 7 yr core helium burning 10 6 yr core carbon burning 300 yr core oxygen burning 200 days core silicon burning 2 days
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