Protostar continues to contract gravity pressure raises core temp so pressure

Protostar continues to contract gravity pressure

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Protostar continues to contract (gravity > pressure); raises core temp (so pressure increases); energy from fusion adds heat/pressure; when gravity=pressure, contracting stops; becomes stable, main sequence starLow-mass stars are more common than high mass stars; most massive than 150 Msun would blow apart; less massive than .08 Msun can’t sustain fusionBrown Dwarfs (Failed Stars); very common, hard to detect; less massive than .08 Msun; still have thermal energy from their formation (infrared light), not hot for fusionMain Sequence Stars; fusing H into He into their cores (hydrogen burning); 90% of star’s life cycle; remains until hydrogen runs outStellar Life Spans; high mass stars have shorter lifespans: more fuel, much greater luminosity (Mass^3.5); high mass burn faster than low mass, 10 times as much fuel = 10^4 times as fast; 10Msun will last only 10 million years, compared to 10 billion yr lifetime of SunHigh Mass (Hummers): “gas guzzlers,” high luminosity, large, blue, short lives, millions of years (higher pressure, higher temp, rapid fusion, higher luminosity)Low Mass (Priuses): “fuel efficient,” low luminosity, small, red, long-lived, hundreds of billions of years (lower pressure, lower temp, slower fusion, lower luminosity)Life of Main Sequence Star: generate energy by hydrogen fusion, long/stable part of star’s life, energy generated keeping interiors hot; resulting pressure balances gravity and presents star from collapse (gravity and pressure are in balance); “quiet adulthood” evolves very slowly as it fuses hydrogen in its core; grows larger, brighter, hotterScorched Earth; sun increases in luminosity by 10%, Earth warms up; oceans evaporate away, H20 destroyed by solar UV light, hydrogen gasFinal Stages Depend on its Mass; when stars run out of Hydrogen, position on HR diagram moves up and rightoRed Dwarf stars (<~0.4 Msun); H fuses to He in core; convection mixes helium throughout; slowly converts H to He; never becomes red giant; no red dwarf used all HoSun-like stars (~0.4 - ~10 Msun); when H runs out, core fusion stops, core contracts and heats up, gravity>pressure, Fusion ignites in shell around core, luminosity up; outer layers expand and cool, pressure>gravity, becomes red giant; growing luminosity will evaporate oceans, drive away atmosphere, Earth dies Helium Fusion = shrinking core heats until the core reaches a temp of about 100 mil K, then helium fusion begins, fuses into carbon (and some oxygen); energy from He fusion adds pressure, stops contracting of star’s core; larger charge leads to greater repulsion (high temp)A new state of balance: energy from helium fusion stops core contracting, gravity-pressure balance restored; star dims, shrinks, gets hotterBecoming a “yellow giant”: helium fusion turns star into yellow giant, helium doesn’t last as long as fuel; star more luminous, helium fusion less efficientHelium Runs Out: core fusion stops again, core contracts, shell fuses helium and hydrogen, star exands and cools again into red giant (w/ double shell fusion)
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