Here's a breakdown:

1. Protostar: A star begins as a cloud of gas and dust that collapses under its own gravity. This collapsing cloud is called a protostar.

2. Gravitational Collapse: As the protostar collapses, it heats up. This is because the gravitational potential energy is converted into thermal energy.

3. Nuclear Fusion Ignition: Eventually, the core of the protostar gets hot and dense enough (around 10 million Kelvin) for nuclear fusion to start. This is when hydrogen atoms fuse to form helium, releasing a tremendous amount of energy.

4. Main Sequence Star: The star now enters the main sequence phase. This is the longest stage of a star's life, during which it is in a stable state, with the outward pressure from nuclear fusion balancing the inward force of gravity. The star's luminosity and temperature are determined by its mass.

So, to answer your question directly, a star becomes a main sequence star when it reaches the point where it can sustain nuclear fusion in its core.