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20. Stellar Death
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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 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
• Stage 2 dredge-up
• Stage 3 dredge-up
After core H fusion ends
After core He fusion ends
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
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
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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 ~ 22,000 mph to ~ 66,000 mph
Carbon Star & Its CO Shell: Photo
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
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”
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
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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
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
The Interior of Old High-Mass Stars
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
– 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
The Death of Old High-Mass Stars
Supernova: The First 20 Milliseconds
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
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
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
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
Important Concepts
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Death of low-mass stars
– ZAMS mass < 4 M☉
– Red giant phases
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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.
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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
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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