The Sun is a massive sphere of hot plasma that serves as the gravitational anchor and energy source for our solar system. Containing over 99.8% of the solar system’s total mass, its constant light and heat result from complex physical processes. Understanding the Sun requires examining the specific elemental materials that make up its bulk.
Defining the Sun’s Primary Elements
The Sun’s composition is overwhelmingly dominated by the lightest elements in the universe. Hydrogen is the most plentiful element, making up roughly 73.4% of the Sun’s total mass in its visible outer layer, the photosphere. The second most abundant element is helium, accounting for about 25% of the mass. Together, these two gases constitute almost 98.4% of the star’s material.
When measured by the number of individual atoms, the hydrogen content is even higher, closer to 92% of all atoms present. This difference exists because a single helium atom is nearly four times more massive than a single hydrogen atom. The remaining fraction of the Sun’s mass consists of trace amounts of heavier elements, which astronomers collectively refer to as “metals.” These include oxygen, carbon, neon, and iron.
How Hydrogen is Distributed Across Solar Layers
The elemental percentages determined from the Sun’s surface are not uniform throughout the star’s interior. The Sun is structured into distinct layers, and the concentration of hydrogen varies dramatically across them. The core is the innermost region, where temperatures reach approximately 15 million Kelvin and matter exists as a superheated plasma. This is the only location where hydrogen is actively being converted into helium.
Because of this constant conversion, the core has become enriched with helium over the past 4.6 billion years. The proportion of helium in the core has increased from its original 24% to about 60% today. Moving outward, the hydrogen content increases again in the surrounding radiative and convective zones. The hydrogen in these outer layers remains largely in its primordial state, reflecting the composition from which the Sun first formed.
The Fusion Process Powered by Hydrogen
Hydrogen holds the central role in the Sun’s existence because it serves as the fuel for the star’s energy output. The source of this energy is nuclear fusion, which occurs exclusively in the dense, high-temperature environment of the core. Fusion is a reaction where lighter atomic nuclei are forced together to form heavier ones, releasing enormous amounts of energy. The primary mechanism in the Sun is known as the Proton-Proton Chain.
This chain begins when two hydrogen nuclei (single protons) collide and fuse to create a deuterium nucleus, releasing a positron and a neutrino. The deuterium then captures another proton to form a helium-3 nucleus. Finally, two helium-3 nuclei combine to produce a stable helium-4 nucleus, simultaneously expelling two protons that can restart the cycle. The total reaction converts four protons into one helium nucleus.
The mass difference between the initial four hydrogen nuclei and the final helium nucleus is converted directly into energy, predominantly as gamma-ray photons. This conversion occurs at a staggering rate, with the Sun’s core fusing approximately 600 billion kilograms of hydrogen into helium every second. This continuous consumption of hydrogen generates the light and heat that sustains life on Earth.
The Sun’s Evolution as Hydrogen Depletes
The continuous consumption of hydrogen in the core dictates the Sun’s evolution. The Sun is currently in its main-sequence phase, a period of stable hydrogen fusion expected to last about 10 billion years. Scientists estimate the Sun has approximately 5 billion years remaining before its core hydrogen supply is exhausted.
Once the hydrogen fuel in the core runs out, fusion will cease. Gravity will cause the inert helium core to contract and heat up drastically. This increased heat will ignite a shell of hydrogen fusion in the layer immediately surrounding the core. The energy from this shell fusion will cause the Sun’s outer layers, which are still rich in hydrogen, to expand dramatically, transforming the star into a Red Giant. Eventually, the Sun will shed its outer layers and leave behind a dense, cooling core known as a white dwarf.