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Exam 3 book notes - Exam 3 Chapter 12-Stars are born in...

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Exam 3 Chapter 12 -Stars are born in cold, dense clouds of gas whose pressure cannot resist gravitational contraction. -a molecular cloud fragment heats up and spins faster as gravity makes it contract, producing a rapidly rotating protostar at its center. -Conservation of angular momentum ensures that protostars rotate rapidly and are surrounded by spinning disks of gas. -Neighboring protostars sometimes end up orbiting each other in binary star systems. -A protostar becomes a main-sequence star when it can sustain hydrogen fusion in its core. - Stars more massive than 150M sun blow themselves apart, while protostars smaller than 0.08Msun become brown dwarfs that never get hot enough for efficient hydrogen fusion. - Brown dwarfs are supported again gravity by degeneracy pressure, which does not weaken with decreasing temperature -Stars spend about 90% of their lives shining steadily as main sequence stars. -After exhausting its core hydrogen, the Sun will expand to become a red giant, powered by rapid hydrogen fusion in a shell surrounding the core. -The Sun’s core will continue to shrink and hydrogen shell burning will continue to in and intensify as the Sun grows into a red giant. -The sudden onset of helium in the Sun’s core will stop the core shrinkage, and the Sun will actually become smaller and less luminous than it was as a red giant. -Core shrinkage will resume after the core helium burning ends, while both helium-burning and hydrogen- burning shells make the Sun bigger and more luminous than ever. -When it dies, the Sun will eject its outer layers into space as a planetary nebula, leaving its exposed core behind as a white dwarf. -A star’s life track in an H-R diagram shows how its properties change with time. - A high mass star lives a short life, rapidly fusing its core hydrogen into helium via the CNO cycle. -Near the end of its life, a high-mass star expands to become a super giant as fusion proceeds furiously in its core and surrounding shells. -the core of a high-mass star eventually becomes hot enough for fusion to produce the elements of which we and Earth are made. - A high-mass star’s death is imminent when iron piles up in its core, because fusion of iron releases no energy. -Measurements of elements abundances in the cosmos confirm our models of how high mass stars produce heavy elements. -When gravity overcomes degeneracy pressure in the iron core, the core collapses into a ball of neutrons and that star explodes in a supernova. -The supernova scatters the elements produced by the star into space and leaves behind either a neutron star
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or a black hole. -Observations of supernova remnants confirm many aspects of our models of how high-mass stars live and die. -Massive stars live short but brilliant lives and die in supernova explosions.
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