The Sun SSL July 2006

Sun Needs Fuel

Hydrostatic Equilibrium Gravity and pressure must balance If not, unstable

Energy Generation Two methods –Gravitational energy –Conversion of mass to energy

Energy Transport Two methods –Radiation –Convection Sun does both

Gravitational Energy Takes energy to lift mass Releases energy when gas falls Shrinking body will release gravitational energy and heat up

Relativity Energy and matter are equivalent E = mc 2

Gravitational contraction Can release copious energy However, no evidence the Sun is shrinking Would only last a few thousand years

Nuclear Energy Very large amounts of energy released Not very efficient –Needs high densities –Needs high temperatures

Eddington Proposed nuclear reactions to fuel the Sun First successful models Two main chains –P-P chain –CNO cycle

P-P chain

CNO Chain C, N and O act as catalysts in reaction More complicated reactions also happen but not as important

Solar Structure

Sun at many energies

Probing the Sun Helioseismology –Apply same principles as studies of the Earth –Disturbances travel through gas of the Sun –Resonances tell of structure –Sun quakes

Solar Interior Core –Energy transport is through radiation Convective zone –Energy transport is through convection Photosphere –Energy transport is through radiation into space

Solar Exterior Chromosphere is hotter than photosphere –Magnetic fields probably involved Corona –Million degrees –X-ray bright –Seen during eclipses

Solar Activity 22 year magnetic cycle 11 year cycle of strength