During the hydrogen shell-burning phase:

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During the hydrogen shell-burning phase, the outer envelope of the star expands and cools, causing it to grow more luminous. This is because hydrogen is being burned in a shell around the core, providing more energy to the star. The core itself remains relatively stable during this phase. During the hydrogen shell-burning phase, the outer layers of the star expand and cool, causing the star to grow more luminous. This is because the energy released from the burning hydrogen is transferred outward to the outer layers, causing them to heat up and emit more light.

During the Hydrogen Shell-Burning Phase