How Is A Stars Energy Produced

1
How Is A Stars Energy Produced?

• Nuclear Fusion
• Combine multiple smaller nuclei into a single
  larger one

2
How Is A Stars Energy Produced?

• Nuclear Fusion
• Very hot in stars core
• So ions are moving very fast
• If collide strongly enough, can stick together
• Have to overcome repulsion between two positively
  charged ions

3
How Is A Stars Energy Produced?

• Must get close enough for nuclear strong force to
  overcome repulsion

4
Nuclear Fusion

• In Sun, fuse 4 hydrogen (1 proton each) into 1
  Helium (2 protons 2 neutrons)

5
Nuclear Fusion

• Multi-step process proton-proton chain

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Nuclear Fusion

• In Sun, fuse 4 hydrogen (1 proton each) into 1
  Helium (2 protons 2 neutrons)
• Why does this produce energy?
• Mass of 1 Helium is about 0.7 LESS than mass of
  four protons
• Emc2 that tiny mass gets turned into a lot of
  energy

7
Nuclear Fusion

• Emc2 that tiny mass gets turned into a lot of
  energy
• Sun converts about 600 million tons of hydrogen
  into 596 million tons of helium every second
• 4 million tons (4109 kg) of matter is turned
  into energy each second !!!
• Still negligible fraction of Suns mass
• (21030 kg )
• QUESTION At this rate, how long would it take
  the Sun to use up all of its mass?

8
Getting the Energy Out

• Nuclear fusion only occurs in core
• Need to get the produced energy out
• Most of energy in photons (light)
• Takes a long time to get out due to lots of
  interactions
• Near core, a photon can only go
• before interacting with other stuff

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Getting the Energy Out

• So path out in radiation zone is a long zigzag,
  called random walk

10
Getting the Energy Out

• Top of radiation zone aboout 2,000,000K
• Photons can be absorbed
• Get convection - blobs of hot gas move
• outward, expand and cool, sink down, repeat

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Getting the Energy Out

• Convection zone

12
Convection at Suns Surface
Actual movie of convective cells at solar
surface Size of movie area 10000 km on a side
13
General Properties of Stars

• Weve seen one star up close - the Sun
• We observe other stars in our galaxy, and see
  them for only a tiny fraction of their lives - a
  snapshot
• From millions of stellar snapshots, astronomers
  figure out how stars work, live and die

14
General Properties of Stars

• Three basic properties determine the life of a
  star
• 1. Luminosity
• 2. Surface temperature
• 3. Mass
• How do we measure these?

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Luminosity and Brightness

• Luminosity Total power emitted by star into
  space
• What are units of power?

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Luminosity

• Recall Stefan-Boltzmann Law
• Each square meter of a hotter objects surface
  emits more light at all wavelengths
• J Emitted power per unit surface area
• ? a constant T Temperature
• To get the TOTAL power (luminosity) for a star,
  multiply by stars area

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Luminosity and Brightness

• Brightness How bright a star appears to us
• Depends on stars luminosity and distance from us
• Brightness is the power received from a star per
  unit area at the receiver

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Luminosity and Brightness
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How does Brightness Change with Distance?

• Follows an inverse square law
• A star twice as far away from us appears 4 times
  dimmer
• What other physical process follows a similar law?

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How does Brightness Change with Distance?

• Star emits all of its power into a spherical area
• The same power must go through ever larger
  spherical areas as you get further from star
• Area of a sphere is.....?

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Brightness and Distance

• Inverse square law for light
• brightness

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Brightness and Distance

• Inverse square law for light

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Brightness and Distance

• Measure the brightness
• IF we know Luminosity, can get distance
• IF we know distance, can get Luminosity