36. Stellar Evolution Main Sequence Evolution

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Astronomy 242: Foundations of Astrophysics II 36. Stellar Evolution Main Sequence Evolution As the hydrogen in the core is slowly used up, a star’s ‘thermostat’ gradually raises the core’s temperature. With a higher temperature, more energy ﬂows out, and luminosity increases. For example, the Sun had only ~70% of its present luminosity when it formed. Becomes a Red Giant inert helium Broken Thermostat core contracts photosphere A Main-Sequence Star... photosphere hydrogen- ‘burning’ core hydrogen- ‘burning’ shell H begins fusing to He in a shell around the contracting He core — but this can’t stop the contraction, so the core gets hotter and the star gets even more luminous. After the Main Sequence Once hydrogen starts burning in a shell, a star becomes larger, redder, and much more luminous — at least for a while. Triple-Alpha Process When the core temperature hits 108 K, helium nuclei can overcome their repulsion and fuse to make carbon. Compared to hydrogen fusion, this is not very efﬁcient. Helium ‘Burning’ Star helium fusing in core hydrogen fusing in shell The central thermostat once again stabilizes the star’s energy production, and the total luminosity decreases. Horizontal-Branch Stars Models predict that stars HR diagrams of old star get smaller, bluer, and less clusters reveal stars with luminous after He ignition. the predicted properties. Alpha Capture Reactions High core temperatures enable helium to fuse with other nuclei, producing 16O, 20Ne, 24Mg, . Asymptotic Giant Branch Double-shell Supergiant 1982ApJ...260..821I C shell: 4H→He shell: 3He→C After the helium in the This situation is unstable. core is gone, burning Thermal pulses eject 4 continues in two shells. mass (up to 10 M⊙/yr). Fate of the Sun Evolution of High-Mass Star (M ≥ 8 M⊙) Main-Sequence Star Red Supergiant Helium-burning Star core: inert He photosphere photosphere core: 4H→He shell: 4H→He Initial evolution follows same shell: 4H→He core: 3He→C sequence as a low-mass star. Continued Double-shell Supergiant Carbon Ignition core: C burning C shell: 4H→He shell: 3He→C shell: 4H→He shell: 3He→C After the helium in the Until the core contracts core is gone, burning enough to ignite carbon. continues in two shells. Late Stages of Evolution H, He By the end of its life, a high-mass star has multiple shells of nuclear burning, making a wide range of elements. He Reactants Temperature Products 12C + 12C 6×108 K 24Mg, 23Mg + n, 23Na + p, 20Ne + 4He, 16O + 2 4He 20Ne + 4He 1.2×109 K 24Mg C, O, Mg 16O + 16O 1.5×109 K 32S, 31S + n, 31P + p, 28Si + 4He, 24Mg + 2 4He 32S, 28Si, He ~3×109 K 56Fe, 56Co, 56Ni S, Si However, the iron-group elements represent Fe the end of this process..

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