Here's a breakdown:

* Main Sequence: This is the longest and most stable phase of a star's life. During this phase, the star fuses hydrogen into helium in its core, producing energy that creates outward pressure balancing the inward pull of gravity.

* Hydrogen Depletion: Over time, the hydrogen fuel in the core gets depleted. This means the fusion process slows down, causing the outward pressure to decrease.

* Gravitational Collapse: With less pressure pushing outwards, the star's core begins to contract under its own gravity. This contraction heats up the core significantly.

* Shell Burning: The increased heat causes the hydrogen in a shell surrounding the core to begin fusing, expanding the star into a red giant or red supergiant, depending on its initial mass.

Key Points:

* Star Mass Determines Lifetime: More massive stars have higher core temperatures, causing them to burn through their hydrogen fuel much faster. This means they spend less time on the main sequence compared to less massive stars.

* No Abrupt End: The transition from the main sequence to the next phase isn't an abrupt event. The star gradually expands and changes its spectral type over time.

Example:

Our Sun, a relatively small star, is currently in its main sequence phase. It is expected to stay on the main sequence for about 10 billion years.