Chapter 10 Notes: Olivia Ward

• Solar neutrino problem: Discrepancies in the measurements of actual solar neutrino types and what the Sun's interior model predict.
• Former standard model: Neutrinos should have been mass-less according to the then-accepted theory; this means that the type of neutrino would be fixed when it was produced. The Sun should emit only electron neutrinos as they are produced by H-He fusion.
• Observation: Only 1/3 to 1/2 of predicted number of electron neutrinos were detected; neutrino oscillation explains the difference but requires neutrinos to have mass.
• Resolution: Neutrinos have mass and can change type.

Our Star
10.1 A Closer Look at the Sun
Why does the Sun shine?

• Is it on fire? No.
  
  • Chemical Energy Content / Luminosity ≈ 10, 000 years
• It is contracting? No.
  
  • Gravitational Potential Energy / Luminosity ≈ 25 million years
• Is it powered by nuclear energy? Yes. (core)
  
  • Nuclear Potential Energy / Luminosity ≈ 10 billion years
• Gravitational Equilibrium: Gravity pulling in balances pressure pushing out.
  
• Energy Balance: Thermal energy released by fusion in core balances radiative energy lost from surface.
  
  • The outward push of pressure precisely balances the inward pull of gravity.
  • Pressure is greatest deep in the Sun where the overlying weight is great.
• Gravitational Contraction: Provided energy that heated the core as the Sun was forming
  
  • Contraction stopped when fusion started replacing the energy radiated into space.

What is the Sun's structure?

•  Radius: 6.9 X 10^8 m (109 times Earth)
  
• Mass: 2 X 10^30 kg (300,000 times Earth)
  
• Luminosity: 3.8 X 10^26 watts
  
• Solar Wind: The stream of charged particles that are continually blown outward in all directions form the sun. (Creates Aurora)
• Corona: Outermost layer of solar atmosphere,
  
  • ≈ 1 million k
• Chromosphere: Middle layer of solar atmosphere
  
  • ≈ 10^4 - 10^5 k
• Photosphere: Visible surface of the Sun
  
  • ≈ 600 k
• Convection Zone: Energy transported upward by rising hot gas
• Radiation Zone: Energy transported upward by protons
• Core: Energy generated by nuclear fusion
  
  • ≈ 15 million k

10.2 Nuclear Fusion in the Sun
How does nuclear fusion occur in the Sun?

• Fission: Big nucleus splits into smaller pieces (nuclear power plants)
  
• Fusion: Small nuclei stick together to make a bigger one (Sun and stars)
  • High temperatures enable nuclear fusion to happen in the core.
    
    • At low speeds, electromagnetic repulsion prevents the collision of nuclei.
    • At high speeds, nuclei come close enough to form together.
    • The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.
  • Proton - Proton chain is how hydrogen fuses into helium in the Sun.
    
    • In: 4 protons
    • Out: 4 He nucleus, 2 gamma grays, 2 positrons, 2 neutrinos
    • Total mass: 0.7% lower
• Hydrogen Fusion by the Proton - Proton Chain
  
  • Step 1: Two protons fuse to make a deuterium nucleus (1 proton and 1 neutron). This step occurs twice in the overall reaction.
  • Step 2: The deuterium nucleus and a proton fuse to make a nucleus of helium-3 (2 protons and 1 neutron). This step also occurs twice in the overall reaction.
  • Step 3: Two helium-3 nuclei fuse to form helium-4 (2 protons, 2 neutrons), releasing two excess protons in the process.
• Solar Thermostat
  
  • Decline in core temperatures causes fusion rate to drop, so core contracts and heats up.
  • Rise in core temperatures causes fusion rate to rise, so core expands and cools down.

How does the energy from fusion get out of the Sun?

• Convection: Takes energy to the surface (rising hot gas)

How do we know what's happening inside the Sun?

• We learn about the inside of the Sun by...
  
  • Making mathematical models
  • Observing solar vibrations
  • Observing solar neutrons
• Patterns of vibration on the surface tell us about what the Sun is like inside.
  • Data on solar vibrations agree with mathematical models of solar interior
• Neutrinos created during fusion fly directly through the Sun.
  
  • Observations of solar neutrinos can tell us what's happening in the core.
  • Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted numbers.
    
    • More recent observations find the right numbers of neutrinos but some have changed forms.

10.3 The Sun-Earth Connection
What causes solar activity?

• Solar activity is like 'weather' on Earth.
  • Sunspots, star flares, solar prominences are all related to magnetic fields.
• Sunspots: Cooler than other parts of the Sun's surface (4,000 K)
  
  • Much smaller than the 8 major planets
  • Not a gas giant like the outer planets
  • Has very elliptical inclined orbit
  • Pluto has more in common with comets than the 8 major planets.
  • 2006: Pluto was named a dwarf planet.
• Zeeman Effect: We can measure magnetic fields in sunspots by observing the splitting of spectral lines.
  
• Magnetic activity causes solar flares that send bursts of x-rays and charged particles into space.
  • Magnetic activity also cause solar prominences that erupt high about the Sun's surface.
• The Corona appears bright in x-ray photos in places where magnetic fields trap hot gas.
  
  • Coronal mass ejections send bursts of energetic charged particles out through the solar system.
• Charged particles streaming from the Sun can disrupt electrical power grids and disable communications satellites.
  

How does solar activity vary with time?

• The number of sunspots rise and fall in 11-year cycles.
  
  • The sunspot cycle is in relation to the winding and twisting of the Sun's magnetic field.