Here's why:

* Main Sequence: Stars spend the majority of their lives on the main sequence. During this phase, they fuse hydrogen into helium in their cores, generating energy and outward pressure that balances gravity.

* Hydrogen Depletion: Eventually, the hydrogen fuel in the core is depleted. This means the nuclear fusion process stops, and the outward pressure from the fusion decreases.

* Gravity Takes Over: With the fusion pressure gone, gravity begins to pull the star's core inward. This causes the core to heat up and compress, eventually triggering a new phase of nuclear fusion, this time using helium as fuel.

* Leaving the Main Sequence: This change in core structure and the onset of helium fusion marks the star's departure from the main sequence. The star will then enter a new evolutionary stage depending on its mass.

For example:

* Sun-like stars: They become red giants, expanding significantly in size and cooling down.

* Massive stars: They evolve into supergiants and eventually become supernovae.

So, the key to understanding a star's evolution is to recognize the role of hydrogen fusion in its core. When it runs out, the star undergoes significant changes and moves off the main sequence.