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Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
Here is a way to think about it.
Outside of
star
Plenty of hydrogen
Shrinking core
Where core
use to be.
And where
conditions
were right for
fusion.
Result…
• Because the core is shrinking there is
hydrogen that is introduced into the area
around the core where temperatures and
pressures are high enough for hydrogen
fusion to take place.
• Hydrogen begins to fuse into helium, in a
shell around the shrinking helium core.
• Now there are two energy sources in the
star.
Two energy sources.
• Gravitational potential energy is being
used to make radiant energy in the core.
• The shell around the core is producing
energy from the fusion of hydrogen.
• The result of all this energy is that the
outer envelope of the star expands
enormously. The star becomes a red
giant. (luminosity class III)
Age of M13: 14 billion years. Mass of stars
leaving the main-sequence ~0.8 solar masses
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
Helium core burning
• The core contraction and hydrogen shell
burning until at last the temperature is high
enough in the core to begin helium fusion.
This is around 100 million degrees.
• When this happens the star is fusing
Helium into Carbon in its core, and still is
fusing Hydrogen into Helium is a shell
around the core.
The triple alpha process
Core and shell burning produces more energy than
the star produced on the main sequence so the Hecore burning stars are more luminous than when
they were main-sequence stars.
Helium
coreburning
stars
Giants
Subgiants
Main
Sequence
Helium gone in the core
• Helium fusion rate is much faster than the
Hydrogen fusion rate was. Within a few hundred
million years the supply is gone in the core.
• The core once again shrinks, releasing
gravitational potential energy.
• The material in a shell closest to the core begins
to fuse helium into carbon, in bursts, as the
temperature increases.
• Above this shell, hydrogen is being converted
into helium.
Here is a way to think about it.
Outside of
star
Helium shell
burning
Plenty of hydrogen
Shrinking core
Hydrogen
shell burning
Three energy sources.
• At this point there are three sources of
energy in the star, the shrinking carbon
core, and two shells.
• The star rapidly expands and heads back
up to the giant stage. This is called the
asymptotic giant branch, because it
asymptotically approaches the red giant
branch.
Asymptotic Giant branch phase
• During this phase the helium core burning
is not stable. It rapidly turns on and off in
bursts. Small explosions.
• The results of these explosions is to eject
shocks into the outer envelope of the star.
Material in the envelope is lifted off the
star, over and over again.
• When the carbon core can no longer
contract, everything stops.
• The lost envelope becomes an expanding
planetary nebula.
• The exposed carbon core is a white dwarf
star.
Planetary nebula & White Dwarf
White
Dwarf star
Summary of evolution of lower mass stars
• Star is on main-sequence – Core converting hydrogen
into helium.
• Star is a Sub-giant -- Core is contracting releasing
gravitational potential energy
• Star is a Giant (III) – Core is contracting releasing
gravitational potential energy and hydrogen into helium
in a shell around the core.
• Helium core burning phase – Star is converting helium
into carbon in the core and hydrogen into helium in a
shell.
• Asymptotic Giant branch phase – Core is contracting
releasing potential energy, Helium into Carbon in a shell,
and hydrogen into helium is a shell around Helium shell.
Notice a pattern
• Whenever a star has an inert core that is
shrinking, the star is moving up the giant
branch. The star grows in radius
• Whenever there is nuclear fusion in the
core the star shrinks back down. Smaller
radius.
• This will be important in high mass stars.
Inert core
Core
fusing
elements
Low mass stars cannot fuse Carbon
• Core temperature is too low to fuse Carbon into
other elements.
• The core shrinks until all the free electrons are
trapped in spaces between the Carbon nuclei.
They set up energy levels and the core acts like
a giant atom. Core cannot shrink any more.
• The core is similar in size to the radius of the
Earth, but has a mass of as high as 1.4 times
the Sun’s mass.
• From here on the core will just slowly cool off.
Like a hot piece of metal, slowly cools down.
Planetary Nebula
• During the Helium shell burning phase, there are
helium flashes occurring. The helium in the shell
doesn’t “burn” at a constant rate. It burns in
spurts. Each time helium shell burning turns on,
there is an eruption.
• The result is the outer envelope of the star gets
shocked, over and over. The outer shell is lifted
off in layers.
• The result is a planetary nebula. The exposed
Carbon core is a white dwarf.
The Ring nebula – M57
Cat’s Eye Nebula
M57 through a small telescope
Boomerang Nebula
Butterfly Nebula – Central White Dwarf has
T = 250,000 K.
Cat’s Eye in optical and X-ray light
Eskimo Nebula
NGC 2440 – Central White Dwarf has
T = 200,000 K
Ring Nebula – Multiple mass ejections.
The planetary nebula phase is short lived.
• The radius of a typical planetary nebula is
about 1 light year.
• The gas is glowing, so we see an emission
nebula.
• Typical elements in at planetary nebula
are hydrogen, helium, carbon, oxygen and
nitrogen. Also some neon present.
Spectrum of Ring nebula
Sirius – The Dog Star
Sirius is a binary star
Sirius A
Sirius B
Which star is older in this binary system
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1. Sirius B because it is
already a white dwarf
2. Sirius A because it is
more luminous
3. They are in a binary
system, they must be
the same age
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Which star was originally the most massive?
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1. Sirius B because it is
a white dwarf
2. Sirius A because it is
more luminous
3. They formed at the
same time so they
must have the same
mass
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Globular cluster M 4
• The stars with masses higher than about
0.8 solar masses have died.
• There should be a lot of white dwarfs in
the a globular star cluster.
White Dwarfs in M 4
• White dwarfs are just the leftover core of the
star. It is not producing energy. It is simply
cooling off.
• As a WD cools it becomes less luminous
because the temperature is decreasing.
• The cooling follows a very simple cooling
relation that depends primarily on time. The
older the white dwarf, the cooler it is.
• There is a cutoff in the WD temperature. No WD
are found that are cooler than the cut off.
Cooling
Cut-off
Why is there a cut-off in the white dwarf
population?
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1. Cooler WD are
impossible to detect
2. At a certain temperature,
WD explode
3. The universe isn’t old
enough to have cooler
white dwarfs
4. WD come into
temperature equilibrium
with the universe and
remain that temperature
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Age of the universe using WD cooling
• To date hundreds of thousands of White
Dwarfs have been observed.
• There is a temperature cut-off beyond
which no white dwarfs are found.
• This is because there hasn’t been enough
time since the start of the universe for WD
to cool any further.
• The age of the universe computed from
WD cutoff is about 12 billion years.
The Death of High Mass Stars
• When a high mass star runs out of hydrogen in
its core, the core begins to shrink. The outside
of the star expands and the star moves right on
the H-R diagram.
• The temperature is cooling and the radius is
growing, but the luminosity is virtually constant.
• Since L = σT4(4πR2); T4 must be changing at
the same rate as R2
• The star becomes a supergiant
(luminosity class I star)
The Death of High Mass Stars
• As the star tracks to the right for the first
time the inert helium core is contracting
and hydrogen shell burning is occurring.
• At the farthest right, helium core burning
begins, converting helium into carbon.
And still hydrogen shell burning.
• The star begins to move to the left on the
H-R diagram.
The Death of High Mass Stars
• When the helium runs out in the core, the
core begins to contract again, there is
helium shell burning into carbon, and
hydrogen shell burning into helium.
• The star moves right again, toward cooler
temperatures and larger radii.
The Death of High Mass Stars
• Finally the carbon core is hot enough to
fuse carbon into oxygen and nitrogen.
• The star moves back to the left on the H-R
diagram. There is a core changing carbon
into oxygen and nitrogen, a shell changing
helium into carbon, and a shell changing
hydrogen into helium.
A rule of thumb.
• Every time a high mass star moves to the
right (cooler temp) on the H-R diagram,
the core is inert, but contracting.
• Every time a high mass star moves to the
left, the core is fusing one element into
another.
• Throughout all of this there is shell burning
going on.
Final stage.
• The core of the high mass star fuses:
• hydrogen into helium
•
helium into carbon
•
carbon into oxygen and nitrogen
• oxygen and nitrogen into sulfur and silicon
•
And finally silicon into IRON.
• At last the core is iron. This is where everything
stops with a bang!
The final core and shells of a
high mass star
Fusing Iron does not release energy.