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UW ASTR 101 - Lecture Notes

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Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW1Questions of the Day•Why don’t white dwarfs collapse?•How does degeneracy pressure behave?•Why do white dwarfs have maximum mass? How can white dwarfs become more massive than the Chandrasekhar mass?•Why do white dwarfs explode in supernovae ?•Why do massive stars become supernovae?•What kind of star is left behind after a massive star supernova, and why?•Why do neutron stars rotate rapidly?•What are pulsars and how are they related to neutron stars?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, UW2Nuclei ElectronsStationaryMoving!Speeds set by quantum mechanics, not temperature!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW3Nuclei ElectronsStationaryMoving!Only source of pressureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXThe “degenerate electron gas” in a white dwarf provides the necessary pressure to support the star!Normal Gas Weird “Degenerate” Gas1.Nuclei provide most of the pressure.2.Higher temps make the nuclei move faster.1.Electrons provide the pressure.2.Electron speeds are set by quantum mechanics, not temperatureThe “degeneracy pressure” of this new state of matter will behave very differently than normal “thermal” gas pressure, however!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW4Rules of Electron Degeneracy Pressure•Depends only on the density of electrons.•Independent of and unaffected by temperature!Pressure TemperatureDensityAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW5Differences between thermal and degeneracy pressurePressure TemperatureDensityNormal Gas:Pressure TemperatureDensityDegenerate Electron Gas:These cause white dwarfs to behave very differently than normal stars!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW6If mass increased, but nothing changed, gravity would pull star inwards…M2M2M…but big energy increase causes temperature and pressure to increase, and pushes the star back out.For normal matter in a main sequence starAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW7If mass increased, but nothing changed, gravity would pull star inwards…M2MFor degenerate matter in a white dwarf2M1.No increase in fusion!2.Only weak increase in pressureAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW8These differences make Massive White Dwarfs ! Smaller!Gravity beats degeneracy pressure more and more with increasing massFor main sequence stars, the increased gravity increased the rate of fusion, heating the star and making it swell due to higher pressure. Doesn’t work for white dwarfs!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXWhy smaller?Pressure TemperatureDensityDegenerate Electron Gas:The higher mass smooshes the star, but the increase in pressure isn’t enough to fight back!Normally, as stuff falls in, it heats up.But, most of the mass (i.e. the nuclei) is locked in place, so the temperature doesn’t change.Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW9The size of the white dwarf goes to zero!!!What happens when the mass increases even more?Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW10White dwarfs have an upper mass limit! “Chandrasekhar Mass” ~ 1.4M"Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW11Why is there a maximum mass?Low Density High Density•Faster Speeds increase pressureMaximum pressure when velocities reach the speed of light!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXWhy an upper mass limit?•Quantum mechanics drives the electrons to high velocity.•The closer electrons are packed together, the stronger the quantum mechanical effects•But, nothing can go faster than the speed of light!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UWXWhy an upper mass limit?•Quantum mechanics drives the electrons to high velocity.•The closer electrons are packed together, the stronger the quantum mechanical effects•But, nothing can go faster than the speed of light!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW12At very high masses:•Electrons start to move at the speed of light!•They can’t move faster than this!•Electron degeneracy pressure FAILS!BAD STUFF HAPPENS!!!!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW13When are white dwarf masses higher than the 1.4M" Chandrasekhar mass?•Born big•Born small, but got big later…Massive starsBinary starsAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW14What are binary stars?Almost 50% of all stars are gravitationally bound to another star! These stars orbit around each other, like planets around the SunBinary stars have been crucial for measuring the masses of starsAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW15What are binary stars?The stars in the binary are born:However, the stars in the binary can have:• Different masses• Different lifetimes•At the same time•From the same molecular cloudNote: this is similar to a stellar cluster but for only 2 stars!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW16Binaries can change with time:2 main sequence starsHigher mass star has evolved into a white dwarfLower mass star expands into a red giantIf the two stars are close enough, matter can jump from the red giant onto the white dwarf!Astronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW17Before “mass transfer”•Lattice of Helium & Carbon nuclei•Electrons moving at the speed of light•Electrons can’t supply any more pressure•Lattice breaks!•Star collapses!1.4M" white dwarfAfter “mass transfer”>1.4M" star formally known as white dwarfAstronomy 101, Winter 2010Copyright@2010 Julianne Dalcanton, UW18After “mass transfer”•Electrons can’t supply any more pressure•Lattice breaks•Star collapsesStuff falling in = hotter + denser>1.4M" star formally known as


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UW ASTR 101 - Lecture Notes

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