Which element marks the beginning of fusion in stars?

Which element marks the beginning of fusion in stars?

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

Answer: (A)

Hydrogen is the element that starts fusion in stars. In stellar cores, temperatures reach millions of kelvin, enough to overcome the electrical repulsion between protons and fuse hydrogen into helium through reactions like the proton–proton chain (dominant in sun-like stars) or the CNO cycle (in more massive stars). This hydrogen-burning phase provides the energy that supports the star against gravity for most of its life. Helium fusion happens later, after the core has exhausted its hydrogen and contracts and heats further, allowing helium to fuse into heavier elements such as carbon via the triple-alpha process. Lithium isn’t a primary stellar fuel and is quickly destroyed in stellar interiors, and carbon is produced during helium burning, not at the beginning.

Hydrogen is the element that starts fusion in stars. In stellar cores, temperatures reach millions of kelvin, enough to overcome the electrical repulsion between protons and fuse hydrogen into helium through reactions like the proton–proton chain (dominant in sun-like stars) or the CNO cycle (in more massive stars). This hydrogen-burning phase provides the energy that supports the star against gravity for most of its life.

Helium fusion happens later, after the core has exhausted its hydrogen and contracts and heats further, allowing helium to fuse into heavier elements such as carbon via the triple-alpha process. Lithium isn’t a primary stellar fuel and is quickly destroyed in stellar interiors, and carbon is produced during helium burning, not at the beginning.