Signatures of the RGB Phase
In the RGB phase there is a hydrogen shell source, rapid
contraction of the inert helium core, and rapid expansion and cooling of the outer envelope.
The lowering of the envelope temperature increases the opacity and the temperature gradient becomes
so steep that the star becomes convective in much of the outer envelope.
Thus, the vertical ascent of
the expanding and highly convective
star into the red giant region is in some sense the reverse of the descent of the fully
convective and contracting protostar on the Hayashi track to form the original main
sequence star that was discussed in Chapter 20.