AST-L19-ch12__3

AST-L19-ch12__3 - AST 1002 Planets Stars and Galaxies...

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1 Stellar Midlife/Aging 1) Stellar Stability/ Main Sequence life 2) Stellar lifetimes 3) Red Giant 4) White Dwarf 5) Supernova 6) More massive stars Today’s Lecture: purpose & goals AST 1002 Planets, Stars and Galaxies Review y) (luminosit burning hydrogen of rate mass) (solar hydrogen of amount x (years) 1x10 10 = MS t t MS = 1/M 2.5 x 10 10 years Gases and Dust Molecular clouds Mostly cold and very low density But enough mass in 50-150 l.y.-wide cloud to form 20,000 solar-mass stars Parts hotter ( Emission nebulae , HII ) – heated by nearby Blue giant stars Protostars/Birth of a Star Same process as formed our solar system Original source of energy is gravity (infall of matter) heat if over 10 million K at core, nuclear fusion turns on (if fails <0.08M brown dwarf ) Stellar Lifetime Review: Main Sequence Lifetime Main Sequence Lifetime t = 10 10 x (M/L) years = 10 10 x (M/M 3.5 ) years (more general form) t ms = 1/M 2.5 x 10 10 years Remember: Mass-Luminosity Relation L ~ M 3.5 0.2M 2 trillion years 1M 10 billion years 10M 32 million years 80M 175 thousand years We estimate that a star 10 times the Sun’s mass will remain stable for only about 30 million years ! (Much shorter than the Sun!!) Example3: Choose a star that is 0.12 solar masses (among the least massive stars, but not right 0.08M ). How l ong w ill i t surv i ve as a ma i n sequen c e star ? M = 0.12 so l ar masses L ~ 3.6x 10 -3 so l ar l um i nos i t i es solving: t = 10 10 x(1/ M 2.5 ) yr t = 10 10 x(1/ 0.12 2.5 ) yr = 10 10 (1/0.12 2 )x(1/0.12 0.5 ) yr = 10 10 /(0.0144)x(0.3464) yr = 2.004 trillion years
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2 How Luminous, How Long? Most luminous stars live shortest lives also, largest (most massive ) stars Less luminous stars live longer These are the less massive stars no star less than 0.8 solar masses has burned all its hydrogen yet!!! (the universe hasn’t been around long enough for any of them to get to that stage) Why are there so many red dwarves ? Every one ever created is still here as a red dwarf! Why are there to f ew b l ue g i ants ? Only those created very recently (within last few hundred thousand – million years) are still here. Blue Giants red dwarves Luminosity & Temperature Luminosity depends on surface temperature size can increase luminosity by increasing surface temperature or size Temperature surface temperature – what we see internal temperature – much higher hydrogen burning only occurs at or above 10 million K remember: Stefan-Boltzmann Law: total
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