1White dwarfs & supernovae — Oct 19• What causes pressure in white dwarfs?• Where were the elements in the baby made?• Carbon was made and expelled by giants• Iron was made in massive stars and expelled by supernovae• Heavier elements were made in supernovae & in giants, where there are free neutrons. Nuclei capture neutrons.Cygnus LoopSupernova 20,000 yr ago Sirius A, a main-sequence starSirius B, an earth-sized white dwarf • Observing• Wed & Thurs, Oct 21 & 22, 8:00-10:00pm.• Elevator cannot go up after 10:00pm.• Attend only if stars are visible. See angel at 6:00pm, if weather is ambiguous.• Quiz. You will be asked to locate a star using the Abrams Planetarium star chart. Quiz counts as one clicker assignment.• Go to the south end of the building (toward Wilson Rd.) & take the elevator up to the penthouse.• Test 2 on Wed.• Through Adams’ discover of a white dwarf (12 Oct)• Energy production in sun not included (12 Oct).• One 8 ½ x 11” cheat sheet.• Open house nights at the observatory• Friday and Saturday, Oct. 23 and 24, 9-11pm, weather permitting.• Observatory:• Go south on Farm Lane to the end• Turn right. Observatory is 100 yards to the west.2Normal/degeneracy pressureWhite dwarfs• Pressure on the walls of the box is caused by the gas hitting the walls. Gas transfers momentum to the walls.• Mental picture: Marbles hit the walls; wall pushes back.• Normal gasP V = n k Tmv2=kT1. Pressure is greater at higher temperature because ___. I. more marbles are hitting the wall every second. II. the marbles are moving faster and each marble has more momentum.A. I onlyB. II onlyC. I & IIPressure in a “degenerate gas”• Normal gasP V = n k T• Pressure is greater at higher temperature because the marbles are moving faster. mv2=kT• Degenerate gas. If the gas is confined to a very small space, Newton’s 2ndlaw becomes invalid.• New laws of motion, called quantum mechanics, apply.• Heisenberg’s uncertainty principle. Suppose a particle is allowed to move within a region of length x.m v x > hmomentum x > hv is speed. h is Planck’s constant.• A particle must move if it is confined to a small space.• If you confine an electron to 10-8m, it moves at 70km/s.30.2 0.4 0.6 0.8 1.0100150200250region s ize@10-8mDv@kmêsD1. Plot shows the speed of a normal gas made of electrons with a temperature of 1000K and a degenerate gas of electrons with a temperature of 0K. A. I is a NG. II is a DG.B. I is a DG. II is a NG.IIIPressure in a “degenerate gas”• Normal gasP V = n k T• Pressure is greater at higher temperature because the marbles are moving faster.• Degenerate gas. If the gas is confined to a very small space, Newton’s 2ndlaw becomes invalid.• New laws of motion, called quantum mechanics, apply.• Heisenberg’s uncertainty principle. Suppose a particle is allowed to move within a region of length x.m v x > hmomentum x > hv is speed. h is Planck’s constant.• A particle must move if it is confined to a small space.• If you confine an electron to 10-8m, it moves at 70km/s.• Pressure of a degenerate gas•P V5/3= constant n5/3• constant = h2/m• Pressure does not depend on temperature!!!4Other fusion reactions?• Sun has one more trick after He is exhausted in core.• Burn He in a shell• Sun is not massive enough to shrink further and get hotter• Core is supported by pressure of degenerate electrons.• Temperature does not rise to burn anything else.• End of the road: planetary nebula & white dwarf coreMin. Temp.Reaction2000MKO Î Mg, S1500MKNe Î O, Mg3000MKSi ÎFe peak800 MK12C +4He Î16O, Ne, Na, Mg200 MK3 4He Î12C10 MK4 1H Î4HeTriple-alpha processSupernova 1987A• Exploded in Large Magellanic Cloud• LMC is small galaxy that orbits our own Milky Way Galaxy.Pre-existing circumstellarring lit up first by photons from SN, now by blast wave from SN. BeforeDuringLarge Magellanic Cloud5Guest star of 1054• Records of Sung Dynasty• In the first year of the period Chih-ho, …, a guest star appeared several degrees SE of Thien-kuan. After more than a year it gradually became invisible.−p578.• Gas expelled in 1054AD, still glowing•Other SN• 1572 Tycho• 1604 KeplerSupernova remnantsCygnus Loop20,000 yrs old.2500 LY away.IC 4438000 yrs oldWe expect one supernova in Milky Way every 25-100 yrs.Crab1,000 yrs old6Supernovae• Explosion releases enormous energy• Luminosity in photons temporarily exceeds that of whole galaxy full (100 billion) of stars.What is a supernova? Why sun becomes a white dwarf, not a supernova• In future double-shell burning sun, hot enough to burn34HeÆ12C• When He exhausted, gravity wins, and core contracts.• Temperature rises.• Electrons are so tight that they become degenerate.• New source of pressure to resist gravity.• Temperature not hot enough to burn carbon.Min. Temp.Reaction2x109O Î Mg, S1.5x109Ne Î O, Mg3x109Si ÎFe peak8x10812C + 4He Î16O, Ne, Na, Mg2x1083 4He Î12C107 oK4 1H Î4He7What is a supernova? Why massive star becomes a supernova• In future double-shell burning massive star, hot enough to burn34HeÆ12C• When He exhausted, gravity wins, and core contracts.• Temperature rises by larger amount b/c gravity is stronger.• Temperature hot enough to burn carbon.4He + 12C Æ16O, etcMin. Temp.Reaction2x109O Î Mg, S1.5x109Ne Î O, Mg3x109Si ÎFe peak8x10812C + 4He Î16O, Ne, Na, Mg2x1083 4He Î12C107 oK4 1H Î4HeWhat is a supernova? Why massive star becomes a supernova• Hot enough to burn4He + 12C Æ16O, etc• When C exhausted, gravity wins, and core contracts.• Temperature rises.• Temperature hot enough to burn neon.20Ne + 4He Æ24Mg• Disaster with iron• Burning releases energy• Fusing iron takes up energy• Gravity finally wins.Min. Temp.Reaction2x109O Î Mg, S1.5x109Ne Î O, Mg3x109Si ÎFe peak8x10812C + 4He Î16O, Ne, Na, Mg2x1083 4He Î12C107 oK4 1H Î4He8What is a supernova? Why massive star becomes a supernova• Disaster with iron• Burning releases energy• Fusing iron takes up energy• Gravity finally wins.• Star collapses in few seconds• Rebounds as supernova• Reason for rebounding is topic of current research• Expel outer layersMin. Temp.Reaction2x109O Î Mg, S1.5x109Ne Î O, Mg3x109Si ÎFe peak8x10812C + 4He Î16O, Ne, Na, Mg2x1083 4He Î12C107 oK4 1H
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