* Nuclear Fusion: Stars like our Sun generate energy through nuclear fusion, primarily converting hydrogen into helium in their core. This is the main process that fuels a star throughout its main sequence lifetime.
* Continuous Helium Production: As a star ages, it continues to fuse hydrogen into helium, steadily increasing the helium content in its core. This process is ongoing throughout the main sequence phase.
* Limited Hydrogen: A star's main sequence lifetime is determined by the amount of hydrogen fuel it has. As a star ages, it consumes its hydrogen fuel, leading to a decrease in hydrogen and a corresponding increase in helium.
* Old Stars, More Helium: Older main sequence stars have had more time to fuse hydrogen into helium, resulting in a higher proportion of helium in their core compared to younger stars.
In summary: An old main sequence star has more helium because it has been fusing hydrogen into helium for a longer period, leading to an accumulation of helium in its core.
