In the core of a star, hydrogen nuclei (protons) combine to form helium nuclei through a process called nuclear fusion.

Here's a simplified explanation:

1. High Temperature and Pressure: The intense heat and pressure in a star's core overcome the electrostatic repulsion between protons.

2. Proton-Proton Chain: A series of reactions occur, starting with two protons colliding to form a deuterium nucleus (one proton and one neutron). This deuterium then collides with another proton to form a helium-3 nucleus (two protons and one neutron). Finally, two helium-3 nuclei fuse to form a helium-4 nucleus (two protons and two neutrons) and release energy.

This process, known as the proton-proton chain reaction, is the primary way stars generate energy and produce helium.

It's important to note that while this is a simplified description, the actual process is much more complex and involves various intermediate steps and isotopes.