Which element makes up most of the sun's mass?

The Star That Runs on Hydrogen

About 73% of the sun's total mass is hydrogen — making it, by a significant margin, the dominant ingredient in our nearest star. Helium accounts for roughly 25%, and everything else — oxygen, carbon, neon, iron, and all the other elements combined — makes up less than 2%. In cosmic terms, the sun is essentially a giant ball of hydrogen doing something extraordinary with itself.

Why So Much Hydrogen?

Hydrogen is the oldest and simplest element in the universe. It formed within the first few minutes after the Big Bang, roughly 13.8 billion years ago, during a period called Big Bang nucleosynthesis. Because it requires only one proton and one electron, hydrogen was the natural first product of a cooling, expanding universe. The sun formed about 4.6 billion years ago from a collapsing cloud of gas and dust — and that cloud was mostly hydrogen, a direct inheritance from the early universe.

What the Sun Actually Does With It

The sun doesn't burn hydrogen the way a candle burns wax. Instead, it fuses it. Deep in the sun's core, where temperatures reach around 15 million degrees Celsius and pressures are almost incomprehensible, hydrogen nuclei are forced together to form helium nuclei. This process — nuclear fusion — releases enormous amounts of energy according to Einstein's famous equation, E=mc². A tiny amount of mass is converted into a staggering amount of energy, which eventually reaches Earth as light and heat.

Every second, the sun converts approximately 600 million tonnes of hydrogen into helium. That sounds like a lot, but the sun is so massive — about 1.989 × 10³⁰ kilograms — that it has enough hydrogen fuel to keep going for another five billion years or so.

What Happens When the Hydrogen Runs Out?

When the sun's core hydrogen is exhausted, it will begin fusing helium, causing the outer layers to expand dramatically. The sun will swell into a red giant, potentially engulfing Mercury and Venus, and possibly reaching Earth's orbit. Eventually, it will shed its outer layers and leave behind a dense, Earth-sized remnant called a white dwarf — a cooling ember made mostly of carbon and oxygen.

Hydrogen Beyond the Sun

This hydrogen-heavy composition isn't unique to our sun. Hydrogen is the most abundant element in the entire observable universe, making up roughly 75% of all normal (baryonic) matter. It's the building block of stars everywhere. When astronomers study distant galaxies or vast nebulae, they're largely mapping hydrogen — the raw material from which stars, and ultimately planets and life, are assembled.

So the next time you feel the sun on your face, you're really feeling the product of hydrogen atoms colliding and merging at unimaginable temperatures, 93 million miles away. It's a deeply simple element doing something profoundly powerful.