Question

when either low-mass or high-mass main sequence stars run out of hydrogen in the core, the core will contain

Explanation:

To answer this, we analyze stellar evolution: when main - sequence stars (both low - and high - mass) exhaust hydrogen in their cores, the core, which was previously fusing hydrogen into helium, will now be composed of helium. This is because the hydrogen fusion process in the core produces helium as a by - product, and when the hydrogen fuel is used up, the core is left with the helium that was created during the hydrogen - burning phase.

Answer:

helium