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Chapter 12

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Natural Science
NATS 1740
Randy Hoffman

Chapter 12 Star Stuff 12.1 Star Birth How do stars form? - A star is born when gravity causes a cloud of interstellar gas to contract to the point at which the central object becomes hot enough to sustain nuclear fusion in its core - Stars are born in cold dense clouds of gas whose pressure cannot resist gravitational contraction - Molecular clouds: star forming clouds that are cold and dense  Allow hydrogen atoms to pair with hydrogen molecules (10-30K temperature range) From clouds to protostar - A molecular cloud fragment heats up and spins faster as gravity makes it contract, producing rapidly rotating protostar at its centre Disks and Jets - Conservation of angular momentum ensures that protostars rotate rapidly and are surrounded by spinning disks of gas Single star or binary? - Neighbouring protostars sometimes end up orbiting each other in binary star systems - Close binary: 2 stars end up quite close to each other (0.1 AU apart, orbit every few days around each other) From protostar to main sequence - A protostar becomes a main-sequence star when it achieves energy balance between hydrogen fusion in its core and radiation from its surface How massive are newborn stars? Limits on stellar masses - Stars more massive than 300M sun blow themselves apart while protostars smaller than 0.08M sun become brown dwarfs that never get hot enough for efficient hydrogen fusion (failed star) Pressure in brown dwarfs - Brown dwarfs are supported against gravity by degeneracy pressure which does nto weaken with decreasing temperature • Thermal pressure: ordinary gas pressure 12.2 Life as a low mass star • Low-mass star: stars with masses less than 2 Solar Masses What are the life stages of a low mass star? Main sequence stage - Stars spend about 90% of their lives shining steadily as main-sequence stars Red-giant stage - After exhausting its core hydrogen, the sun will expand to become a red giant, powered by rapid hydrogen fusion in a shall surrounding the core • Hydrogen shell fusion: hydrogen fusion in a shall around the core of a star - The suns core will continue to shrink and hydrogen shell fusion will continue to intensify as the sun grows into a red giant Helium core fusion • Helium fusion: occurs when nuclei slam into one another at much higher speeds than those needed for hydrogen fusion (requires more temperature) - The sudden onset of helium fusion in the suns core will
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