Here's why:

* Hydrogen is the primary fuel source. Nuclear fusion occurs in the core of a star, where hydrogen atoms are forced together to form helium, releasing immense energy in the process.

* Helium is the product of hydrogen fusion. While it's not a fuel in the same way as hydrogen, helium is a byproduct that contributes to the star's energy output.

The process of nuclear fusion in stars can be simplified as:

4 Hydrogen atoms (H) -> 1 Helium atom (He) + Energy

This process is responsible for the light and heat we see from stars.