The primary substance that makes up stars is hydrogen. Stars are celestial bodies that generate energy through nuclear fusion, primarily converting hydrogen into helium in their cores. This process releases immense amounts of energy, shining the stars brightly. Hydrogen is the most abundant element in the universe, constituting about 74% of its normal matter. As stars age, they can fuse heavier elements, but hydrogen remains the fundamental building block for their formation. The study of stellar evolution also enhances our understanding of the universe's structure and the life cycles of celestial bodies.