Stars Part Two:

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Stars Part Two

• Stellar Evolution

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Overview of the life of a star

• Formation of protostar from a cloud of mostly
  Hydrogen gas.
• Main sequence star
• Red giant
• White dwarf or
• Supernova -
• Neutron star or
• Black hole

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Formation of protostar

1. Gaseous clouds contract under their own gravity.
2. Regional areas of initial high density accrete
   more and more gas.
3. Gravitational potential turns to heat.
4. Heat and pressure start fusion.

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Birth of a star IP Demo Star_Birth.ip
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Birth of a star

1. As the cloud of gas and dust collapses, a small
   rotation becomes big (Conservation of angular
   momentum)
2. The rapidly spinning protostar often needs to get
   rid of angular momentum before it can start
   fusion.
3. The magnetic field channels rapidly spinning
   material out of polar jets

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Birth of a star

1. Eventually, the spin slows enough to allow fusion
2. The newly born star often blows away the nebula
   it came from with its radiation.
3. The remaining material (still spinning) stays
   around the newly formed star in an accretion disk.

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Birth of a solar system
Accretion Disk
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The Solar System
National Geographic Magazine
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The Inner Planets
Mercury Venus Earth Mars
Asteroids

• Close together (Relatively)
• Terrestrial (made of rock like Earth)

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The Outer Planets
Jupiter Saturn Uranus Neptune
Pluto

• Spread out (Relatively)
• Gas giants

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Life on the Main Sequence

• Energy comes primarily from the Proton-Proton
  cycle
• 1H 1H 2H e ?
• 1H 2H 3He ?
• 3He 3He 4He 1H 1H
• (requires heat and pressure)

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Thermal Agitation balances the tendency of
gravity to crush a star
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1. The rate of burn depends roughly on the cube of
   the mass
2. Even though larger stars have more fuel, they
   burn the fuel they have at a much faster rate.
3. Big stars are Brief, Bright, and Blue
4. Diminutive stars are Durable, Dim and reD

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From Robert Garfinkles Star Hopping
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From Jay Pasachoffs Contemporary Astronomy
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A Star trying to be too big
From Jay Pasachoffs Contemporary Astronomy
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4He accumulates in the core of the star
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The death of a star

1. When most of the Hydrogen in the core has been
   used up, leaving a Helium core, the star cools
   down. (The Helium displaces the fusing Hydrogen)
2. Heat energy no longer balances gravity.
3. Gravity collapses the He core.
4. The heat generated by the implosion of the core
   spurs more fusion of the remaining Hydrogen.
5. The outer envelope of the star expands, and
   cools. It is now a Red Giant

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Collapse of the He Core
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Turning into a Red Giant

1. A star the size of the sun would expand to the
   orbit of Venus, or maybe the earth.
2. As a red giant, the star blows off a great deal
   of its mass into space.
3. A star 8 time as massive as the sun will have a
   residual mass of 1 or 2 times the mass of the sun
   after its red giant stage.
4. Stunning image from the Hubble

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Helium Fusion

• When the core gets hot and dense enough, He
  begins to fuse
• 4He 4He 8Be ?
• 4He 8Be 12C ?
• The star contracts slightly and heats up, moving
  along the horizontal branch
• Before the He is used up these reactions also
  occur
• 4He 12C 16O ? (mainly)
• 4He 16O 20Ne ?
• 4He 20Ne 24Mg ?

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Helium Fusion
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Carbon Fusion

1. When most of the Helium in the core has been used
   up, leaving a Carbon core, the star cools down.
2. Heat energy no longer balances gravity.
3. Gravity collapses the Carbon core.
4. The heat generated by the implosion of the core
   spurs more fusion of the remaining Helium.
5. The outer layer of the star expands, and cools
   briefly.

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Collapse of the Carbon Core
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Carbon Fusion

• If the remaining part of the star is more than .7
  times the mass of the sun, the core gets hot and
  dense enough to start Carbon fusion
• 12C 12C 24Mg ?
• 16O 16O 28Si 4He
• Nuclei as heavy as 56Fe and 56Ni can be created
  if the star core is hot enough.
• Nucleosynthesis and fusion stop with 56Fe and
  56Ni as larger nuclei would require the input of
  energy, because of binding energy

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From Douglas Giancolis Physics
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So far
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How do we know all this?
By observing Globular clusters
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How do we know all this?

• Globular clusters are thousands of stars that all
  formed at more or less the same time.
• Globular clusters are much smaller than galaxies.
• Galaxies create stars in an on-going process.
• The stars in a globular cluster accrete suddenly
  and nearly simultaneously.

By observing Globular clusters
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Planetary Nebulas

1. Some stars with mass 1-7 times the suns mass.
2. While the star is fusing carbon, it shrinks and
   gets hotter.
3. The material blown off by the red giant phase is
   overtaken by the material blown off by the carbon
   core collapse.
4. The rapidly spinning core creates a strong
   magnetic field that channels the expulsion of the
   outer envelope.
5. Some planetary cores might have a companion.

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If the residual mass of the star is less than 1.4
times the current mass of the sun, our story ends
here. A star with the mass of the sun becomes a
White dwarf about the size of the earth. The
Pauli exclusion principle prevents the star from
collapsing any further. It gradually runs out of
Carbon fuel, getting dimmer and dimmer, until it
becomes a black dwarf.
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If the residual mass of the star is less than 1.4
times the current mass of the sun, our story ends
here. A star with the mass of the sun becomes a
White dwarf about the size of the earth. The
Pauli exclusion principle prevents the star from
collapsing any further. It gradually runs out of
Carbon fuel, getting dimmer and dimmer, until it
becomes a black dwarf.
If the residual mass of the star is less than 1.4
times the current mass of the sun, our story ends
here. A star with the mass of the sun becomes a
White dwarf about the size of the earth. The
Pauli exclusion principle prevents the star from
collapsing any further. It gradually runs out of
Carbon fuel, getting dimmer and dimmer, until it
becomes a black dwarf.
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Now for something completely different.
Wanna hear a scary story?
Do not adjust your television set
We are on a special schedule
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Life After the Main Sequence Starring Marcela
Supernova Joe Neutron Star Bob Quasar Mary
Pulsar Freda Black Hole
Music by Warped Space Time
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If the mass of the star is greater than 1.4 times
the mass of the sun. (This is called the
Chandrasekhar limit) it dont care about no Pauli
exclusion principle. When the Carbon Fusion
fires burn down, gravity crushes the star. The
collapse of the star releases an incredible
amount of energy. The star becomes a supernova,
increasing in brightness by billions of times
for a few days, and then dies out.
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The terrific energy released by the collapse of
the star creates elements heavier than Iron, and
forces electrons and protons to combine creating
neutrons. Dogs become cats. Republicans support
campaign finance reform. Democrats vote for tax
cuts. In February of 1987, a supernova occurred
in the Large Magellenic Cloud, 170,000 ly from
Earth. It was briefly visible to the naked
eye. (Assuming your eye was naked in Australia)
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Neutron Stars

1. The remnant of the supernova is composed almost
   entirely of neutrons.
2. White Dwarfs are the size of planets.
3. Neutron stars are the size of towns.
4. Some Neutron stars spin a thousand times a
   second.
5. The pressure is so high in the core atomic nuclei
   cannot exist.
6. The outer envelope is about a mile thick - a
   crust of nuclei and electrons.
7. The core is a super-fluid.

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1. In 1967, Antony Hewish of Cambridge University in
   England was studying the scintillation of radio
   sources due to the solar wind.
2. A graduate student named Jocelyn Bell Burnell
   discovered a strong night time source of
   twinkling.
3. Its location was fixed with respect to the stars.

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Pulsars

1. Pulsars emit pulses some as short as 1/40th of a
   second.
2. There are many things they could not be.
3. The only thing small enough, and rotating fast
   enough was a neutron star

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From Jay Pasachoffs Contemporary Astronomy
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Pulsars Movies Real photos from hubble Animation
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Black Holes

1. If the mass of the neutron star is bigger than
   about 2 or 3 solar masses, it dont care about no
   neutron exclusion principle.
2. Gravity collapses the neutron star even further.
3. The star becomes a black hole - an object from
   which even light cannot escape.
4. Light is really fast.
5. The curvature of space-time becomes infinite.
6. General relativity doesnt work.
7. Um we dont yet have a quantum theory of gravity.

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Black Holes

1. Black holes actually do radiate energy from the
   event horizon due to the Heisenberg uncertainty
   principle.
2. When stars orbit a black hole, we can see their
   orbit, but not the black hole. We can infer the
   mass from the mass of the star and its orbit.
3. The Andromeda galaxy has stars orbiting a dark
   object that is 30 to 70 million times the mass of
   the sun.

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Picture of a Black Hole
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Quasars (Quasi-stellar radio source)

1. Massively bright.
2. Intense radio source.
3. Red shifted radiation.
4. Black holes eating matter.
5. Usually located in the centers of galaxies

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Quasars

1. In falling material forms an accretion disk.
2. Quasars are ravenous beasts.
3. The black holes magnetic field pumps energy into
   the accretion disk.
4. The accretion disk gets hot.
5. The accretion disk has tornadoes that create jets
6. Predictions
7. Old bright Quasars are rare, young ones common
8. Recently disturbed galaxies should have bright
   quasars.

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