1. 20. Stellar Death Low-mass stars undergo three red-giant stages Dredge-ups bring material to the surface Low-mass stars die gently as planetary nebulae Low-mass stars end up as white dwarfs High-mass stars synthesize heavy elements High-mass stars die violently as supernovae Supernova 1987A Supernovae produce abundant neutrinos Binary white dwarfs can become supernovae Detection of supernova remnants

2. Low-Mass Stars: 3 Red Giant Phases Low-mass definition –< ~ 4 M ☉ during main-sequence lifetime Red giant phases –Initiation ofshellhydrogenfusion Red giant branch on the H-R diagram –Initiation ofcoreheliumfusion Horizontal branch of the H-R diagram –Initiation ofshellheliumfusion Asymptotic giant branch of the H-R diagram

3. The Sun’s Post-Main-Sequence Fate

4. Interior of Old Low-Mass AGB Stars

5. Stellar Evolution In Globular Clusters

6. Dredge-Ups Mix Red Giant Material Main-sequence lifetime –The core remains completely separate No exchange of matter with overlying regions –Decreasing HIncreasing Hein the core Overlying regions retain cosmic chemical proportions –~ 74 % H~ 25% He~ 1% “metals”[by mass] Red giant phases –Three possible stages Stage 1 dredge-upAftercoreHfusionends Stage 2 dredge-upAftercoreHefusionends Stage 3 dredge-upAftershellHefusionbegins –Only if M Star > 2 M ☉ –One possible result A carbon star –Abundant CO ejected into space –Same isotopes of C & O that are in human bodies

7. Low-Mass Stars Die Gently He-shell flashes produce thermal pulses –Caused by runaway core He fusion in AGB stars Cyclical process at decreasing time intervals –313,000 years –295,000 years –251,000 years –231,000 years –All materials outside the core may be ejected ~ 40% of mass lost from a1.0 M ☉ star > 40% of mass lost from a>1.0 M ☉ star Hot but dead CO core exposed –At the center of an expanding shell of gas Velocities of ~ 10 km. sec -1 to ~ 30 km. sec -1 Velocities of ~ 22,000 mph to ~ 66,000 mph

8. Carbon Star & Its CO Shell: Photo

9. Carbon Star & Its CO Shell: Sketch

10. Thermal Pulses of 0.7 M ☉ AGB Stars

11. One Example of a Planetary Nebula

12. Helix Nebula: 140 pc From Earth

13. An Elongated Planetary Nebula

14. Low-Mass Stars End As White Dwarfs UV radiation ionizes the expanding gas shell –This glows in what we see as a planetary nebula Name given because they look somewhat like planets No suggestion that they have, had, or will form planets –This gas eventually dissipates into interstellar space No further nuclear fusion occurs –Supported by degenerate electron pressure –About the same diameter as Earth~ 8,000 miles –It gradually becomes dimmer Eventually it becomes too cool & too dim to detect

15. White Dwarfs & the Earth

16. The Chandrasekhar Limit White dwarf interiors –Initially supported by thermal pressure Ionized C & O atoms A sea of electrons –As the white dwarf cools, particles get closer Pauli exclusion principle comes into play Electrons arrange in orderly rows, columns & layers –Effectively becomes one huge crystal White dwarf diameters –The mass-radius relationship The larger the mass, the smaller the diameter The diameter remains the same as a white dwarf cools –Maximum mass degenerate e – pressure can support ~ 1.4 M ☉ After loss of overlying gas layers –White dwarf upper mass limit is the Chandrasekhar limit

17. Evolution: Giants To White Dwarfs

18. White Dwarf “Cooling Curves”

19. High-Mass Stars Make Heavy Elements High-mass definition –> ~ 4 M ☉ as a ZAMS star Synthesis of heavier elements –High-mass stars have very strong gravity Increased internal pressure & temperature Increased rate of core H-fusion into He Increased rate of collapse once core H-fusion ends Core pressure & temperature sufficient to fuse C –The CO core exceeds the Chandrasekhar limit Degenerate electron pressure cannot support the mass The CO core contracts & heats –Core temperature > ~ 6.0. 10 8 K –C fusion into O, Ne, Na & Mg begins

20. Synthesis of Even Heavier Elements Very-high-mass definition –> ~ 8 M ☉ as a ZAMS star Synthesis of still heavier elements –End of core-C fusion Core temperature > ~ 1.0. 10 9 K Ne fusion into O & Mg begins –End of core-Ne fusion Core temperature > ~ 1.5. 10 9 K O fusion into S begins –End of core-O fusion Core temperature > ~ 2.7. 10 9 K Si fusion into S & Fe begins –Start of shell fusion in additional layers

21. The Interior of Old High-Mass Stars

22. Consequence of Multiple Shell Fusion Core changes –Core diameterdecreaseswith each step Ultimately about same diameter as Earth~ 8,000 miles –Rate of core fusionincreaseswith each step Energy changes –Each successive fusion step produces less energy –All elements heavier than iron require energy input Core fusion cannot produce elements heavier than iron All heavier elements are produced by other processes

23. Evolutionary Stages of 25-M ☉ Stars

24. High-Mass Stars Die As Supernovae Basic physical processes –All thermonuclear fusion ceases The core collapses –It is too massive for degenerate electron pressure to support The collapse rebounds Luminosity increases by a factor of 10 8 –As bright as an entire galaxy –> 99% of energy is in the form of neutrinos –Matter is ejected at supersonic speeds Powerful compression wave moves outward Appearance –Extremely bright light where a dim star was located –Supernova remnant Wide variety of shapes & sizes

25. The Death of Old High-Mass Stars

26. Supernova: The First 20 Milliseconds

27. Supernova 1987A Important details –Located in the Large Magellanic Cloud Companion to the Milky Way ~ 50,000 parsecs from Earth Discovered on 23 February 1987 –Near a huge H II region called the Tarantula Nebula –Was visible without a telescope First naked-eye supernova since 1604 Basic physical processes –Primary producer of visible light Shock wave energy< 20 days Radioactive decay of cobalt, nickel & titanium> 20 days Dimmed gradually after radioactivity was gone> 80 days –Luminosity only 10% of a normal supernova

28. Unusual Feature of SN 1987A Relatively low-mass red supergiant –Outer gaseous layers held strongly by gravity –Considerable energy required to disperse the gases –Significantly reduced luminosity Unusual supernova remnant shape –Hourglass shape OuterringsIonized gas from earlier gentle ejection CentralringShock wave energizing other gases

29. Supernova 1987A: 3-Ring Circus

30. White Dwarfs Can Become Supernovae Observed characteristics –No spectral lines of H or He These gases are gone The progenitor star must be a white dwarf –Strong spectral line of Si II Basic physical processes –White dwarf in a close-binary setting Over-contact situationCompanion star fills Roche lobe –White dwarf may exceed the Chandrasekhar limit Degenerate electron pressure cannot support the mass Core collapse begins, raising temperature & pressure Unrestrained core C-fusion begins –White dwarf blows apart

31. White Dwarf Becoming a Supernovae

32. The Four Supernova Types Type Ia Type Ib Type Ic Type II No H or He lines Strong Si II line No H lines Strong He I line No H or He lines Strong H lines

33. Type Ia & II Supernova Light Curves

34. Gum Nebula: A Supernova Remnant

35. Pathways of Stellar Evolution

36. Death of low-mass stars –ZAMS mass < 4 M ☉ –Red giant phases Start of shell H fusion Start of core He fusion Start of shell He fusion No elements heavier than C & O –Gentle death Dead core becomes a white dwarf Expelled gases become planetary neb. Death of high-mass stars –ZAMS mass > 4 M ☉ –Red supergiant phases No elements heavier than Fe –Catastrophic death Dead core a neutron star or black hole Supernova remnant Elements heavier than Fe produced Pathways of stellar evolution – Low-mass stars Produce planetary nebulae End as white dwarfs – High-mass stars Produce supernovae End as neutron stars or black holes Important Concepts