20. Stellar Death. Interior of Old Low-Mass AGB Stars

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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 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 The Sun s Post-Main-Sequence Fate Interior of Old Low-Mass AGB Stars Stellar Evolution In Globular Clusters Dredge-Ups Mix Red Giant Material Main-sequence lifetime The core remains completely separate No exchange of matter with overlying (non-core) regions Increasing He & decreasing H 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 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!!!

2 Low-Mass Stars Die Gently Carbon Star & Its CO Shell: Photo 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 ~ 94.2% ~ 85.1% ~ 92.0% 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 ~ 36,000 km. hr 1 to ~ km. hr 1 Velocities of ~ 22,000 mph to ~ 66,000 mph Carbon Star & Its CO Shell: Sketch Thermal Pulses of 0.7 M AGB Stars One Example of a Planetary Nebula Helix Nebula: 140 pc From Earth

3 An Elongated Planetary Nebula 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 White Dwarfs & the Earth 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 Evolution: Giants To White Dwarfs White Dwarf Cooling Curves

4 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 > ~ K C fusion into O, Ne, Na & Mg begins The Interior of Old High-Mass Stars 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 > ~ K Ne fusion into O & Mg begins End of core-ne fusion Core temperature > ~ K O fusion into S begins End of core-o fusion Core temperature > ~ K Si fusion into S & Fe begins Start of shell fusion in additional layers 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 Evolutionary Stages of 25-M Stars High-Mass Stars Die As Supernovae Basic physical processes All thermonuclear fusion ceases The core collapses Core is too massive for degenerate electron pressure to support The collapse rebounds The 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 hypersonic speeds Powerful compression wave moves outward Appearance Extremely bright light where a dim star was located Supernova remnant Wide variety of shapes & sizes

5 The Death of Old High-Mass Stars Supernova: The First 20 Milliseconds Supernova 1987A Unusual Feature of SN 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 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 Luminosity only 10% of a normal supernova Supernova 1987A: 3-Ring Circus 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

6 White Dwarf Becoming a Supernovae The Four Supernova Types Type Ia No H or He lines Strong Si II line Type Ib No H lines Strong He I line Type Ic No H or He lines Type II Strong H lines Type Ia & II Supernova Light Curves Gum Nebula: A Supernova Remnant Pathways of Stellar Evolution Death of low-mass stars ZAMS mass < 4 M Red giant phases Start of shell H fusion Start of core He fusion Important Concepts 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

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