Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•Why can high mass stars use more varieties of fuel over their lives, compared to low mass stars?•How are planetary nebula formed?•Why are planetary nebulae colorful?•What kind of star is leftover after the formation of a planetary nebula?•Why are white dwarfs different than main sequence stars?•Why do white dwarfs eventually fade?•Why don’t white dwarfs collapse?•How does degeneracy pressure behave?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWAdministrative Stuff• Midterm a week from Wednesday!• 50 multiple choice questions in 50 minutes.• Bring a scantron form!• PDFs of old midterms are now posted on the syllabus, under 2/10• Remember all those office hours? Great for reviewing old midterms!• CLUE review session 2/9 in the evening.• Will cover everything we cover this week -- keep in mind that we’re 1 lecture ahead of the syllabusXAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW2Rules for the Day•When Stuff falls inwards, it heats up.•When Stuff falls inwards, it gets denser too.•Higher temperatures and densities may allow more positively charged nuclei to interact and fuse.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXAn example: The main sequence brightens with time.•4 H ! 1 He means fewer particles.• Fewer particles = less pressure• Gravity wins a little more all the time!• Stuff falls in (i.e. star contracts)• Core gets hotter!• Fusion happens faster!• More energy is released per secondAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3 Like everything else, the fate of stars depends almost entirely on MASS!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4The Thought Process of a Desperate Star…• Oh crap, I’m out of fuel.• There goes my pressure support, I’d better collapse my core.• Oh wait, now I can start burning my old fuel just outside the core. Whew.InertAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5But, the inert core still keeps collapsing!Shell burning can’t heat up the core effectively.The core still doesn’t have enough pressure support!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6But, the inert core still keeps collapsing!But, as the core collapses, it heats up!It also gets denser!Maybe it gets hot & dense enough for this? (with He, not H)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7The Thought process of a Desperate Star…•Well, the shell burning helps, but my core keeps collapsing.•Hmmm. My core is heating up.Lower mass stars say:• My gut isn’t getting hot enough to burn any other kind of fuel.• Dang. I’m screwed.• Denial. Rage. Acceptance. Death.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8The Thought Process of a Desperate Star…•Well, the shell burning helps, but my core keeps collapsing.•Hmmm. My core is heating up.Higher mass stars say:But only while the next fuel lasts! Cycle repeats!• Dang, my core is hot!• Hey, I can burn up this next heavier element in my core.• Wahoo! Reprieve!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9This process works no matter what the original fuel was• Oh crap, I’m out of Hydrogen• There goes my pressure support, I’d better collapse my core.• Oh wait, now I can start burning my old fuel just outside the core. Whew.InertHelium Carbon.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10Where the cycle ends depends upon mass.Very Low Mass: Can’t burn anything by H.Solar Mass: Can burn He, but nothing else.Very High Mass: Can burn anything!Heavier StarDenser, Hotter Collapsing CoreCan Fuse More Positively Charged NucleiAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11Low Mass Stars (<1M") Stop Fusion HereAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12Stars like the Sun can burn He when the core shrinks and heats enough…3 Helium nuclei ! 1 Carbon nucleusAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13Stars like the Sun stop Fusion here…Note that there are now two shells! Extra luminous!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14The “evolutionary track” of a 1M" starTrack shows where a single star is in the HR diagram at different points in its lifetime.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15High mass stars keep right on going!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16Like all desperate measures, each fix is less successful than the first…•Core is hotter and hotter each time.•Burn rate is faster and faster…•…but less and less energy is released per fusion reaction (i.e. energy difference between H and He is particularly large).Each Cycle is shorter than the one before!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17For a massive star…•H-burning: 7 million years (107 K)•He-burning: 500,000 years (109 K)•C-burning: 600 years (1011 K)•Ne-burning: 1 year (1012 K)•O-burning: 6 months (1013 K)•Si-burning: 1 day! (1014 K)Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18“Nucleosynthesis”•We’re made of all that stuff made while powering desperate stars!!!!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW19This process is where most iron and carbon come fromAstronomers refer to all the elements heavier than Helium as “metals”We see metals everywhere we look in the Universe. How did they get out of stars?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW20One way is through “stellar winds”The star V838 MonocerotisAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW21Strong stellar winds are driven by stars with masses >1M"These stars have:•High luminosity•Very little mass above the fusing shellsAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW22Core collapse sinks the center.Shell burning puffs the outskirts.Outer layers are only weakly bound to the star! Easy to push them off completely.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW23The swollen outer layers get pushed off! “Stellar Wind”• ~20% of the star’s mass is returned to the galaxy! Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW24The swollen outer layers get pushed off! “Stellar
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