10/15/2010AST207 F2010 1Life of the Sun—15 Oct• Energy production in the sun • Sun will use up the hydrogen in the center in 5Byr • Center of sun must shrink to get hotter to balance gravity – Sun will become a red giant. Surface expands.• Sun will become a planetary nebula• Sun will become a white dwarfAnnouncements• Test 2 is Wed, Oct 20.– Covers material though HR diagram of star clusters (11 Oct). Does not cover energy production.– Covers homework 5.– Mostly on material since first test.– One cheat sheet.– See practice test on angel.– Missouri “Show me” Club• Tues, Oct 19, 7:40-8:40pm• BPS 1420• Homework 5 is due at start of class on Mon, Oct 18. No late papers.• Open house at the MSU Observatory– Today and Saturday (October 15 and 16), 9-11pm, weather permitting– Bring your friends, parents, siblings, children10/15/2010AST207 F2010 2Review• The sun produces energy by fusing hydrogen into helium 4p4He.– For this to occur, the protons must collide vary fast to overcome the electric repulsion.– Repulsion force depends on the product of the charges .•1for H•2for He• Half of the hydrogen in the center is used up.• For pressure and gravity to balance – Fusion maintains the temperature.1. If the sun runs out of hydrogen, there is no more fusion to maintain the pressure. What happens?A. The sun gets hotter because ___.B. The sun gets cooler because ___.1H4He3He100Sun as a main-sequence star•H→He in the core• T=15MK• Fuel will last another 5 Byr.HHe burningcoreInert H TemperatureLuminosity Radiusphotosphere10/15/2010AST207 F2010 3The sun’s choice• Sun does a balancing act.• Sun must produce energy to replenish the energy radiated away.•If HHe shuts off, source of energy to maintain pressure shuts off, and gravity wins.• What does the sun do to adjust for gravity’s victory?• Core of the sun shrinks.• Core of sun gets hotter•H→He in the a shell surrounding inert core• Balance restored.Inert He coreHHe burningshellInert HSun as a subgiant• H is gone in the core• The never-ending battle between gravity and pressure. How does the sun adjust?• Without burning fuel to keep temperature up, pressure would fall and gravity would win.• Core shrinks, gets hotter•H→He in the a shell surrounding inert core• Balance restored. TemperatureLuminosity Radiusphotosphere5 Byr from nowInert He coreHHe burningshellInert H10/15/2010AST207 F2010 4Sun as a giant• H is gone in the core• The never-ending battle between gravity and pressure. How does the sun adjust?• Without burning fuel to keep temperature up, pressure would fall and gravity would win.• Core shrinks, gets hotter•H→He in the a shell surrounding inert core• Balance restored.• Inert He core expands TemperatureLuminosity RadiusphotosphereA few 100Myr laterInert He coreHHe burningshellInert HSun as a giant• H is gone in the core• The never-ending battle between gravity and pressure. How does the sun adjust?• Without burning fuel to keep temperature up, pressure would fall and gravity would win.• Core shrinks, gets hotter•H→He in the a shell surrounding inert core• Balance restored.• Inert He core expands TemperatureLuminosity RadiusphotosphereLaterInert He coreHHe burningshellInert H10/15/2010AST207 F2010 5The sun’s choice• Sun does a balancing act.• Sun must produce energy to replenish the energy radiated away.• Without burning fuel to keep temperature up, pressure would fall and gravity would win.• Core shrinks, gets hotter T=200MKReaction Min. Temp.4 1H 4He 10 MK3 4He 12C 200 MK12C +4He 16O, Ne, Na, Mg800 MKNe O, Mg 1500MKO Mg, S 2000MKSi Fe peak 3000MKSun Burns Helium• H is gone in the core & shell is exhausted• The never-ending battle between gravity and pressure. How does the sun adjust?• Without burning fuel to keep temperature up, pressure would fall and gravity would win.• Core shrinks, gets hotter T=200MK•3He→C in the core (triple alpha process)• Balance restored. TemperatureLuminosity RadiusphotosphereLaterHe C in coreHHe burningshellInert H10/15/2010AST207 F2010 6The sun’s choice1. Why does fusion of helium require a higher temperature?A. Helium is heavierB. Helium has 2 protonsC. Helium has two neutrons• With more charge, it takes higher speeds to bring two He nuclei close enough to fuse.• Carbon has 6 protons.Reaction Min. Temp.4 1H 4He 10 MK3 4He 12C 200 MK12C +4He 16O, Ne, Na, Mg800 MKNe O, Mg 1500MKO Mg, S 2000MKSi Fe peak 3000MKOther 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 coreReaction Min. Temp.4 1H 4He 10 MK3 4He 12C 200 MK12C +4He 16O, Ne, Na, Mg800 MKNe O, Mg 1500MKO Mg, S 2000MKSi Fe peak 3000MK10/15/2010AST207 F2010 7Normal/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.10/15/2010AST207 F2010 80.2 0.4 0.6 0.8 1.0100150200250region size10-8mvkms1. 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
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