Our Star

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Our Star
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Why does the Sun shine?
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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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E mc2 Einstein, 1905
It is powered by NUCLEAR ENERGY!
Nuclear Potential Energy (core)
10 billion years
Luminosity
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Why does the Sun shine?

• Chemical Gravitational energy cant explain how
  Sun could shine for gt 25 million years.
• Sun must be stable, balancing gravity pulling in
  with forces pushing out from light and heat.

Chemical Energy?
Gravitational Energy?
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Why does the Sun shine?

• Gravitational equilibrium balance of forces in
  out
• Sun shines because gravitational equilibrium
  keeps core hot dense generate energy through
  nuclear fusion.

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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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What is the Suns structure?
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What is the Suns structure?

• From what we can see going outwards
• Photosphere
• Chromosphere
• Corona

These we CAN see measure with Visible-light,
UV, X-ray telescopes in orbit on Earth
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Photosphere
Radius 6.9 ? 108 m (109 times Earth) Mass
2 ? 1030 kg (300,000 Earths) Luminosity
3.8 ? 1026 watts
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• Photosphere
• Visible surface 6,000 K
• Seen in Visible Light
• Shows
• Dark limb
• Granules
• Sunspots!
• Absorption lines

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Granules seen in the Photosphere
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Granules seen in the Photosphere are the visible
top of a current of hot gas
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Sunspots seen in the Photosphere
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The Chromosphere seen during a total solar
eclipse

• Credit Chromosphere image courtesy Luc Viatour
  and Windows to the Universe, http//www.windows.uc
  ar.edu

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Chromosphere Middle layer of solar atmosphere
104105 K Seen in UV Light Shows emission
lines Features Prominences, Flares, Spicules
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Spicules seen in the Chromosphere (UV)
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Prominences seen in the Chromosphere
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Prominences seen in the Chromosphere
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Look carefully you can see Chabot!
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Corona Outermost layer of solar atmosphere 1
million K Seen in X-rays Shows emission lines
faint continuous spectrum
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The Corona Visible Light
The Corona X-ray Light
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Solar wind A flow of charged particles
(electrons, protons, some helium nuclei) from the
surface of the Sun Creates Aurora
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How do we know what is happening inside the Sun?
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What is the Suns structure?

• Going inwards to layers we cannot see
• Convection zone
• Radiation zone
• Core

These we cant see we model with computer
simulations, observations of surface features to
suggest structure, and observations of neutrinos
to hint at fusion reactions taking place in the
core.
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Convection zone Beneath Photosphere Creates
granules we see Energy transported upward by
rising hot gas
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Bright blobs on photosphere where hot gas reaches
the surface
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Convection (rising hot gas) takes energy to the
surface.
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Patterns of vibration on the surface tell us
about what the Sun is like inside.
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Radiation zone Energy transported upward by
Gamma Ray X-ray photons Determine extent based
on computer models
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Core Energy generated by nuclear fusion 15
million K Generates gamma rays Inner 10 of sun
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Data on solar vibrations agree with mathematical
models of solar interior.
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How does nuclear fusion occur in the Sun?
Fusion is the UNITING of light atomic nuclei into
heavier nuclei, releasing binding energy in the
form of gamma ray radiation and other particles
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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 temperatures enable 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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Hans Bethe

• Deduced Proton-Proton Sequence for Hydrogen
  Fusion to Helium inside Stars
• Predicted neutrinos to balance equations
• Nobel Prize in 1967 for contributing to discovery
  of neutrinos
• Active in Non-Nuclear Proliferation post WWII

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Hans Bethe

• The Temperature of the Sun. Web of Stories.
  YouTube http//www.webofstories.com/play/4536?oMS
  
• Hans Bethe, who calls himself the "H-bomb's
  midwife", reflects on Hiroshima. Web of Stories.
  YouTube. http//www.youtube.com/watch?vapgB_NR59s
  s
• Kalilialouise. Hans Bethe The Moral
  Responsibility of the Scientist NHD 2009 .
  YouTube. http//www.youtube.com/watch?vc7iJtT41yy
  8

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Cecelia Payne

• Deduced from Spectra Sun was primarily Hydrogen
• Hired as technical assistant to Harlow Shapley
• First Woman to Head Department at Harvard (Chair
  of Astronomy)

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Protonproton 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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Neutrinos created during fusion fly directly
through the Sun. Observations of these solar
neutrinos can tell us whats happening in the
core.
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Ray Davis Homestake Mine Experiment discovered
neutrinos but failed to find the predicted
number.
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Solar neutrino problem More recent observations
find the right number of neutrinos, but some have
changed form.
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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.

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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
Structure of the Sun
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What causes solar activity?
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Solar activity is like weather

• Sunspots
• Solar flares
• Solar prominences
• All are related to magnetic fields.

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Sunspots Are cooler than other parts of the
Suns surface (4,000 K) Are regions with strong
magnetic fields
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Zeeman Effect We can measure magnetic fields in
sunspots by observing the splitting of spectral
lines
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Charged particles spiral along magnetic field
lines.
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Loops of bright gas often connect sunspot pairs.
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Magnetic activity causes solar flares that send
bursts of X-rays and charged particles into space.
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Magnetic activity also causes solar prominences
that erupt high above the Suns surface.
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The corona appears bright in X-ray photos in
places where magnetic fields trap hot gas.
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How does solar activity affect humans?
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Coronal mass ejections send bursts of energetic
charged particles out through the solar system.
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Charged particles streaming from the Sun can
disrupt electrical power grids and disable
communications satellites.
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How does solar activity vary with time?
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The number of sunspots rises and falls in 11-year
cycles.
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The sunspot cycle has something to do with the
winding and twisting of the Suns magnetic field.