Chapter 16: The Sun - Summary Notes

Physical Properties of the Sun

• Radius: $700,000$ km, Mass: $2.0 × 10^{30}$ kg, Density: $1400$ kg/m$^3$.

• Differential rotation with a period of about a month.

• Surface temperature: $5800$ K, Core temperature: $15 × 10^6$ K.

• Apparent surface is the photosphere.

• Escape velocity: $617$ Km/s.

• The sun is a fusion reactor held together by gravity.

• Chemical composition: 70.5% Hydrogen, 27.5% Helium, 2% other elements.

• Luminosity: $L_{sun} = 3.9 × 10^{26}$ Watts.

Solar Interior

• Core: Nuclear fusion occurs.

• Mathematical models help understand the interior by comparing model outcomes with experiments like solar oscillation and neutrino experiments.

• In equilibrium, inward gravity balances outward pressure.

• Complex vibration patterns are observed via Doppler shifts of solar spectral lines.

• Hydrogen content increases from the center to the surface.

• Energy transport occurs via radiation in the radiation zone and convection in the convection zone.

• Photons take ~10 million years to escape the sun's surface, while neutrinos take only 2 seconds.

The Sun’s Atmosphere

• Spectral analysis identifies elements in the chromosphere and photosphere.

• Spectral lines form when light is absorbed before escaping the Sun.

• The chromosphere is above the photosphere and is cooler.

• The corona is hotter than lower layers, likely due to electromagnetic interactions.

Solar Magnetism

• Sunspots are cooler areas due to intense magnetism.

• Sunspots are linked by magnetic field lines and originate from the Sun's differential rotation.

• The sunspot cycle is 11 years, with polarity switching every 11 years (22-year cycle).

The Active Sun

• Areas around sunspots are active, leading to solar prominences (large gas sheets) and solar flares (large explosions).

• Coronal mass ejections involve large bubbles detaching from the Sun.

• Solar wind escapes through coronal holes.

• The corona varies with the sunspot cycle.

The Heart of the Sun

• Nuclear fusion (proton-proton chain) is the energy source: $4(1H) → 4He + energy + 2 neutrinos$.

• Energy is released as gamma rays.

• Mass-energy conversion follows $E = mc^2$.

• The sun converts about 600 million tons of hydrogen per second.

Solar Neutrinos

• Neutrinos provide a direct view of the core.

• Detectors use large volumes to observe single interaction events.

• Neutrino observations confirm standard solar model predictions after accounting for neutrino type changes.