Life Cycle of Stars

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Life Cycle of Stars
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Stars are born in Nebulae

• Vast clouds of gas and dust
• Composed mostly of hydrogen and helium
• Some cosmic event triggers the collapse of a
  portion of the nebula
• Gravity condenses the matter into a central area
• Heat and pressure build in the core
• Protostar is formed

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Eagle Nebula
Helix Nebula
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Protostar
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A Star is Born

• When fusion of hydrogen begins in the core, the
  protostar becomes a star
• High Mass Star a star whose mass is gt3 solar
  masses (gt3 times the mass of our sun)
• Low Mass Star A star whose mass is lt3 solar
  masses

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Nuclear Fusion
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Hertzsprung-Russell Diagram
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A Stars Balancing Act

• A star stays on the Main Sequence as long as
  there is a balance of inward and outward forces
• Inward Force Gravity
• Outward Force Heat Pressure generated by
• nuclear fusion

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Nuclear fusion
Star Equilibrium Balance of Forces
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Life Span Low Mass Star

• Hydrogen fusion in the core continues for
  billions of years, forming helium
• Once the hydrogen in the core is used up, the
  star begins to collapse and moves to the next
  stage in its life.
• Heat and pressure in the core cause the helium to
  fuses rapidly, causing a helium flash.
• Outer layers of the star begin to expand

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Final Stages Low Mass Star

• Outer layers expand and star becomes a Red Giant
• Eventually the outer layers disperse, leaving a
  very hot, dense core (maybe the size of Earth)
  called a White Dwarf

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Red Giant
Red Giant
White Dwarf
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Life Span High Mass Star

• Hydrogen fuses to helium in the core but is used
  up much faster may last only 100 million years
• Once the hydrogen in the core is used up, the
  star begins to fuse heavier elements.
• Heavier elements continue to fuse until iron
  (element 26) is formed.
• Iron cannot be fused in the core of a star
  fusion stops and gravity takes over, collapsing
  the star

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Final Stages High Mass Star

• With decreasing outward pressure, gravity causes
  the star to collapse.
• The collision of atoms in the core causes a
  Supernova explosion.
• All the elements heavier than iron are formed in
  a supernova explosion.
• Remnants of a supernova depends on the stars
  mass
• Neutron Star a dense star body made of neutrons
• Black Hole an object so dense even light cannot
  escape.

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Black Hole
High Mass Star
Supernova