Final answer:
During a star's energy production, hydrogen nuclei are fused into helium nuclei in a nuclear fusion process, which generates a significant amount of light energy.
Step-by-step explanation:
As a star produces energy, hydrogen nuclei, or protons, are "pushed together" in the core through a process called nuclear fusion. In this process, four protons bond together, forming a helium nucleus. Two protons in the group convert into neutrons during this process. The mass of the helium nucleus produced is less than the combined mass of the four protons due to the mass-energy equivalence principle, and the "lost" mass is converted into a tremendous amount of energy, mostly in the form of light energy, or photons. This is the basic operating mechanism of the proton-proton chain that powers stars like our Sun.