helium-to-hydrogen ratio

The universe is primarily composed of hydrogen and helium, with hydrogen being the lightest and most abundant element. The fusion of hydrogen nuclei into helium nuclei is the fundamental reaction that powers stars like our Sun, releasing vast amounts of energy. This process, known as nuclear fusion, is central to stellar evolution and the creation of heavier elements up to iron.1

In the early universe, shortly after the Big Bang, nuclear reactions established an initial chemical composition dominated by hydrogen and helium. Current estimates suggest that primordial helium makes up about 25% of the universe's mass, with hydrogen accounting for the remaining 75%. This ratio is a key piece of evidence supporting the Big Bang theory. As stars evolve, they further process these elements, altering the chemical composition of the interstellar medium over cosmic time.12