Here's a breakdown of how it happens:

1. Gravitational Collapse: A cloud of gas and dust in space, called a nebula, starts to collapse under its own gravity.

2. Protostar Formation: As the cloud collapses, it becomes denser and hotter. This hot, dense core is called a protostar.

3. Nuclear Fusion Ignition: The core of the protostar eventually reaches a temperature and pressure high enough to overcome the electrostatic repulsion between protons. This allows nuclear fusion to begin, where hydrogen atoms are fused into helium.

4. Star Formation: The energy released by nuclear fusion provides outward pressure that balances the inward pull of gravity. This marks the birth of a star.

So, while the collapse of the nebula is the first step in the process, the ignition of nuclear fusion is the defining moment when a star is truly born.