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 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 fusionends

• Stage 2 dredge-up After core He fusionends

• Stage 3 dredge-up After shell He fusionbegins

– 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

– 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• 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

Type Ib

Type Ic

Type II

No H or He linesStrong Si II line

No H linesStrong He I line

No H or He lines

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• 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