Announcements Angel Grades are updated (but still some assignments not graded) More than half the class has a 3.0 or better Reading for next class: Chapter.
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Announcements Angel Grades are updated (but still some assignments not graded) More than half the class has a 3.0 or better Reading for next class: Chapter 18 Star Assignment 8, due Wednesday April 7 ÜAstronomy Place tutorial “Stellar Evolution” complete and do exercises ÜAstronomy Place tutorial “Black Holes” lessons plus exercises
Mass - Luminosity Relation Larger Mass stars have larger Gravity pulling in Need larger Pressure pushing out Larger Pressure requires higher Temperature Higher Temperature produces much greater Energy Generation Rate Energy Loss balances Energy Generation L ~ M 3. 5
Main Sequence: More massive stars have larger gravity pulling in Need larger pressure pushing out Requires larger temperature Produces faster nuclear fusion reactions Faster energy generation Balanced by larger Luminosity Star must be larger to let photons escape easier
Temperature Luminosity Very massive stars are rare Low-mass stars are common Why no stars with less than 0.08 M sun ?
What happens as Hydrogen is fused into Helium in core of a star? Number of particles decreases Less Pressure Core contracts & Heats up More Energy Generation Star Expands to let more energy out (greater Luminosity) Star becomes a Giant
What happens when all the H in the core is finally converted to He? He has larger charge, stronger repulsion, requires higher temperature to fuse into C He core is inert, loses energy, can’t maintain pressure, star contracts Converts gravitational PE -> thermal KE heats up star, including H surrounding core Ignites H shell fusion, continued contraction raises temperature of shell, rate of energy generation increases Luminosity increases, star expands to let energy out
Helium fusion requires higher temperatures than hydrogen fusion because the larger charge leads to greater repulsion. Eventually, core gets hot enough to fuse helium. Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon Helium Fusion
Quantum Mechanics Fundamental Principle of Quantum Mechanics: CAN’T OBSERVE WITHOUT DISTURBING Expressed mathematically as Heisenberg’s Uncertainty Principle x v > h/m PositionVelocityA number ass
Pressure of “Degenerate” Gas In words: Uncertainty in Position x Uncertainty in Speed is Greater than h (a small number) divided by the mass Consequence: Speed ~ h/(mass x distance between particles) Squeeze particles closer together -> speed increases -> collide harder -> more Pressure
End of Fusion for Small Mass Stars When core density becomes very high, nuclei & electrons are squeezed so close together that the Uncertainty Principle makes their speed increase They become “Degenerate” Pressure increases with increasing Density (not Temperature), stops further contraction & heating
Life Stages of Low-Mass Star 1.Main Sequence: H fuses to He in core 2.Red Giant: H fuses to He in shell around He core 3.Helium Core Burning: He fuses to C in core while H fuses to He in shell 4.Double Shell Burning: H and He both fuse in shells 5.Planetary Nebula leaves white dwarf behind Not to scale!
Reasons for Life Stages 1.Core shrinks and heats until it’s hot enough for fusion 2.Nuclei with larger charge require higher temperature for fusion 3.Core thermostat is broken while core is not hot enough for fusion (shell burning) 4.Core fusion can’t happen if degeneracy pressure keeps core from shrinking Not to scale!
Large mass stars get hot enough to fuse heavy nuclei
Fusion reactions in late stages of evolution of massive stars make heavier elements up to Iron
Life Stages of High-Mass Star 1.Main Sequence: H fuses to He in core 2.Red Supergiant: H fuses to He in shell around He core 3.Helium Core Burning: He fuses to C in core while H fuses to He in shell 4.Multiple Shell Burning: Many elements fuse in shells 5.Supernova leaves neutron star behind Not to scale!
Death of Stars 1)White Dwarf 2)Neutron Star 3)Black Hole 4)Nothing
Small Mass Stars End Life as WHITE DWARFS Core becomse so dense can not contract and heat any more Star supported by pressure of degenerate electrons Size about size of Earth Star slowly cools
More Massive White Dwarfs are Smaller More Mass -> More gravity Need larger Pressure Must squeeze electrons more to increase their speed and pressure Smaller White Dwarf
Maximum Mass for White Dwarfs Pressure of “degenerate” electrons can only support so much mass before electron speed would need to be the speed of light. Maximum mass of White Dwarfs 1.4 M sun
Maximum Mass of Star becomes WD Stars larger than 1.4 Msun can become WD because they throw off mass as Planetary Nebula
What happens to Stars with too much mass to become White Dwarfs? Core contracts Gets hotter Can fuse elements up to Iron and release energy Iron is most tightly bound nucleus (has smallest mass per nucleon of all) To make heavier nuclei -> must provide energy
Iron builds up in core until electrons get squeezed onto protons to make neutrons. Degeneracy pressure goes away and no longer resists gravity Core then suddenly collapses, creating supernova explosion Neutrons collapse to the center, forming a neutron star