Ch.14 The Sun

Hydrostatic equilibrium

Outward pressure force balances inward gravity in a stable star.

What supports Sun against gravity?

Gas pressure produced by hot, dense material.

Gas pressure dependence

Increases with temperature and particle density.

Sun's primary energy source

Nuclear fusion in its core.

Hydrogen fusion

Hydrogen nuclei ultimately form helium and release energy.

Fusion conditions in Sun

Very high core temperature (over 10 million K) and density.

Mass–energy equivalence

E = mc²; small lost mass becomes released energy.

Why does fusion release energy?

Helium product has less mass than starting hydrogen nuclei.

Solar neutrinos

Particles detected from fusion reactions in Sun's core.

Sun's age

About 4.6 billion years.

Sun's main-sequence lifetime

Roughly 10 billion years total.

Solar core

Dense, hottest inner region where fusion occurs.

Radiative zone

Energy moves outward via photons repeatedly absorbed and re-emitted/scattered.

Photon travel through radiative zone

About 50,000 years in lecture's estimate.

Convection zone

Energy carried outward by rising hot gas and sinking cooler gas.

Photosphere

Sun's visible surface layer.

Photosphere temperature

Approximately 6,000 K in lecture.

Solar granulation

Bright cells reveal hot gas rising to photosphere.

Sunspot

Cooler, darker surface region with strong magnetic fields.

Sunspot temperature

About 4,000 K in lecture.

Why are sunspots dark?

They emit light but less per area than hotter surroundings.

Chromosphere

Layer above photosphere; temperature can reach about 20,000 K.

Corona

Extended outer atmosphere; typically 1–2 million K.

Solar wind

Stream of charged particles flowing outward from Sun.

Solar flare

Sudden release of magnetic energy in Sun's atmosphere.

Coronal mass ejection

Large eruption of magnetized plasma into space.

Solar activity cycle

Approximately 11-year cycle in sunspot and magnetic activity.

Solar storms can affect

Power grids, communications, and satellites.

How does energy reach Sun's surface?

Radiation through inner zone, convection through outer zone.

Why can't chemical burning power Sun for billions of years?

It provides far too little energy compared with nuclear fusion.