A solar mass star will evolve off the main sequence when it has exhausted roughly 10% of its hydrogen fuel in its core.

Here's why:

* Main Sequence: Stars spend the majority of their lives on the main sequence. During this stage, they fuse hydrogen into helium in their core, generating energy through nuclear fusion and creating outward pressure that balances the inward force of gravity.

* Hydrogen Depletion: As a star fuses hydrogen, it gradually depletes the fuel in its core. When around 10% of the hydrogen is used up, the core starts to shrink and heat up.

* Evolution Off Main Sequence: This shrinking and heating causes the star's outer layers to expand and cool, becoming a red giant. This marks the end of the main sequence stage and the beginning of a new evolutionary path.

Note: The exact percentage of hydrogen depletion and the timing of evolution off the main sequence can vary slightly based on the star's initial mass and composition.