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Class 7 : Post main sequence evolution of stars

Explore the evolution of low-mass stars similar to the Sun and high-mass stars that undergo core collapse supernova. Learn about core transformations, fusion processes, and final states like white dwarfs, neutron stars, and black holes. Unveil the mysteries of cosmic evolution!

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Class 7 : Post main sequence evolution of stars

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  1. Class 7 : Post main sequence evolution of stars • Evolution of “low-mass” stars • Essentially follow same path as Sun (discussed last semester) • Will recap briefly • Evolution of “high-mass” stars

  2. I : Low mass stars (M<8Msun) • Main sequence corresponds to HHe burning… lasts for a time that we estimated last class • Once hydrogen runs out… • Core contracts; envelope expands  Red Giant • If M>0.4Msun, start to burn HeC through the triple-alpha process (occurs explosively if 0.4Msun<M<2Msun;gradually if M>2-3Msun) • The expels stellar envelope in series of explosive events (novae)… form a planetary nebula • He or C core remains as a white dwarf… dense (stellar mass but size of Earth) and hot

  3. Triple-alpha process Kicks in at about 100 million K

  4. II : Evolution of a high-mass star • Stars with M>8Msun take a different path… core gets hot enough that nuclear burning can proceed beyond Carbon • There is a sequence of reactions that go all of the way from H to Fe (iron) • The fusion reactions get less and less efficient as the sequence proceeds… mass must be processed as a progressively faster rate in order to satisfy stars demand for energy • Iron is the end of the road… it has the most stable nucleus and so you cannot extract energy by fusing it • End up with a shell-like (or onion-like structure)… an iron core surrounded by a shell of SiFe burning, which is surrounded by a shell of OSi burning etc.

  5. What happens next? • Once iron is reached, fusion stops in core • Without energy production, core is slowly crushed • When Mcore~1.4Msun, pressure forces become incapable of supporting core… core undergoes catastrophic gravitational collapse (takes a few seconds) • Energetics of core collapse… • releases about 1046J • 99% emerge as neutrinos • Star is blown apart… core collapse supernova • 1% of energy (1044J) emerges as radiation and kinetic energy • Fusion reactions during the supernova responsible for all elements heavier than iron

  6. What happens to the core? • If M<20Msun • Becomes neutron star (M~1.5-2Msun, R~10km) • Matter gets “neutronized” • If M>20Msun • Core collapses all of the way to a black hole • M~3-10Msun, R=5-30km • More about these in later classes

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