20. Stellar Death

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 of shell hydrogen fusion • Red giant branch on the H-R diagram • Initiation of core helium fusion • Horizontal branch of the H-R diagram • Initiation of shell helium fusion • 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 H Increasing He in 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-up After core H fusion ends • Stage 2 dredge-up After core He fusion ends • Stage 3 dredge-up After shell He fusion begins • Only if MStar > 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 a 1.0 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 ~ 22,000mphto ~ 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 . 108 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 . 109 K • Ne fusion into O & Mg begins • End of core-Ne fusion • Core temperature > ~ 1.5 . 109 K • O fusion into S begins • End of core-O fusion • Core temperature > ~ 2.7 . 109 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 diameter decreases with each step • Ultimately about same diameter as Earth ~ 8,000 miles • Rate of core fusion increases with 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 108 • 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 • Outer rings Ionized gas from earlier gentle ejection • Central ring Shock 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 situation Companion 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. No H or He lines Strong Si II line Type Ia No H lines Strong He I line Type Ib Type Ic No H or He lines Type II Strong H lines The Four Supernova Types

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