The theory of stellar evolution is reviewed in a simple fashion, with emphasis on topics for which some rethinking may be required. It is suggested that evolution of stars off the main sequence in the H-R diagram into the area occupied by the giants is caused, not by fuel exhaustion, but simply by instability following the ignition of thermonuclear energy release in the interior. The physics of a degenerate gas is discussed. It is pointed out that the equation of state remains the same as for a normal gas, but the pressure is exerted only by that fraction of the particles which occupy states withinkTof the Fermi level. Degeneracy of the electron gas causes a large reduction of opacity, leading to an increased rate of loss of energy. The higher luminosity is fist fed by gravitational potential energy released during a phase of rapid contraction. However, when the proton Fermi level reaches a certain threshold, thermonuclear processes take over. During intial contraction angular momentum is transferred from orbital motion to eddies. Two eddies, revolving about the centre of mass produce a binary star system. In the case when a number of eddies take up nearly all the orbital angular momentum further contraction into a single star occurs. Later this angular momentum appears to be transferred back into orbital motion of matter near the surface.