The Stellar Fusion Process:

* Hydrogen Burning: Main sequence stars, like our Sun, primarily fuse hydrogen into helium in their cores. This is the main source of their energy.

* Hydrogen Depletion: Over time, the hydrogen fuel in the core gets used up.

* Helium Accumulation: As hydrogen fuses into helium, the helium concentration in the core steadily increases.

The Aging Process:

* Core Contraction: As the hydrogen fuel dwindles, the core begins to contract under its own gravity.

* Temperature Rise: This contraction causes the core temperature to rise.

* Helium Fusion (Eventually): Eventually, the core becomes hot enough (around 100 million Kelvin) to ignite helium fusion, creating heavier elements like carbon and oxygen. This marks the transition to the red giant phase.

Key Point: The presence of more helium in the core of an old main sequence star is a direct result of the ongoing hydrogen fusion process that fuels the star's energy output. The accumulation of helium is a sign of the star's aging and its eventual transition to a different evolutionary stage.