What change in hydrostatic balance defines the red giant phase compared to the main sequence phase for a Sun-like star?
Answer
The entire hydrostatic balance is re-established with the engine running on a hotter, tighter jacket of fuel (the shell)
The main sequence maintains hydrostatic balance via steady, central hydrogen fusion. When core fusion stops, the balance is broken, leading to core collapse and subsequent shell ignition. The new equilibrium defining the red giant phase relies on the intense, localized energy output from the hydrogen shell burning. This rapid increase in shell fusion rate forces the entire structure to expand dramatically until a new, cooler equilibrium temperature is found across the immense surface area, balancing the radiation output against the gravity acting on the much larger volume.
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