About three-fourths of the mass of the sun consists of which element?

The sun shines by acting as a colossal natural nuclear reactor, and its primary fuel is the simplest and most abundant element in the universe. This element makes up roughly 75% of the sun's mass. Stars are born from immense, collapsing clouds of interstellar (Review) gas and dust, and these clouds are overwhelmingly composed of this light gas, providing the raw material for star formation.

Inside the sun's incredibly hot and dense core, the force of gravity is so strong that it smashes these atoms together. This process, known as nuclear fusion, forces them to combine and form the next-lightest element, helium. This is why helium makes up nearly all of the remaining 25% of the sun's mass. This constant conversion process releases the tremendous amount of energy that radiates out into the solar system as light and heat, making life on Earth possible.

All of the heavier elements we are familiar with, such as oxygen, carbon, and iron, account for less than 2% of the sun’s total mass. For over 4.5 billion years, our star has been steadily burning through its massive fuel supply, and it has enough to continue shining for another five billion years before its life cycle begins to change.