Life Cycle of a Star

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Life Cycle of a Star

• Objectives
• how stars are created
• how they live
• what happens to them at the end of their lives

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How Stars Are Created

• cloud of hydrogen is compressed to the right
  density
• force of gravity is more powerful than the
  tendency for it to disperse
• cloud then irresistibly collapses
• resultant heating of its interior should
  eventually ignite the process of thermonuclear
  fusion thought to power stars

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Eagle Nebula - which astronomers think is the
remnants of an exploded star
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A Typical Star
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Equilibrium Life Goal of a Star

• This 5-step process works like this
• Nuclear fusion. Gravity gas pressure
  (equilibrium)
• Out of fuel.
• Fusion stops, temperature drops.
• Core contracts (gravity pulling atoms in).
• Increased temperature (more atoms, more
  collisions) and density in the core reinitiates
  nuclear fusion, equilibrium is achieved, and the
  cycle begins again at step 1.

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Proton-Proton Fusion
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Herzsprung-Russell (HR) Diagram
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Death of a Star

• A low mass star becomes a white dwarf
• Medium-mass stars become neutron stars
• The largest mass stars may become black holes

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White Dwarf
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Artists Conception of a Black Hole
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Summary

• Stars form from mostly hydrogen gas gravity
• 2 forces in equilibrium gravity (inward)
  nuclear pressure (outward)
• Once proton fusion stops, star begins to die
• Size determines death fate of the star