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Notes:
Back to the main picture: one needs to balance gravity with pressure, and normally this requires high temperature. The high temperature implies one looses energy, so an energy source is needed.
On the main sequence, this is nuclear fusion. This suffices for a long time, and the only small change that occurs is that fusion reduces the number of particles (4 H and 4 electrons go to 1 He and 2 electrons). To maintain the pressure, the temperature has to rise. Hence, the star will loose more energy (its luminosity increases) and the fusion rate will accelerate.
The figure is Fig 14.2 from the book.