Two major reactions:

Proton-Proton

5*10^6 < T < 2*10^7

Or if T higher (assuming heavier elements are present)

CNO Cycle

2*10^7 < T < 10^8

Note: a star with little CNO can’t do this.

Above 10^8

Can really be written

Must be very dense for three He atoms to coe close enough together all at once

Important implications for cosmology --> onely way to make elements heavier than He is to bring these nuclei together -- only in stars/

Core in thermal/gravitational equilibrium. Above the core structure is quite different. Force of the photons escaping the core adds to the thermakl enerfy density. Lifts a mantle of cooler materical away from the center of the star.

Energy Transport

a) Radiation

b) Convection

Hydrostatic Equilibrium

P = nkT + Radiative term

Radiative term dominates outside the core

Opacity plays big role

Imagine no opacity

photons escpae -- core sized star

Very high opacity

gets bigger and bigger

longer for energy to escape -- heats and expands