POST-MAIN SEQUENCE EVOLUTION

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POST-MAIN SEQUENCE EVOLUTION

• THE END OF THE MAIN SEQUENCE
• A star leaves the MS when it exhausts H at the
  core. During the MS, there is an excellent
  balance between P and gravity HYDROSTATIC
  EQUILIBRIUM
• When H is gone, the core is essentially all He
  and (at between 6 and 40 million K),
  far too cool to start
  nuclear fusion of He.
• The structure must readjust since the H fusion,
  which had provided the energy and pressure, at
  the center.

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SUBGIANT PHASE

• All H gone in core He "ash" is too cold to
  "burn"
• Pressure provided by energy from fusion in the
  core disappears.
• The He core contracts -- gravity wins over
  pressure again.
• Contraction heats the core.
• Most of this heat is trapped, so core T rises.
• Rising density and T imply core P rises pretty
  fast, so there is a contraction, NOT a collapse.

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Hydrogen Burning Shell (Subgiant)
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Thought Question

• What happens when a star can no longer fuse
  hydrogen to helium in its core?
• A. Core cools off
• B. Core shrinks and heats up
• C. Core expands and heats up
• D. Helium fusion immediately begins

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Thought Question

• What happens when a star can no longer fuse
  hydrogen to helium in its core?
• A. Core cools off
• B. Core shrinks and heats up
• C. Core expands and heats up
• D. Helium fusion immediately begins

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Subgiant, 2

• Increased core T diffuses into the H BURNING
  SHELL -- the layer of H hot enough to fuse
  outside the inert He core.
• This higher T causes a dramatic increase in L
  from that shell (both pp chains and CNO cycle
  fusion rates are VERY SENSITIVE to T)
• Higher L in shell causes the inert H envelope to
  expand.
• Work is done in producing this expansion, so the
  star's surface T declines (an expanding cloud of
  gas cools just as an opaque contracting one
  heats).
• This corresponds to the star moving to the right
  and up on the H-R diagram and it enters the
  SUBGIANT phase.

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Life Track after Main Sequence

• Observations of star clusters show that a star
  becomes larger, redder, and more luminous after
  its time on the main sequence is over

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RED-GIANT PHASE

• As the core continues to contract and heat up, T
  108 K is finally reached
• Then higher electric repulsion of Helium nuclei
  can be overcome
• AND He CAN FUSE INTO CARBON
  3 4He ? 12C ?
  (the TRIPLE-ALPHA REACTION).
  
• Really, 4He 4He ? 8Be but Be-8 is unstable, so
  3 He-4's are needed to come together nearly
  simultaneously.
• This generates more energy, and both L and T in
  core increases.

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Helium Flash

• For M lt 2 M? this occurs while the He core is
  degenerate (yet more about this later when we
  discuss White Dwarfs)
• As P doesn't rise with T for degenerate matter,
  the thermostat is broken
• So the core temperature rises fast when He fusion
  begins and the Luminosity from He goes up even
  faster HELIUM FLASH
• until thermal pressure is large again and expands
  core again, again dropping the core temperature
• This causes a very fast expansion of the star's
  envelope, and a further cooling of its surface,
  yielding a RED GIANT (with size 100's of that of
  Sun on MS but lower Ts ).

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Life Track after Helium Flash

• Models show that a red giant should shrink and
  become less luminous after helium fusion begins
  in the core

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THE HORIZONTAL BRANCH

• He Flash ends quickly, once core pressure has
  grown, causing the core radius to rise, thus,
  yielding a decline in Tc to just about 108 K.
• Now He burns smoothly in the core -- producing
  the He BURNING MAIN SEQUENCE -- which is visible
  on an H-R diagram as the HORIZONTAL BRANCH (lower
  L but higher Ts than during the He flash).
• Stars are again in HYDROSTATIC EQUILIBRIUM
  throughout the thermostat works again
• These are still RGs, and on HB the higher masses
  are to the left part of the HB.
• Most stars spend most of their POST-MS life on
  the HB, but this is typically lt 10 of their MS
  life.

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Back up to the Red-Giant Branch on the H-R
Diagram (Asymptotic Giant Branch)
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Thought Question

• What happens when the stars core runs out of
  helium?
• A. The star explodes
• B. Carbon fusion begins
• C. The core cools off
• D. Helium fuses in a shell around the core

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Thought Question

• What happens when the stars core runs out of
  helium?
• A. The star explodes
• B. Carbon fusion begins
• C. The core cools off
• D. Helium fuses in a shell around the core

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AGB for Lower Mass Stars

• Increased core T diffuses into the He BURNING
  SHELL -- the layer of He hot enough to fuse
  outside the inert C core.
• This higher T causes a dramatic increase in L
  from that shell.
• Higher L in shell causes the inert He envelope,
  as well as the H burning shell and inert H
  envelope to expand.
• Work is done by the gas in producing this
  expansion, so the star's surface T declines by a
  bit.
• Star is hotter inside and more luminous than
  before

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Helium Burning Shell
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Double Shell Burning

• After core helium fusion stops, He fuses into
  carbon in a shell around the carbon core, and H
  fuses to He in a shell around the helium layer
• This double-shell burning stage never reaches
  equilibriumfusion rate periodically spikes
  upward in a series of thermal pulses
• With each spike, convection dredges carbon up
  from core and transports it to surface

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ON TO WHITE DWARFS

• For stars with MS masses less than about 7 to 8
  M? AGBs or Supergiants lose a good bit of mass,
  and some of these pulsations become so powerful
  that massive shells (of 0.1 to 0.2 M?) are
  ejected.

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End of Fusion

• Fusion progresses no further in a low-mass star
  because the core temperature never grows hot
  enough for fusion of heavier elements (some He
  fuses to C to make oxygen)
• Degeneracy pressure -- electrons fill up all
  quantum mechanically allowed energy levels
  --supports the white dwarf against gravity

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Planetary Nebulae

• Double-shell burning ends with a pulse (or
  pulses) that eject most of the H and He into
  space as a planetary nebula
• The core left behind becomes a white dwarf

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Ejected Shells Planetary Nebulae
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CENTRAL STARS OF PN

• The cores of the RGs/SGs are very hot and excite
  the PN gas.
• These Central Stars of PN have C or CO cores,
  and He envelopes (All the H was expelled as winds
  or PN).

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Dead Core Evolution

• They are not massive enough to compress the C
  core to T gt 7 x 108 K at which it could fuse, so
  these CSPN's just cool off and fade in power,
  slowly shrinking in size
• BUT, when density of the core reaches 106 g/cm3
  (or one ton / teaspoon!) the PAULI EXCLUSION
  PRINCIPLE takes over
• no 2 electrons can be in the same energy state
• this Quantum Mechanical effect provides a HUGE
  DEGENERACY PRESSURE that stops the continued
  contraction at a radius of about 1/100th of R?
  (nearly the same as R? ).

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White Dwarfs

• Once it is held up by degeneracy pressure
• we call it a WHITE DWARF.
• The MAXIMUM MASS electron degeneracy pressure can
  support is about 1.4 M?-- the CHANDRASEKHAR
  LIMIT.
• So 7-8 M? stars on the MS leave WDs close to the
  Chandrasekhar limit
• But the more common 0.8-2 M? stars leave WDs
  around 0.6-0.7 M? (the typical mass of a WD).

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Observed White Dwarfs

• Sirius B is a bound companion to the nearby very
  bright star Sirius (A) M1.1 M? R5100 km
• M4 the nearest globular cluster, about 16 pc
  across at 2100 pc distance
• Nearly 100 WDs are seen in a small region

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Size of a White Dwarf

• White dwarfs with same mass as Sun are about same
  size as Earth
• Higher mass white dwarfs are smaller!
• (Higher density needed to support more mass)

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Earths Fate
Errors on Scale 100?
10?
1?

• Suns radius will grow to near current radius of
  Earths orbit

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Earths Fate

• Suns luminosity will rise to 1,000 times its
  current leveltoo hot for life on Earth
• Life and Death of the Sun Applet

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Summary for Low Mass Stars

• What are the life stages of a low-mass star?
• H fusion in core (main sequence)
• H fusion in shell around contracting core (red
  giant)
• He fusion in core (horizontal branch)
• Double-shell burning (red giant)
• How does a low-mass star die?
• Ejection of H and He in a planetary nebula leaves
  behind an inert white dwarf

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Asymptotic Giant Branch (for Massive Stars)
?Supergiants

• Once the He in the core is all burned up, we
  reach the end of the He BURNING MAIN SEQUENCE.
• As at the end of the He MS Pressure provided by
  energy from fusion in the core disappears.
• The Carbon core contracts -- gravity wins over
  pressure again.
• Contraction heats the core Most of this heat is
  trapped, so core T rises.
• Rising density and T imply core P rises, so again
  there is a contraction, not a collapse.

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Supergiants for Higher Mass Stars

• For more massive stars the same thing happens,
  but the star starts way up on the H-R diagram,
  and it enters the SUPERGIANT phase.
• The ESCAPE VELOCITY from such big stars gets low
  Vesc (2 G M / R)1/2 as R increases while
  M stays the same.
• They lose a lot of mass via winds.
• Also, RGs and SGs are subject to opacity driven
  instabilities which cause the outer layers to
  expand and cool and contract and heat up.
• This produces VARIABLE STARS if the atmosphere
  lies in the INSTABILITY STRIP.
• Important classes of variable stars are the RR
  LYRAE (horizontal branch) and two types of
  CEPHEID VARIABLES (supergiants), since they are
  wonderful DISTANCE INDICATORS

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Massive Star Post-MS Evolution

• Stars starting the MS with more than 8M? are
  unlikely to leave behind WDs
• Why? They leave cores w/ M gt 1.4M? the
  Chandrasekhar limit.
• Evolutionary History
• MS
• H is exhausted in core
• H shell burning starts, with modest increase in L
  and fast decrease in TS (fast move to right on
  H-R diagram)
• He fusion starts (non-degenerately, so no flash)
• Modest increase in L and core He burning -- a
  SUPERGIANT
• He is exhausted in core

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Massive Post-MS Evolution, 2

• So far, pretty similar to lower mass stars
  studied already, but
• Now we feel the big difference higher M means
  gravity can crush the C core until it reaches T gt
  7 x 108 K so
• Carbon CAN ALSO FUSE
• 12C 4He ? 16O ?
• Some 16O 4He ? 20Ne ?
• Also some 12C 12C ? 24Mg ?
• This fuel produces less energy per mass so C is
  burnt quickly.
• Loops in the H-R diagram.

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Massive Post MS Evolution on H-R Diagram
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Massive Post-MS Evolution, 3

• The more massive the star the more nuclear
  reactions will occur
• Most such stars will have Oxygen cores that can
  also fuse, typically needs T gt 1 x 109 K!
• 16O 4He ? 20Ne ?
• 20Ne 4He ? 24Mg ?
• Well come back to this type of onion-layer model
  star when we talk about supernova explosions and
  neutron stars.
• The elements cooked here are needed for life

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Massive Stars Have Powerful Winds
HST picture of AG Carinae 50 solar masses Light
echoes showing shells from V838 Monocerotis
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Binary Star Evolution

• Many stars are in binary or multiple systems
• If the binary is close enough, evolution is
  affected
• More massive stars still can be on MS while less
  massive has evolved off (like Algol)
• Only possible if there is mass transfer through
  Lagrangian point (L1) between Roche lobes

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Binary Evolution Depends on Separation

• Detached, evolve separately
• Semi-detached, one fills Roche lobe, dumping on
  other
• Contact or common-envelope, both overflow single
  star w/ two fusion cores

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Binary Evolution Algol Type

• Start detached
• More massive leaves MS, overflows Roche lobe
• Now 2nd star is more massive but still on MS, so
  smaller!