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The Future Sun • • Homework 5 is due 6:30am on Friday, 23 March. Public viewing sessions at MSU campus observatory. • Fri & Sat, 9-11pm, if it is not cloudy. • Mar 23 & 24 • Apr 20 & 21 • May 18 & 19 • Giants are dying stars; white dwarfs are dead stars • Why does the sun die? • What will the sun become when it dies? Giants are dying stars; white dwarfs are dead stars • Evidence on giants from star clusters • Compare members of a population • M80 All stars in a cluster are born at once. • Formation time is the collapse time of the cluster, which is very short. • I am a G star like the sun. I have 100,000 fraternal twins, some weighing 30 times my mass, some 1/10 of my mass. Pleiades 1 Star cluster Pleiades HR Diagram of Clusters • Not all fainter stars were observed. • • • For all star clusters, main sequence extends to cooler stars beyond observations In what ways are HR diagrams of H+χ Perseus, Pleiades, Hyades, & NGC188 different? Q Which is false a. Perseus has hotter stars than Pleiades. b. Most stars are one the main sequence. c. Perseus has small range of luminosity d. Some clusters have giants. 2 Stars with high mass live a short life 106 O star: 30M~; 200,000L~ Lifetime=amount of fuel/consumption rate Lifetime=21gallons/(3gallons/hr)=7hr Lifetime=30M~/ 200,000L~ =Lifetime~/7000 =1.3Myr Human scale = 4days → 102 Luminosity 104 100 10-2 Sun Lifetime=10Byr Human scale 80yr Barnard’s star: 0.1M~; 0.004L~ 10-4 Lifetime=0.1M ~/ 0.004L~ =25Lifetime~ =250Byr Human scale = 20,000yr 25,000 10,000 5,000 Temperature 3,000 H-R Diagrams of star clusters • Q2 There are no A stars in M80 because a. they never formed. b. they died and disappeared c. all stars became redder as they get older. d. they are too faint to see. 3 H-R Diagrams of star clusters • Q3 The hottest dwarfs in Pleiades are A stars. The hottest dwarfs in M15 are F stars. Pleiades is ___ than M15. a. older b. younger Life of the sun • Heat source moves closer to surface. • Layers below surface swell up. • Star becomes larger • Surface becomes cooler Î Red giant. Luminosity Î Present sun HÎHe burning core Radius photosphere Inert H Í Temperature 4 Life of the sun • Heat source moves closer to surface. • Layers below surface swell up. • Star becomes larger • Surface becomes cooler Î Red giant. Luminosity Î 5 Byr from now Inert He core HÎHe burning shell Inert H Radius photosphere Í Temperature Life of the sun • Heat source moves closer to surface. • Layers below surface swell up. • Star becomes larger • Surface becomes cooler Î Red giant. Luminosity Î Few 100 Myr later Inert He core Radius photosphere HÎHe burning shell Inert H Í Temperature 5 Life of the sun • Heat source moves closer to surface. • Layers below surface swell up. • Star becomes larger • Surface becomes cooler Î Red giant. Luminosity Î Later Inert He core Radius photosphere HÎHe burning shell Inert H Í Temperature After helium is used up Reaction Min. Temp. 107 o K 4 1H Î 4He 3 4He Î 12C 12C + 4He Î 2x108 16O, Ne, Na, Mg Triple-alpha process 8x108 Ne Î O, Mg 1.5x109 O Î Mg, S 2x109 Si ÎFe peak 3x109 • Contraction heats center • Helium starts to burn. 6 Luminosity Î The future of the Sun Earth’s orbit. Notice changes in scale Radius Î Time (billions of yrs.) Î • Cat’s Eye. Harrington & Borkowski, (UMd) & NASA 7 • Helix nebula • Gas & dust ejected by star in the middle. • Ejection occurred several times. • Wind blows gas into previous ejecta. • Colors • Blue: O • Red: H & N NASA, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO). • NGC 6826 • Wind from hot star blows on material expelled by pulses. Balick (U Wash), et al & NASA 8 The dead sun • • • • White dwarf Sirius B White dwarf is initially very hot. As it cools, the white dwarf grows redder and fainter. Q4 Put these in order: MS, WD, giant, PN a. b. c. d. MS, WD, G, PN MS, WD, PN, G MS, PN, G, WD MS, G, PN, WD NASA, H E Bond (STSCI) et al. 9