Stellar Evolution

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Stellar Evolution

• Astrophysics Lesson 12

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Learning Objectives

• To know-
• How stars form from clouds of dust and gas.
• How main sequence stars evolve as they run out
  of hydrogen.
• How this evolution appears on the HR diagram.

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Homework

• Remember to bring the completed open book exam
  this Friday.

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Some terms

• Radiation pressure - the pressure exerted upon
  any surface exposed to electromagnetic radiation.
• Hydrogen burning actually refers to the
  fusion of hydrogen into helium not reacting with
  oxygen.

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Pillars of Creation
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Pillars of Creation
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Star Formation

• Stars are formed from great clouds of gas and
  dust, most of which is the remnants from previous
  supernovae.
• The denser clumps collapse slowly contract under
  the force of gravity.

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Protostars ? Hydrogen Fusion

• When the clumps get dense enough, the cloud
  fragments into regions called protostars that
  continue to contract and heat up.
• Eventually the temperature at the centre of the
  protostar reaches a few million degrees and
  hydrogen nuclei start to fuse together to form
  helium.

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Protostars in Orion I
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Protostars in Orion II
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Protostars in Orion III
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Observe this with naked eye
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On the Main Sequence

• The fusion of hydrogen releases enough energy to
  create enough radiation pressure to stop the
  gravitational collapse.
• The star has now reached the main sequence and
  will remain there while it fuses hydrogen to
  helium.
• Core hydrogen burning

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Leaving the Main Sequence

• Stars spend most of their lives as main sequence
  stars.
• As the star ages more and more helium builds up
  in the core.
• Eventually all the hydrogen is gone and you are
  left with a core of only helium.

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Shell Hydrogen Burning

• When the hydrogen in its core runs out, the
  outward radiation pressure stops, gravity wins
  and the core starts to contract.
• As the core contracts it heats up. This raises
  the temperature of hydrogen surrounding the core
  enough for it to fuse.
• This is shell hydrogen burning very low mass
  stars stop here.

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Red Giant

• The core continues to contract until it is hot
  and dense enough for helium to fuse into carbon
  and oxygen.
• ? Core helium burning.
• This releases a huge amount of energy which
  pushes the outer layers of the star outwards
  which then cool.
• ? Red Giant.

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Shell Helium Burning.

• When the helium runs out, the carbon-oxygen core
  contracts again ? shell helium burning.
• For stars with mass similar to the of the Sun,
  the carbon-oxygen core isnt hot enough for
  fusion.
• The core continues to contract until electrons
  exert enough pressure to stop it collapsing
  further.

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Ejecting the Outer Layers

• The helium shell becomes more and more unstable
  as the core collapses.
• This causes the star to pulsate and eject its
  outer layers into space
• ? planetary nebula.
• A hot dense solid core is left behind
• ? white dwarf.

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White Dwarf ? Black Dwarf

• Within a million years the nebula fades and the
  core will simply continue to cool and finally the
  star is said to be dead.
• This is the fate that awaits our Sun in about 5
  billion years.
• Animations

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Planetary Nebula I
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Planetary Nebula II
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Planetary Nebula III
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The Whole Story
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The HR Diagram Evolutionary Track for a 1 Mo star