Download MAIN SEQUENCE STARS, Red Giants and White Dwarfs

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.
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.
Hydrogen Burning Shell (Subgiant)
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
Thought Question
What happens when a star can no longer fuse
hydrogen to helium in its core?
A.
B.
C.
D.
Core cools off
Core shrinks and heats up
Core expands and heats up
Helium fusion immediately begins
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.
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
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.
Helium Flash
• For M < 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 ).
Life Track after Helium Flash
• Models show
that a red giant
should shrink
and become
less luminous
after helium
fusion begins in
the core
THE HORIZONTAL BRANCH
o 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.
o 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).
o Stars are again in HYDROSTATIC EQUILIBRIUM
throughout: the thermostat works again
o These are still RGs, and on HB the higher masses
are to the left part of the HB.
o Most stars spend most of their POST-MS life on the
HB, but this is typically < 10 % of their MS life.
Back up to
the RedGiant Branch
on the H-R
Diagram
(Asymptotic
Giant
Branch)
Thought Question
What happens when the star’s core runs out of
helium?
A.
B.
C.
D.
The star explodes
Carbon fusion begins
The core cools off
Helium fuses in a shell around the core
Thought Question
What happens when the star’s 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
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
Helium Burning Shell
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
equilibrium—fusion rate periodically spikes upward in
a series of thermal pulses
• With each spike, convection dredges carbon up from
core and transports it to surface
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.
End of Fusion
• Fusion progresses no further in a lowmass 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
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
Ejected Shells = Planetary Nebulae
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 C+O
cores, and He
envelopes (All the H
was expelled as
winds or PN).
Dead Core Evolution
• They are not massive enough to compress the C
core to T > 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 ).
White Dwarfs
•
Once it is held up by degeneracy pressure:
Pdegen  
5/3
• 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).
Observed White Dwarfs
• Sirius B is a bound
companion to the
nearby very bright
star Sirius (A):
M=1.1 M
R=5100 km
• M4 the nearest
globular cluster,
about 16 pc across
at 2100 pc distance
• Nearly 100 WDs are
seen in a small
region
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)
Earth’s Fate
Errors on
Scale:
100
10
1
•
Sun’s radius will grow to near current radius
of Earth’s orbit
Earth’s Fate
•
•
Sun’s luminosity will rise to 1,000 times its
current level—too hot for life on Earth
Life and Death of the Sun Applet
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
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.
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
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 > 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
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 > 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.
Massive
Post MS
Evolution
on H-R
Diagram
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 > 1 x 109 K!
• 16O + 4He  20Ne + 
• 20Ne + 4He  24Mg + 
• We’ll 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
Massive Stars
Have Powerful
Winds
HST picture of AG Carinae:
50 solar masses
Light echoes showing shells from
V838 Monocerotis
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
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
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!