Stellar Evolution

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Stellar Evolution

• Lab 5

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What is a Star?

• The basic difference between a star and a planet
  is that
• a star emits light produced in its interior by
  nuclear 'burning
• a planet only shines by reflected light

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The Hertsprung-Russell Diagram

• In 1912, Danish astronomer Ejnar Hertzsprung and
  American astronomer Henry Russell independently
  graphed the luminosity vs. temperatures for
  thousands of stars and found a surprising
  relationship
• The Hertzsprung-Russel (H-R) diagram shows the
  evolution of stars based on their characteristics
  of surface temperature, luminosity and mass

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H-R Diagram and Main Sequence Stars

• The H-R Diagram shows that most of the stars lie
  along a smooth diagonal curve called the main
  sequence where
• Hot, luminous stars appear on the upper left
• Cool, dim stars in the lower right
• 90 of the stars fall on the main sequence

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Off Main Sequence Stars

• Off the main sequence, there are
• cool, bright stars in the upper right
• hot, dim stars in the lower left

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Main Sequence Stars

• A large number of stars are
• chemically homogeneous
• burn hydrogen to helium in their cores
• emit luminosity inversely proportional to the
  4th power of their Temp
• emit luminosity directly proportional to the
  4th power of their mass
• These stars are called main-sequence stars

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Luminosity vs radius

• the radius of the stars increases as proceed
  bottom left diagonally to top right
• Sirius B 0.01 solar radius
• Sun 1 solar radius
• Spica 10 solar radii
• Rigel 100 solar radii
• Betelgeuse 1000 solar radii

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Mass vs luminosity

• stars along the main sequence vary from the
  highest (30 solar masses) at the top left to the
  lowest ( 0.1 solar mass) at the bottom right
• our Sun is an average star

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Classes of Stars by Luminosity

• The H-R diagram summarizes the types of stars in
  the universe
• Class Description Familiar Examples
• Ia Bright Supergiants Rigel, Betelgeuse
• Ib Supergiants Polaris (the North star),
  Antares
• II Bright Giants Mintaka (delta Orionis)
• III Giants Arcturus, Capella
• IV Sub-giants Altair, Achenrar
• V Main sequence Sun, Sirius
• not classified White dwarfs Sirius B, Procyon B
• White dwarfs stars are not classified because
  their stellar spectra are different from most
  other stars

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Life on the Main Sequence

• Stars on the main sequence burn by fusing H into
  He
• Large stars tend to have higher core temperatures
  than smaller stars, so they burn the hydrogen
  fuel in the core quickly and have shorter
  lifetimes

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HYDROGEN!!!!!!

• Small stars burn H more slowly.
• The length of time a star spends on the main
  sequence depends upon how quickly the hydrogen
  gets used up
• Once the hydrogen in the core is gone the mass of
  the star determines what happens next
• The Sun is halfway through its life of 10
  billion years

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Limits Shown on H-R

• H-R shows a definite upper limit for Main
  Sequence stars, all stars more massive than this
  will have ended their hydrogen burning existence
  already.
• Maximum mass for a Main Sequence is 60 times
  the Solar Mass
• Minimum mass of a star is 0.1 of a Solar Mass,
  the mass required to produce nuclear reactions in
  the core.
• The surface temperature of stars varies from 2000
  to 35,000 degrees
• Smaller stars are much more numerous than other
  types of stars.

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Explanation
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Another Version of the H-R
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Yet Another Version
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Interrelationships