Nuclear fusion of hydrogen begins inside a protostar when the core reaches a temperature of approximately 10 million Kelvin (18 million degrees Fahrenheit).

Here's why:

* Gravitational Collapse: A protostar forms from a cloud of gas and dust. Gravity pulls this material inward, causing the core to heat up and become denser.

* Fusion Threshold: At a specific temperature and pressure, the nuclei of hydrogen atoms have enough energy to overcome their electrostatic repulsion and fuse together. This is the temperature threshold for nuclear fusion.

* Proton-Proton Chain: In the core of a protostar, the primary fusion process is the proton-proton chain, which involves the fusion of hydrogen nuclei (protons) to form helium. This process releases a tremendous amount of energy, which stabilizes the star and prevents further gravitational collapse.

Note: The exact temperature and pressure required for fusion to begin can vary slightly depending on the mass and composition of the protostar.