The Sun is a G-type main-sequence star, an immense ball of superheated gas. Its composition is dominated by hydrogen and helium, which together make up approximately 98% of its mass. While the Sun is predominantly hydrogen, about 28% of its mass is helium. This significant quantity of helium did not all originate from the star’s own activity, requiring an examination of both the universe’s earliest moments and the star’s current internal processes.
The Ancient Source: Helium from the Big Bang
The largest component of the Sun’s helium content is an inheritance from the universe’s initial conditions, forged during Big Bang Nucleosynthesis. This event occurred mere minutes after the universe began when the cosmos was a homogeneous plasma of extremely high temperature and density. The intense heat and pressure allowed the formation of the simplest atomic nuclei.
Within the first 200 seconds, protons and neutrons fused together before the rapidly expanding universe cooled enough to halt the process. The ratio of neutrons to protons, approximately one to seven, determined the final elemental proportions. These particles combined to form deuterium, which quickly reacted further to produce helium-4 nuclei.
This intense period of cosmic fusion established a universal baseline of light elements. The resulting primordial composition across the entire universe settled at a ratio of approximately 75% hydrogen and 25% helium by mass. When the cloud of gas and dust collapsed to form the Sun 4.6 billion years ago, it drew material from this pre-existing interstellar medium.
The star was thus born with a foundational helium mass fraction of around 25%. This ancient, universally distributed helium accounts for the vast majority of the helium currently present in the Sun.
The Engine Room: Creating Helium Today
While the initial helium was inherited, the Sun currently manufactures additional helium in its core. This ongoing process of stellar nucleosynthesis is the source of the star’s radiant energy, slowly adding to the inherited helium mass fraction. The extreme conditions necessary for this reaction are confined to the core, where the temperature reaches roughly 15 million Kelvin.
The mechanism responsible for this energy production is primarily the Proton-Proton (P-P) Chain reaction. This multi-step nuclear process converts four separate hydrogen nuclei, which are single protons, into a single helium-4 nucleus. The entire reaction sequence releases a tremendous amount of energy.
The P-P Chain begins when two protons collide and one transforms into a neutron, forming a deuterium nucleus while emitting a positron and a neutrino. This first step is governed by the weak nuclear force and is the rate-limiting part of the reaction. Subsequent, much faster reactions combine the deuterium with other protons to eventually form helium-4.
Every second, the Sun converts approximately 600 billion kilograms of hydrogen into helium through this process. This steady conversion means the Sun’s total helium content is gradually increasing over its 10-billion-year main-sequence lifetime, supplementing the primordial helium it began with.
Where the 28% Resides: Distribution and Measurement
The quoted figure of 28% helium by mass is an average that describes the Sun’s total current composition. This average disguises a significant difference in elemental distribution between the star’s core and its outer layers. The helium nuclei created through fusion are denser than the surrounding hydrogen plasma.
Because the Sun’s core lacks the convective churning found in its outer layers, this newly formed, heavier helium accumulates at the center. Consequently, the star’s core has become significantly enriched in helium over its lifetime. The core is now composed of approximately 60% to 70% helium by mass, a substantial increase from its starting value.
Conversely, the Sun’s outer atmosphere, or photosphere, where the light we see is emitted, is not affected by the fusion occurring deep inside. The helium in the photosphere remains close to the initial primordial mix, reflecting a composition of roughly 25% helium by mass. This compositional difference between the surface and the core is a direct consequence of the physical processes at play inside the star.
Scientists determine the Sun’s composition by analyzing the light it emits, a technique called spectroscopy. This method works by separating the star’s light into a spectrum, which reveals dark absorption lines that act as unique fingerprints for different elements. Early analysis of the solar spectrum led to the discovery of helium itself, named for the Greek sun god Helios.
Directly measuring helium in the Sun’s atmosphere is challenging because the element does not produce easily observable spectral lines at the photosphere’s temperature. Modern quantification of the Sun’s helium content often relies on indirect methods, such as studying the way the Sun vibrates, known as helioseismology, to map the density and composition of the interior.