Download Chapter 16 Lives of the Stars (Low Mass)

Chapter 16
Lives of the Stars (Low Mass)
Post Main Sequence life for low and
intermediate mass stars
• Low mass star (M < 0.2
M ) can burn hydrogen for
extremely long and we
haven't observed them
running out yet
• Observations of star clusters
show that intermediate mass
stars ( 0.2 M to 8 M )
becomes larger, redder, more
luminous after their time on
the main sequence is over:
they become first subgiants,
then red giants
SUN
SUN
Getting older: for an intermediate mass star
SUN
Mirror Principle:
core contracts,
envelope expands
• After most H burning is done, the He produced by fusion remains in core (He
•
•
•
•
is heavier than H)
As density of hydrogen becomes too low for fusion, temperature decreases,
fusion rate drops…
Pressure decreases, gravity wins : the core contracts with He falling inward (as
He is heavier than H)
H gathers in a shell around the inert (= non-fusing) newly-formed Helium
core and H fusion resumes
Because H fusion happens in shell, its thermal pressure does not stop the core
from contracting, and the envelop expands: luminosity increases
Subgiant: the way to Giant
• Subgiant: slightly more luminous than the original
main-sequence star but not as luminous as the future
giant star. H burning in shell around the forming inert
Helium core.
• Early subgiant: diameter and brightness have
increased, but the star has yet to cool down or change
color significantly.
• Later subgiants: closer to becoming a true giant, now
has larger diameter and lower temperature, becomes
orange.
• Subgiant branch on HR diagram: horizontal evolution
right of the immediate main sequence (luminosity changes
very little during the whole subgiant stage, but the
temperature significantly decreases)
Red giant phase
• Huge increase in luminosity giant star, H is still burning
in shell around the forming inert Helium core.
• Much larger diameter, and lower temperature.
• Hydrogen is getting depleted very fast
Next energy source: Helium fusion
The Helium core is initially inert, but gets compacted more
and more, therefore its temperature steadily increases.
Helium fusion does not begin right away because it requires
a very high temperature of 100 million K (compared to H
fusion which needed ~10 million K; Helium larger electrical
charges leads to greater repulsion between atoms so is harder
to fuse). The triple-alpha process combines three He nuclei
to make one carbon.
Thought Question
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts up
B. Hydrogen fusion stops
C. Helium fusion rises very sharply
Hint: Degeneracy pressure is the main form of pressure in
the inert helium core
Thought Question
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts up
B. Hydrogen fusion stops
C. Helium fusion rises very sharply
Hint: Degeneracy pressure is the main form of pressure in
the inert helium core
Broken thermostat in red giants
• Degeneracy pressure, not thermal pressure, supports the
core of low mass and intermediate mass red giants.
Why ? Because the core has been compacted a lot, and
there are not enough “chairs” for electrons to sit, so
degeneracy pressure is very strong.
• Hence it is degeneracy pressure that prevents the core
from contracting further/fights gravity.
• The “thermostat” regulation is broken in red giant as
degeneracy pressure does not increase with temperature =
the core does not expand (and so doesn’t cool off) as
temperature rise.
Intermediate mass stars:
Helium Flash !
• For medium massive red giants (0.5 MSUN to 2 MSUN), as
fusion of He is starting, temperature increases more and
more.
• Fusion rate skyrockets due to higher temperature (more
collision between He): this is called the Helium Flash as it
happens in a matter of seconds !
• The Helium flash marks the end of the red giant phase:
finally, as core temperature become extreme, thermal
pressure pushes back against gravity, finally takes over and
expands the core again to reach a balance
Low mass and high mass stars:
no Helium Flash
• Low mass red giants ( 0.2 MSUN to 0.5 MSUN) had enough
mass to become red giants, but not enough to compress
the He core enough to reach 100 millions K. He fusion
never happens.
• Higher mass red giants (2 MSUN to 8 MSUN), the
collapsing core will reach 100 million K before it is dense
enough to supported by degeneracy pressure, so helium
fusion will begin much more smoothly, and produce no
helium flash.
Life after Helium Flash:
Horizontal branch
The Horizontal branch is a stage of evolution that immediately
follows the red giant branch in stars whose masses are similar to the
Sun's. Horizontal-branch stars are powered by helium fusion in the
core and by hydrogen fusion in a shell around the core.
The thermostat is temporarily fixed.
Horizontal branch
Life after Helium Flash:
Horizontal branch
• A red giant shrinks and
fades after He fusion begins
in the core
• Despite having both He and H
fusing, not much energy is
produced because the fusion
rates are low now: the
density is much lower as the
star expanded so much
• Mirror principle applies
again: as the core expands,
envelope shrinks
• Surface temperature slightly
increases as surface shrinks,
the star becomes more yellow
Life after Helium Flash:
Horizontal branch
• Observations of
star clusters agree
with models
• Helium-burning
stars are found in
a horizontal
branch on the
H-R diagram
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
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
Last stages of nuclear burning:
AGB stars
• After core He fusion begin to runs out, Horizontal giants
finally become Asymptotic Giant Branch (AGB) stars:
– He fuses into C in a shell around the C core
– H fuses into He in a shell around the He layer
• AGB star is the fate of all low- to intermediate-mass stars (0.5
MSUN to 8 MSUN) late in their lives.
• Double-shell burning stage never reaches equilibrium—fusion
rate periodically spikes upward in a series of thermal pulses
• The star is now large, luminous, and unstable
(Sun will grow out nearly to Earth-Sun distance !)
Mira A, an AGB star
UV image of
environment around
Mira A and B. Mira B
has strong winds and
carves out a cavity in
the lost gas
Hubble image of Mira A. The
star has large bright surface
features that vary over time
and cause it to appear to
change shape.
The strong wind
from Mira interacts
with the ISM as
the star travels
through it ,
creating a bowwave that trails
behind it a distance
of 13 light-years
Planetary Nebulae
• A low mass star ends
with a strong pulse
that ejects the H and
He envelope into
space as a planetary
nebula
• The core left behind
The Ring Nebula
becomes a white
dwarf (supported by
degeneracy pressure)
The Eskimo Nebula
Life stages of a low/medium (< 8 MSUN) mass star
AGB star: double shell
burning (both helium and
hydrogen), inert carbon core
Horizontal branch:
helium-burning core
Subgiant/red giant branch:
inert helium core
How much time will these stages last for the Sun ?
The Fate of the Solar System
Current Temperatures
• Even before the red
giant state, increasing
luminosity will affect
Earth
• After an increase in
luminosity of 10%
Earth will be on
verge of a runaway
greenhouse effect!
Ring
Nebula
After a he
10%
increase
in luminosity.
Water boils at 373 K.
The Fate of the Solar System
• As luminosity of the Sun increases, more distant regions
will warm and could support liquid water
The Fate of the Solar System
Thought Question
What effect will mass-loss from the Sun have on the orbits of
the planets?
A. Orbits will expand
B. Orbits will shrink
C. Orbits stay the same
The Fate of the Solar System
Thought Question
What effect will mass-loss from the Sun have on the orbits of
the planets?
A. Orbits will expand
B. Orbits will shrink
C. Orbits stay the same
The Fate of the Solar System
• Mass loss from Sun will cause the orbits of the planets to expand. This
graph shows the radius of the Sun against time, with orbits of inner
planets also plotted at their distances.
The Fate of the Solar System
• But Earth might still not be spared! Being closer to the Sun means more drag
from the solar wind, so Earth might lose momentum and its orbit shrink from
drag. In this graph, the orbit of Earth is the dotted line and drag is taken into
account. When the Sun reaches a certain size, Earth is pulled into the Sun.
The Fate of the Solar System
• Depending on mass loss, outer Solar System bodies will
also be impacted. The outer planets may stay stable but
objects further out may end up ejected!