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Star Lives

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Astronomy 1021
Chris Racknor

Jan 31 12 Star LivesConcept of Stellar Evolution As a star is generating energy through nuclear fusion its chemical composition is gradually changing ex transforming hydrogen into helium the star is evolving Stellar evolution is the change in composition but also changes in stellar parameters over time temperatures size ex how stars change position on the HR diagramTimescales Note that the timescales involved in stellar evolution are extremely long for human understanding We have never seen a full evolution of a star What we know is like taking pictures of thousands of people and figuring out what people look like as they are from themUnderstanding stellar evolution comes from understanding what happens to the pressure in a star as it ages It all comes down to balance Gravitypressure contraction Gravitypressure hydrostatic equilibrium Gravitypressure expansionInitial Mass and Evolution For the purpose of understanding stellar evolution it is useful to divide stars into three groups based on their initial main sequence mass Lowmass starsmasses less than 2 Msun Intermediatemass starsmasses between 2 and8 Msun Highmass starsmasses greater than 8 MsunThere are two paths the lives of stars can take and it all depends on mass 1 Low mass and intermediate mass stars 2 HighmassEvolution of Lowmass Stars Low mass stars on the main sequence have a slow and steady life Fusion transforms 4 hydrogen particles into 1 helium particle since the number of particles drops the pressure drops and thus the core contracts slightly and heats up and thus luminosity increases slowly On HR diagram stars slowly move a little upward as they grow older After the Hydrogen is Gone At some point the hydrogen in the core is completely transformed into helium and thus there is no more fuel for fusion The core is now inert no more fusion and made of helium No more fusionno more energy production No more energyno pressure Gravity is then greater than pressure so it is winning the fight When gravity is winning the star and the core contract As the core contracts it heats up Red Giant Stage When the core heats up so does the area just outside the core Eventually this shell around the helium core is hot enough to fuse hydrogen there This hydrogen shell burning proceeds at a higher rate than main sequence core burning because of the higher temperature thus the luminosity increaseshigher temperature hydrogen fusion is faster than regular energy production Faster energy productionmore thermal pressure
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