Classification of Stars

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Classification of Stars
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H-R Diagram pg. 554
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H-R Diagram

• Plots surface temps of stars against their
  absolute magnitudes.
• As surface temp increases, brightness increases.

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Main-sequence stars

• Most of the visible stars
• Main sequence stars range from hot, bright stars
  to cool, dim stars

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Giants

• Very large
• Cooler bright
• Brightness due to large size

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White Dwarfs

• Small
• Very hot dim
• Dim because of small size

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Stellar Evolution
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Begins as a nebula

• Cloud of gas dust
• Composed mostly of hydrogen helium

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Stellar Evolution

• Particles in the nebula become compressed
• The matter accumulates into a sphere
• Gravity becomes stronger as the sphere of matter
  grows.
• As a result, the sphere flattens into a disk
  (protostar).

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Stellar Evolution

• As gravity continues to pull inward
• (1) matter crashes into the protostar
• (2) pressure rises within
• Both cause an increase in temp

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Stellar Evolution

• A star is created when the temp reaches 10
  million C nuclear fusion begins.

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Nuclear fusion

• Small atomic nuclei combine to form larger atomic
  nuclei
• During this process, energy is released

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Main Sequence Stars

• Longest stage
• Length depends on mass luminosity
• Suns main sequence stage is 10 b.y.

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Main Sequence Stars

• Energy is generated in the core as hydrogen atoms
  fuse into helium atoms.
• Stable in size because gravity pulling inward
  balances the energy going outward.

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Giants Supergiants

• Less fusion less energy being released
• No more balance, gravity is stronger
• Star begins to collapse

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Giants Supergiants

• The temp rises as the core shrinks (increased
  pressure).
• This causes helium to begin fusing into carbon

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Giants Supergiants

• Thus, more energy is released to counteract
  gravity the star expands.
• As it expands, it cools.

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White Dwarfs

• Final stage
• There is no energy for fusion to take place
• Star loses outer gases (planetary nebula) core
  is exposed

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White Dwarfs

• Gravity pulls remaining matter to inward
• This contraction forms a hot, dense core

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White Dwarfs

• A white dwarf that quietly cools becomes a black
  dwarf
• Some white dwarfs become active again may explode.

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Novas

• Temporary brightness
• Occurs when energy is released during cooling.
• Most likely occur when white dwarfs pull in gases
  from nearby stars pressure builds.

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Supernovas

• Explosion that blows the star apart.
• Occur in larger stars compared to those that
  novas occur in (more energy/pressure).

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Nuclear Fusion

• Gravity causes the star to collapse, causing
  higher temp pressure that result in fusion
• The nuclear fusion at this stage involves heavier
  elements.
• As fusion continues, the core of the star will
  become entirely iron.

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Supernova

• The iron core will eventually collapse under
  gravity, causing the star to explode.
• The energy released amount of energy radiated
  by the star during its entire lifetime.

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After a supernova

• The core may contract into small, dense pieces
  neutron star.
• Or produce a black hole

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Black holes

• Leftovers from a supernova that are so big as
  they contract, a hole is left in space.
• The gravity of black holes is so great that even
  light cannot escape.