A star is a massive, luminous sphere of plasma held together by its own gravity, and its chemical makeup is overwhelmingly simple. The immense pressure and temperature within a star’s core are sustained by nuclear fusion, which requires a specific set of raw materials. Hydrogen and Helium serve as the primary fuel and product, respectively, defining the star’s structure, energy output, and lifespan.
The Initial Stellar Composition
A newly formed star, like our Sun when it first ignited, begins its life with a remarkably consistent chemical composition. By mass, approximately three-quarters of the star is made up of Hydrogen, the most abundant element in the cosmos. The second most plentiful element is Helium, which constitutes nearly a quarter of the star’s total mass. This leaves a very small fraction for all other elements, which combined make up roughly two percent or less of the star’s initial mass.
The abundance of these elements is determined by the composition of the interstellar gas cloud from which the star collapses. Even a small variance in these starting material percentages can significantly affect how a star evolves and how long it lives. For instance, stars with a lower fraction of elements heavier than Hydrogen and Helium tend to be older, having formed earlier in the universe’s history.
Tracing the Elements to the Big Bang
The overwhelming dominance of Hydrogen and Helium in stellar composition is a direct consequence of the universe’s earliest moments. The formation of these light elements is attributed to a process called Primordial Nucleosynthesis, which occurred in the first few minutes after the Big Bang.
The universe was initially an extremely hot, dense plasma, but as it expanded and cooled, protons and neutrons began to combine. This brief period allowed for the fusion of protons to form deuterium, a heavy isotope of Hydrogen, which then quickly fused further into Helium-4 nuclei.
The process stopped abruptly after about 20 minutes because the universe had expanded and cooled too much to sustain the necessary nuclear reactions. This limited the formation of heavier atomic nuclei. Trace amounts of Lithium and Beryllium were also created, but the conditions were not right to produce significant quantities of any element beyond Helium.
Defining the Role of Stellar Metals
In astrophysics, the term “metals” is used with a specific meaning that differs significantly from its standard chemical definition. Astronomers use the word as a convenient shorthand to collectively describe all chemical elements heavier than Hydrogen and Helium.
The small percentage of these heavier elements in a star’s composition is referred to as its metallicity. This measurement is important for classifying stars into different populations, which reflects their age and where they formed in the galaxy. Older stars tend to have lower metallicity because they formed from the relatively pure Hydrogen and Helium gas of the early universe.
These heavier elements originate almost entirely from previous generations of massive stars. They are synthesized through nuclear fusion within a star’s core over its lifetime and are then scattered into space through stellar winds or catastrophic supernova explosions. This recycled material enriches the interstellar medium, providing the building blocks for subsequent generations of stars and planets.