  We consider the blue loops in the Hertzsprung-Russell diagram that occur when
intermediate-mass stars begin core helium burning. It has long been known that
the excess of helium above the burning shell, the result of the contraction of
the convective core during core hydrogen burning, has the effect of making such
stars redder and larger than they would be otherwise. The outward motion of the
burning shell in mass removes this excess and triggers the loop. Hitherto
nobody has attempted to demonstrate why the excess helium has this effect. We
consider the effect of the local opacity, which is reduced by excess helium,
the shell fuel supply, which is also reduced, and the local mean molecular
weight, which is increased. We demonstrate that the mean molecular weight is
the decisive reddening factor. The opacity has a much smaller effect and a
reduced fuel supply actually favours blueward motion.
