NGC 3370 Spiral Galaxy

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Age of M13 14 billion years. Mass of stars
leaving the main-sequence 0.8 solar masses
Helium core-burning stars
Giants
Sub-giants
Main Sequence
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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.
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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.

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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)

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Age of M13 14 billion years. Mass of stars
leaving the main-sequence 0.8 solar masses
Helium core-burning stars
Giants
Sub-giants
Main Sequence
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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.

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The triple alpha process
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Core and shell burning produces more energy than
the star produced on the main sequence so the
He-core burning stars are more luminous than when
they were main-sequence stars.
Helium core-burning stars
Giants
Sub-giants
Main Sequence
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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.

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Here is a way to think about it.
Outside of star
Helium shell burning
Plenty of hydrogen
Shrinking core
Hydrogen shell burning
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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.

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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.

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• The lost envelope becomes an expanding planetary
  nebula.
• The exposed carbon core is a white dwarf star.

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Planetary nebula White Dwarf
White Dwarf star
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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.

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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.

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Inert core
Core fusing elements
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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
  Suns mass.
• From here on the core will just slowly cool off.
  Like a hot piece of metal, slowly cools down.

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Planetary Nebula

• During the Helium shell burning phase, there are
  helium flashes occurring. The helium in the
  shell doesnt 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.

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The Ring nebula M57
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Cats Eye Nebula
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M57 through a small telescope
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Boomerang Nebula
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Butterfly Nebula Central White Dwarf has T
250,000 K.
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Cats Eye in optical and X-ray light
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Eskimo Nebula
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NGC 2440 Central White Dwarf has T 200,000 K
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Ring Nebula Multiple mass ejections.
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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.

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Spectrum of Ring nebula
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Sirius The Dog Star
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Sirius is a binary star
Sirius A
Sirius B
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Which star is older in this binary system

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?

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
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• 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.

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White Dwarfs in M 4
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• 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.

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Cooling
Cut-off
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Why is there a cut-off in the white dwarf
population?

1. Cooler WD are impossible to detect
2. At a certain temperature, WD explode
3. The universe isnt 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 hasnt 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.

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The Death of High Mass Stars
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• 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 sT4(4pR2) T4 must be changing at the
  same rate as R2
• The star becomes a supergiant
  (luminosity class I star)

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The Death of High Mass Stars
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• 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.

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The Death of High Mass Stars
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• 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.

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The Death of High Mass Stars
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• 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.

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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.

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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!

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The final core and shells of a high mass star
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Fusing Iron does not release energy.