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
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.
● 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.
● 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.
● 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.
● The life stages of high-mass stars are similar to the life-stages of low-mass stars unti