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Download Eventually, H burns outward in a shell
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Q. Which of these will NOT happen down in the Eventually, H burns outward in a shell core, after the nuclear energy source shuts off? • Heat source moves closer to surface. A. The core will cool down. • Layers below surface swell up. B. Its pressure will decrease. Starlayers becomes larger C. The “weight” of the •outer will then compress the core. • Surface becomes cooler Ho D. The core will become denser and hotter. Red giant. wc E. All of the above will happen. th an Luminosity is b e?? Inert He core HHe burning shell Inert H Diameter Temperature Fission vs. Fusion Iron (Fe) is the most stable nucleus. Fission Fusion [Fig. 10.4] Heavy nucleus (e.g. Uranium) breaks up into lighter nuclei. Light nuclei (e.g. Hydrogen) combine to form heavier nucleus. 1 Nucleosynthesis: where we came from. • H, He, Li are only elements formed in initial formation of universe. • simplest stable combinations of protons, neutrons and electrons Fusion in stars increasingly Periodic Table of the moreElements complicated, but more stable nuclei. hydrogen Fusion Periodic Table is in order of complexity Mass (= energy) per Nucleon • Up until iron (Fe). helium uranium Fission Iron lead [Fig. 12.17] Atomic Number (protons+neutrons) 4 1H 4He 3 4He 12C 12C + 4He 16O, Ne, Na, Mg Ne O, Mg O Mg, S Si Fe peak Min. Temp. 107 o K 2x108 8x108 1.5x109 2x109 3x109 Fusion in stars increasingly more complicated, but more stable nuclei. • Up until iron (Fe). Mass (= energy) per Nucleon Reaction Atomic Number (protons+neutrons) Periodic Table is in order of complexity 2 Reaction Luminosity 4 1H 4He 3 4He 12C 12C + 4He 16O, Ne, Na, Mg Ne O, Mg O Mg, S Si Fe peak Min. Temp. 107 o K 2x108 8x108 1.5x109 2x109 3x109 What we need are: New sources of fuel Renew nuclear burning. More pressure. Halt contraction. Inert He core HHe burning shell Inert H Diameter Temperature Reaction 4 1H 4He 3 4He 12C 12C + 4He 16O, Ne, Na, Mg Ne O, Mg O Mg, S Si Fe peak Min. Temp. 107 o K 2x108 8x108 1.5x109 2x109 3x109 What we need are: New sources of fuel Triple-alpha process H He (core) • Contraction heats center. • Helium starts to burn. 3 Reaction Min. Temp. 4 1H 4He 107 o K 3 4He 12C 2x108 12C + 4He 16O, Ne, Na, Mg Ne O, Mg 8x108 1.5x109 O Mg, S 2x109 Si Fe peak 3x109 2 M or less stars • Following He C burning, core never gets hot enough for further reactions. Planetary nebula shell expelled. Outer layer = inert hydrogen Star continues to shrink. Surface becomes very hot. [Fig.12.12] Solar-mass star Luminosity The future of the Sun Notice changes in scale Earth’s orbit. Radius Time (billions of yrs.) 4 But in more massive stars (>2 M)… nuclear burning in successive shells Reaction 4 1H Min. Temp. 4He 107 o K 3 4He 12C 12C 2x108 + 4He 16O, Ne, Na, Mg Ne O, Mg 8x108 1.5x109 O Mg, S 2x109 Si Fe peak 3x109 “Onion skin” model • Central core is iron • Outer layers correspond to each previous step in nuclear burning chain. Inert H H fusion He fusion C fusion O fusion Ne fusion Mg fusion Si fusion Inert iron core [Fig 12.16] Luminosity Here’s what happens on the H-R diagram for stars with > 2 M . Interactive HR Diagram HR – The Movie Temperature [Fig. 12.14] 5
