Question

What is hydrogen shell fusion?

Answer 1

Hydrogen shell fusion is hydrogen fusion reactions taking place in a shell around a helium fusing core.

Explanation:

When a star has depleted its supply of hydrogen in its core, the core is mainly helium. At this stage the core contacts and the temperature rises. The star enters the red giant stage.

Around the helium core there is a shell of hydrogen. Fusion reactions continue in this shell.

When the core temperature reached `10^8K`. The triple alpha process starts which fuses helium into carbon. Fusion reactions still continue in the shell of hydrogen around the now active core.

So, hydrogen shell fusion is hydrogen fusion reactions taking place in a shell of hydrogen surrounding a core of helium or heavier elements in an ageing star.