A protostar becomes a star when it reaches a point where nuclear fusion begins in its core. This is the defining moment when a protostar starts to generate its own energy and light.

Here's a breakdown of the process:

1. Formation: A protostar is formed from a dense cloud of gas and dust that collapses under its own gravity.

2. Gravitational Collapse: As the cloud collapses, it heats up and spins faster.

3. Protostar Stage: The collapsing cloud forms a protostar, a hot, dense object still accumulating material. This stage can last for tens of thousands to millions of years.

4. Nuclear Fusion Ignition: The core of the protostar becomes so hot and dense that hydrogen atoms begin to fuse together, releasing immense amounts of energy. This is the moment the protostar becomes a star.

Key Factors:

* Temperature: The core of the protostar needs to reach around 10 million degrees Celsius (18 million degrees Fahrenheit) for nuclear fusion to begin.

* Density: The core also needs to be incredibly dense for the atoms to collide with enough force to fuse.

The process of becoming a star is a complex and fascinating one, and it can take millions of years to complete.