Life Stages of Stars

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University of Massachusetts Amherst
Alexandra Pope

CNO Cycle (C: Carbon, N: Nitrogen, O: Oxygen) ● High mass main sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts. ● Greater core temperatures enable hydrogen nuclei to overcome greater repulsion. Life Stages of High-Mass Stars ● The initial post-main-sequence stages of high-mass stars are similar to those of low-mass stars: 1. Hydrogen core fusion (main-sequence) 2. Hydrogen shell burning (supergiant) 3. Helium core fusion (supergiant) ● Big Bang made 75% H, 25% He; stars make everything else. ● Helium fusion can make carbon in low mass stars. ● CNO cycle can change carbon into nitrogen and oxygen. Helium Capture ● High core temperature allow helium to fuse with heavier elements. ● Helium capture builds carbon into oxygen, neon, magnesium, and other elements. Advanced Nuclear Burning ● Core temperatures in stars with >8Msun allow fusion of elements as heavy as iron. ● Advanced reactions in stars make elements like Si, S, Ca, and Fe. Multiple Shell Burning ● Advanced nuclear burning proceeds in a series of nested shells. ● Iron is a dead-end fusion because nuclear reactions involving iron do not release energy. ● This is because iron has lowest mass per nuclear particle. Evidence for helium capture: ● Higher abundances of elements with even number of protons. Death of a High-Mass Star ● Iron builds up in the core until degeneracy pressure can no longer resist gravity. ● The core then suddenly collapses, creating a supernova explosion. Supernova Explosion ● Core degeneracy pressure goes away because electrons combine with protons making neutrons and neutrinos. ● Neutrons collapse to the center, forming a neutron star. ● Energy and neutrons released in supernova explosion enable elements heavier than iron to form, such as gold and uranium. Supernova Remnant ● Energy released by the collapse of the core drives the star’s outer layers into space. ● The Crab Nebula is the remnant of a supernova seen inA.D. 1054. Supernova 1987A ● The closest supernova in the last four centuries was seen in 1987. Rings around Supernova 1987A ● The supernova’s flash of light caused rings of gas around the supernova to glow. REVIEW ● The life stages of high-mass stars are similar to the life-stages of low-mass stars unti
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