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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

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20 stellar death
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 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
The Sun’s Post-Main-Sequence Fate


Interior of old low mass agb stars
Interior of Old Low-Mass AGB Stars


Stellar evolution in globular clusters
Stellar Evolution In Globular Clusters


Dredge ups mix red giant material
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


Low mass stars die gently
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


Carbon star its co shell photo
Carbon Star & Its CO Shell: Photo



Thermal pulses of 0 7 m agb stars
Thermal Pulses of 0.7 M☉AGB Stars



Helix nebula 140 pc from earth
Helix Nebula: 140 pc From Earth


An elongated planetary nebula
An Elongated Planetary Nebula


Low mass stars end as white dwarfs
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



The chandrasekhar limit
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
Evolution: Giants To White Dwarfs


White dwarf cooling curves
White Dwarf “Cooling Curves”


High mass stars make heavy elements
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
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
The Interior of Old High-Mass Stars


Consequence of multiple shell fusion
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



High mass stars die as supernovae
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
The Death of Old High-Mass Stars



Supernova 1987a
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
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
Supernova 1987A: 3-Ring Circus


White dwarfs can become supernovae
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
White Dwarf Becoming a Supernovae


The four supernova types

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



Gum nebula a supernova remnant
Gum Nebula: A Supernova Remnant



Important concepts

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