Stars are immense, self-luminous spheres of plasma whose characteristics, from brightness to lifespan, are determined by their chemical makeup. The composition provides the fuel for the nuclear reactions that define a star’s existence and drives stellar evolution. By understanding the elements that constitute a star like the Sun, we can grasp the physics that allows it to radiate energy for billions of years. The Sun serves as the most accessible and well-studied example for exploring stellar composition.
Hydrogen: The Sun’s Primary Building Block
The most common element in the Sun is hydrogen. By mass, hydrogen accounts for approximately 73.4% of the Sun’s total material. When considering the count of individual atoms, this dominance is even more pronounced, with hydrogen constituting about 92.0% of the Sun’s atoms.
Helium is the second most abundant element, making up around 25.0% of the Sun’s mass and 7.8% of its atoms. Together, these two lightest elements comprise over 98% of the Sun’s entire mass. The small remaining fraction consists of every other element, which astronomers collectively term “metals,” regardless of their actual chemical properties.
The Fusion Process: How Hydrogen Powers a Star
Hydrogen’s abundance represents the fuel source for the star’s immense energy output. The Sun generates its power through nuclear fusion, which occurs in its incredibly hot and dense core. The primary reaction sequence in stars like the Sun is the proton-proton (p-p) chain.
This process involves a series of steps where four individual hydrogen nuclei (single protons) are combined to form one helium nucleus. The high temperature, around 15 million Kelvin in the Sun’s core, provides the necessary kinetic energy for the protons to overcome their electrical repulsion. This allows the strong nuclear force to bind them together.
The resulting helium nucleus has slightly less mass than the four original hydrogen nuclei. This difference in mass is directly converted into energy, released as light and heat. The Sun converts roughly 600 billion kilograms of hydrogen into helium every second, turning about 4 million kilograms of matter into energy. This sustained conversion provides the outward pressure that supports the star against its immense gravity, allowing it to remain stable on the main sequence for billions of years.
Why Hydrogen Dominates Universal Composition
Hydrogen is the most common element in the Sun and the universe because of events immediately following the Big Bang. During the first few minutes of the universe’s existence, a process known as Big Bang Nucleosynthesis (BBN) occurred. At this time, the universe was hot and dense enough for subatomic particles to combine.
The conditions allowed for the formation of the lightest atomic nuclei: hydrogen and helium, from the primordial soup of protons and neutrons. Hydrogen (a single proton) and helium were created in a predictable ratio. Theory and observation confirm that BBN resulted in a mass fraction of approximately 75% hydrogen and 25% helium.
Because there is no stable nucleus with an atomic weight of 5 or 8, the BBN process could not efficiently create elements heavier than helium. Consequently, the vast majority of matter in the universe was established as hydrogen and helium before the first stars had formed. This initial cosmic abundance determined the starting composition for all subsequent stars, including the Sun.
Beyond Hydrogen: Trace Elements and the Stellar Life Cycle
While hydrogen and helium account for nearly all of the Sun’s mass, the remaining trace elements play a significant role in stellar classification and evolution. Trace elements, such as oxygen, carbon, neon, and iron, make up less than 2% of the Sun’s total mass. Astronomers refer to any element heavier than helium as a “metal,” a convention that simplifies the discussion of stellar chemical history.
The presence of these heavier elements places the Sun in the category of a Population I star. This classification indicates that the Sun is a relatively young star that formed from interstellar gas enriched by the remnants of previous generations of stars. Older stars, known as Population II, contain far fewer of these heavier elements.
The ultimate fate of the Sun is tied directly to its hydrogen fuel supply. After roughly 10 billion years, the hydrogen in the core will be depleted, and fusion will cease in that region. This depletion will cause the core to contract and heat up, eventually igniting a new shell of hydrogen fusion around the core. This event marks the end of the Sun’s stable phase and the beginning of its transformation into a red giant.