Jessie Horn: Chapter 10 Notes

Chapter 10: The Sun
Why does the Sun shine? Is it on fire? No. Is it contracting? No. It is powered by NUCLEAR ENERGY!!!

• Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years.
• The Sun shines because gravitational equilibrium keeps it core hot and dense enough to release energy through nuclear fusion.
• Gravitational equilibrium: Energy provided by fusion maintains the pressure.
• Energy Balance: Thermal energy released by fushion in core balances radiative energy lost from surface.
• Gravitational contraction...provided energy that heated the core as the Sun was forming. Contraction stopped when fusion began replacing the energy radiated into space.

What is the Sun's Structure?
    From what we can see going outwards...

• Photosphere: visible surface is approximately 6,000 K. Seen in VISIBLE light. Shows dark "limb," granules, sunspots, absorption lines.
• Chromosphere: Middle layer of solar atmosphere is approxamately 10^4-10^5 K. Seen in UV Light, shows emission lines, feautres prominences, flares, spicules.
• Corona: Outermost layer of solar atmosphere approximately 1 million K. Seen in X-rays, shows emission lines and faint continuous spectrum.
• Solar Wind: A flow of charged particles (electrons, protons, some helium nuclei) from the surface of te Sun. Creates Aurora.

How do we know what is happening inside the Sun?
        What is in the Sun's structure? Going inwards to layers we cannot see...

• Convection zone
• Radiation zone
• Core

*These we can't see-we model with computer simulations, observations of surface feautres to suggest structure, and observations of neutrinos to hint a fusion reactions taking place in the core.

• Convection zone: Beneat photosphere, creates granules we see, energy transported upward by rising hot gas.
• Convection (rising hot gas) takes energy to the surface.
• Patterns of vibration on the surface tells us about what the Sun is like inside.
• Radiation zone: Energy transported upward by Gamma Ray and X-ray photons, determine extent based on computer models.
• Core: Energy generated by nuclear fusion, approximately 15 million K, generates gamma rays, inner 10% of Sun.

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.
• Fission: Big nucleus splits into smaller pieces (Nuclear power plants)
• Fusion: Small nuclei stick together to make a bigger one (Sun, stars)
• High temperatures enable nuclear fusion to happen in the core.
• Sun releases energy by fusin four hydrogen nuclei into one helium nucleus.
• Solar Thermostat: Decline in core temperature causes fusion rate to drop, so core contracts and heats up. Rise in core temperature causes fusion rate to rise, so core expands and cools down

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

• Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons.
• Neutrinos created during fusion fly directly through the Sun.
• Observations of these solar neutrinos can tell us what's happening in the core.
• Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number. Early searches for solar neutrinos failed to find the predicted number. More recent observations find the right number of neutrinos, but some have changed form.

What causes solar activity?
Solar activity is like "weather"

• Sunspots
• Solar flares
• Solar prominences
• All are related to magnetic fields
• The stretching and twisting of magnetic field lines near the Sun's surface causes solar activity.
• Bursts of charged particles from the Sun can disrupt communications, satellites, and electrical power generation.

How does solar activity vary with time?

• Activity rises and falls in 11-year cycles.