Star Stuff

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Chapter 12

• Star Stuff

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Evolution of Low-Mass Stars

• 1. The Sun began its life like all stars as an
  intersteller cloud.
• 2. This cloud collapses due to gravity into a
  dense core.
• 3. In about a million years a small, hot, dense
  core called a protostar forms.

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Rotating Cloud Fragment
Jets of high speed gas
Proto-stellar disk
Protostellar Wind
Artists Conception of Star Birth
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Schematic Illustration of protostellar disk-jet
structure
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• 4. When the temperature reaches 10 million
  Kelvin in the core, fusion begins and transforms
  the protostar into a zero age main-sequence
  star.
• 5. Low mass stars like the Sun remain on the
  main-sequence for about 10 billion years.
  Massive stars stay on the main-sequence for about
  1 billion years.

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Life track of a 1M? star from protostar to
main-sequence star
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Life tracks from protostar to main sequence for
stars of different masses.
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• 6. Hydrogen fusion begins in a shell around the
  core and the star expands into a Red Giant.
• 7. After most of the hydrogen is fused into
  helium, helium fusion begins in an event called
  the Helium Flash.
• 8. Stars can then become unstable and turn into
  pulsating stars like RR Lyrae Variables or
  Cephied Variables.

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After a star ends its main-sequence life, its
inert helium core contracts while a hydrogen
shell begins fusion at a higher rate. This forces
the stars outer layers to expand outward.
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Core Structure of helium burning star.
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The onset of helium fusion. When helium fusion
begins, the stars surface shrinks and heats. The
stars life track therefore moves downward and
left on the H-R diagram
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• 9. As a star burns helium into carbon the
  radiation pressure pushes the star's outer
  atmosphere away from the core creating a
  Planetary Nebula.
• Electron degeneracy pressure halts any further
  collapse. Fusion process in the core stops.
• 10. This leaves an exposed core called a White
  Dwarf. These have about the same diameter as the
  Earth.

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The life track of a 1M? star from
main-sequence to white dwarf.
Core structure at key stages
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Gaseous shells surround the remnant carbon core
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Evolution of High-Mass Stars

• 1 to 5. Same as before
• intersteller cloud ? dense core ?
• ? protostar ? zero-age main-sequence star ?
  main-sequence star
• 6. When a high-mass star exhausts the hydrogen
  fuel in its core the star leaves the main
  sequence and begins to burn helium.

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• 7. The star becomes a Red Supergiant after
  millions of years of helium fusion.
• 8. When helium is depleted, fusion of heavier
  elements begins. This process is called
  nucleosynthesis.
• H ? He ? C ? O ? Si ? Fe

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Stellar Nucleosynthesis
Evolutionary Time Scales for a 15 M? Star
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The Multiple Layers Of Nuclear Burning In The
Core Of A High Mass Star During Its Final Days
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Life Tracks On The H-R Diagram From Main-sequence
Star To Red Supergiant For Selected High Mass
Stars
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• 9. Fusion stops with iron (Fe) and a star with
  an iron core is out of fuel.
• Reason Iron atoms cannot fuse and release
  energy.
• 10. The core collapses due to reduced pressure
  converting the iron core into mostly neutrons.
• 11. The core pressure then surges and lifts the
  outer layers from the star in a titanic explosion
  - a supernova!

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Iron Core degenerates into a neutron core
(neutron degeneracy). Electrons and protons
combine to form neutrons with the release of
neutrinos
Average mass per nuclear particle from hydrogen
to iron decreases and then increases for atomic
masses greater than iron.
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Origin Of The Elements- Stellar
Nucleosynthesis Observed Relative Abundances Of
Elements In The Galaxy In Comparison To The
Abundance Of Hydrogen.
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Changing H-R diagram of a hypothetical star
cluster.
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The Double Cluster h and ? Persei
Only 10 million years old
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Glodular cluster 47 Tucanae.
11 billion years old
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Evolution of a Binary Star System
Each star can be pictured as being surrounded by
a zone of influence or Roche lobe.
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What can happen?
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What can happen?
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End of Chapter