Chapter 14The Role of GravityLow-Mass Stars OverviewHigh-Mass Stars OverviewStellar RecyclingThe Beginning: Interstellar CloudsProtostarsBipolar FlowsT Tauri StarsStellar Mass LimitsMain Sequence StarsMain Sequence LifetimeGiant StarsFusion Past HydrogenYellow Giants and Pulsating StarsPeriod Luminosity RelationDeath of Sun-Like StarsDeath of Sun-Like StarsOld Age of Massive StarsCore Collapse in HMSSupernovaSupernova RemnantsStellar Evolution TheoriesChapter 14Stellar EvolutionThe Role of GravityLow-Mass Stars OverviewHigh-Mass Stars OverviewStellar Recycling•Up to 90% of the original mass of a star returned to space.•The Sun formed from the remnants of several past generations.•Can’t go on forever.The Beginning: Interstellar Clouds•Interstellar clouds: gas and dust•Gas: 71% H, 27% HeDust: Silicates, Carbon, Iron compounds•They are cold (10 K)•Not completely uniform•Video timeProtostars•1500 K, “shine” in infrared•Cycle of gravitational contraction paused when a new fusion pathway is unlocked.•First pathway happens at 1 million Kelvin, 2H —> 4He (limited supply of 2H)Bipolar Flows•Outflowing gas along rotation axis.•Magnetic fields likely involved.•Blows a “hole” in the interstellar cloud.T Tauri Stars•Very young stars•Lie above the main sequence initiallyStellar Mass Limits•Most 0.1 - 30 M⊙, some outside this range•< 0.1 M⊙ = brown dwarfs (between stars and planets)•Stars around 200 M⊙ shine themselves apart.Main Sequence Stars•1H —> 4He fusion can begin at 5 million Kelvin.•HMS are hotter and more luminous, but less dense.•HMS have a different process for H —> He fusion.Main Sequence Lifetime•Can know the lifetime of a star based on its mass and its luminosity.•HMS have higher masses, but higher luminosity by a larger factor —> short lifetimes.lifetime = 1010ℳℒGiant Stars•Luminosity increases by a factor of 3 over 10 billion years.•Hydrogen fuses in a shell around core. Some fuse He in core.Fusion Past Hydrogen•Helium starts fusing at 100 million Kelvin•10% as much energy per gram of fuel as hydrogen fusion•Degeneracy in low mass stars and the “helium flash”Yellow Giants and Pulsating Stars•Many stars vary in brightness.•Yellow giants happen after the helium flash.•“Instability strip”•RR Lyrae and Cepheid variables.Period Luminosity Relation•This means we can find the luminosity based on the period of pulsation.•From the luminosity we can find the distance to the star.Death of Sun-Like Stars•Gravity can’t unlock new fusion pathways.•Gas slowly released into space.•Only a minority of the material actually glows.Death of Sun-Like Stars•All stars that start with > 8M⊙ end their lives as a white dwarf.•Lifetimes for LMS range from under 100 million years to several trillion years.Old Age of Massive Stars•Nucleosynthesis: formation of heavy elements via fusion.•All of the heavy elements in this room (or in the whole universe for that matter) were created by this process.Core Collapse in HMS•Luminosity of HMS relatively constant over whole lifetime.•Iron is a fusion dead-end.•Iron is eventually compressed into neutrons as outer layers collapse.•Pressure surges, causing an explosion.Supernova•An explosion as bright as an entire galaxy.•Creates even heavier elements than iron.•More irregular than planetary nebula.Supernova Remnants• 300 years old 1,000 (Crab nebula) Several thousand•Violent death makes for an irregular corpse.Stellar Evolution Theories•Many ideas were posited and tested. Computer models opened the door to testing of complex models.•Gives us a way to determine the age of a star
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