What causes the core to shrink during a star's evolution?

What causes the core to shrink during a star's evolution?

• A. Hydrogen depletion
• B. Helium accumulation
• C. Core temperature rise
• D. Increased angular momentum

Answer: (A)

When the hydrogen fuel in the core is exhausted, fusion there shuts down. That removes the outward pressure produced by hydrogen burning, so gravity can pull the core inward. As the core contracts, it heats up due to compression, reaching the temperatures needed to ignite helium and continue the star’s evolution. So hydrogen depletion is the trigger for the core to shrink. Helium accumulation happens because hydrogen has been fused into helium, so it’s a consequence, not the cause. A rise in core temperature comes from the contraction itself, not the initial trigger. Increased angular momentum affects rotation and isn’t the main driver of this contraction in this phase.

When the hydrogen fuel in the core is exhausted, fusion there shuts down. That removes the outward pressure produced by hydrogen burning, so gravity can pull the core inward. As the core contracts, it heats up due to compression, reaching the temperatures needed to ignite helium and continue the star’s evolution. So hydrogen depletion is the trigger for the core to shrink.

Helium accumulation happens because hydrogen has been fused into helium, so it’s a consequence, not the cause. A rise in core temperature comes from the contraction itself, not the initial trigger. Increased angular momentum affects rotation and isn’t the main driver of this contraction in this phase.