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Notes:
After the Helium flash, the star shrinks, becoming less luminous and somewhat hotter. This is because the Helium core has expanded, making the gravity on the Hydrogen shell weaker. Therefore, the pressure and temperature in the shell are reduced and its luminosity decreased. The decrease in luminosity of the Hydrogen shell is larger than the added luminosity of Helium burning in the core.
Fig. 17.6a from the book.