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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
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
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
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
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
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
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
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
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
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