What characterizes a star leaving the Main-Sequence?

Answer 1

This basically happens when the star runs out of hydrogen-1 fuel. After that, it starts to fuse helium-4 into heavier isotopes, but this requires a new pressure/temperature balance - outside the main sequence.

Answer 2

A star leaving the Main Sequence indicates that it has exhausted the hydrogen fuel in its core. As a result, the core contracts and heats up, causing the outer layers to expand and cool, leading to the star becoming a red giant. This phase marks the transition to the later stages of stellar evolution.

Answer 3

It has transformed al his hydrogen into helium.