Lecture 10 the Sun

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Reminder
Midterm1 is Tuesday Feb 12 in class hour Material
covered chapters 1, 3, 4, 5, and 6 40 multiple
choice questions. Review for Midterm 1 second
half of lecture 10 Thursday Feb 7. will cover
questions from midterm 1 in 2006 and 2007 which
are available under exams on the course web
site. Class enrollment is 410. Accordingly,
individual help for midterm 1 is available from
Ian and the lab TAs. You may see any lab TA not
just your own. See next slide for their office
hours. You may also see TAs outside office hours
by requesting an appointment.
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TA Contact Information
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The Sun Our Star (Chapter 14)
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14.1 A Closer Look at the Sun

• Why was the Suns energy source a major mystery?
• Why does the Sun shine?
• What is the Suns structure?

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Why was the Suns energy source a major mystery?
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Is it on FIRE?
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Is it on FIRE?
Chemical Energy Content
10,000 years
Luminosity
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Is it on FIRE? NO!
Chemical Energy Content
10,000 years
Luminosity
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Is it CONTRACTING?
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Is it CONTRACTING?
Gravitational Potential Energy
25 million years
Luminosity
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Is it CONTRACTING? NO!
Gravitational Potential Energy
25 million years
Luminosity
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Why does the Sun shine?
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- Einstein, 1905
It can be powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
10 billion years
Luminosity
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Weight of upper layers compresses lower layers
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Gravitational equilibrium Energy provided by
fusion maintains the pressure
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Gravitational contraction Provided energy that
heated core as Sun was forming Contraction
stopped when fusion began
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What is the Suns structure?
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Radius 6.9 x 108 m (109 times Earth) Mass
2 x 1030 kg (300,000 Earths) Luminosity
3.8 x 1026 watts
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Solar wind A flow of charged particles from the
surface of the Sun
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Corona Outermost layer of solar atmosphere 1
million K
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Chromosphere Middle layer of solar atmosphere
104 - 105 K
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Photosphere Visible surface of Sun 6,000 K
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Convection Zone Energy transported upward by
rising hot gas
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Radiation Zone Energy transported upward by
photons
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Core Energy generated by nuclear fusion 15
million K
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Summary

• Why was the Suns energy source a major mystery?
• Chemical and gravitational energy sources could
  not explain how the Sun could sustain its
  luminosity for more than about 25 million years
• Why does the Sun shine?
• The Sun shines because gravitational equilibrium
  keeps its core hot and dense enough to release
  energy through nuclear fusion.

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Summary

• What is the Suns structure?
• From inside out, the layers are
• Core
• Radiation Zone
• Convection Zone
• Photosphere
• Chromosphere
• Corona

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14.2 Nuclear Fusion in the Sun

• How does nuclear fusion occur in the Sun?
• How does the energy from fusion get out of the
  Sun?
• How do we know what is happening inside the Sun?

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How does nuclear fusion occur in the Sun?
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Fusion Small nuclei stick together to make a
bigger one (Sun, stars)
Fission Big nucleus splits into smaller
pieces (Nuclear power plants)
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High temperature enables nuclear fusion to happen
in the core
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Sun releases energy by fusing four hydrogen
nuclei into one helium nucleus
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Proton-proton chain is how hydrogen fuses into
helium in Sun
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IN 4 protons OUT 4He nucleus 2 gamma rays 2
positrons 2 neutrinos Total mass is 0.7 lower
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Thought Question

• What would happen inside the Sun if a slight rise
  in core temperature led to a rapid rise in fusion
  energy?
• A. The core would expand and heat up slightly
• B. The core would expand and cool
• C. The Sun would blow up like a hydrogen bomb

Solar thermostat keeps burning rate steady
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Solar Thermostat
Rise in core temperature causes fusion rate to
rise, so core expands and cools down
Decline in core temperature causes fusion rate to
drop, so core contracts and heats up
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How does the energy from fusion get out of the
Sun?
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Energy gradually leaks out of radiation zone in
form of randomly bouncing photons