What is the most common and useful element found in a star?

Hydrogen is the most common and useful element found in a star, primarily because it is the most abundant element in the universe. In stars, hydrogen serves a critical role as the primary fuel for nuclear fusion. During the process of fusion, hydrogen nuclei (protons) combine under extreme pressure and temperature to form helium nuclei, releasing tremendous amounts of energy in the process. This energy is what powers stars and creates the light we see from them.

In the context of stellar chemistry, hydrogen is essential for the life cycle of a star. During its life, a star fuses hydrogen into helium, which is the foundation for the formation of heavier elements as the star evolves. As stars age and exhaust their hydrogen fuel, they begin to fuse helium and eventually heavier elements, but the initial and most prominent stage of a star's life centers around hydrogen. Because of its abundance and the fundamental processes it facilitates, hydrogen is indeed the most significant element in the life cycle of a star.