Which element is the lightest and most abundant in the universe and participates in stellar fusion processes?

• A. Hydrogen
• B. Helium
• C. Oxygen
• D. Carbon

Answer: (A)

Hydrogen is the lightest element, with just one proton (and one electron), and it is by far the most abundant element in the universe, making up a large majority of the normal matter in stars and galaxies. In stars, hydrogen nuclei fuse in nuclear reactions known as hydrogen burning—the proton-proton chain in cooler stars or the CNO cycle in hotter ones. This fusion converts hydrogen into helium and releases the energy that powers the star. Because it is both the lightest and the most abundant, hydrogen best fits the idea of a substance that participates in stellar fusion while being the dominant component of the universe. Heavier elements like helium, carbon, and oxygen do participate in fusion as stars evolve, but they are far less abundant and not the lightest.