Stars are primarily composed of hydrogen and helium, though they also contain small but significant amounts of heavier elements. A star’s chemical makeup evolves throughout its lifetime, reflecting its birth environment and internal nuclear processes. Understanding stellar chemistry provides insights into the origin of all elements in the universe and the dynamic life cycles of stars.
The Primordial Stellar Ingredients
Hydrogen and helium are the most abundant elements in the universe, a direct consequence of the Big Bang. Big Bang nucleosynthesis, occurring minutes after the Big Bang, formed atomic nuclei of hydrogen, helium, and trace lithium. Approximately 75% of the universe’s ordinary matter is hydrogen by mass, and about 25% is helium, with all other elements constituting less than 2%.
These primordial elements became the fundamental building blocks for the first stars. Vast clouds of gas and dust, known as the interstellar medium, are predominantly hydrogen (around 90% by atom count) and helium (about 9%). These cosmic clouds provide the raw material that gravity gathers to initiate star formation, with hydrogen and helium serving as the initial fuel for stellar fusion.
From Cosmic Clouds to Shining Stars
Star formation begins within dense, cold regions of interstellar gas and dust clouds. Gravitational forces cause pockets within these clouds to collapse inward, drawing together immense amounts of material. As the material collapses, it heats due to increasing pressure and friction, forming a pre-stellar core that accrues mass. This contracting, heating ball of gas is known as a protostar.
A protostar is not yet a true star because its core has not reached the extreme temperatures necessary for nuclear fusion. This process can take millions of years as the protostar contracts and heats. Only when the core temperature reaches approximately 10 million Kelvin does hydrogen fusion begin, marking the birth of a main-sequence star. Hydrogen atoms then combine to form helium, releasing energy that counteracts gravity, stabilizing the star.
Forging Elements Within Stars
Hydrogen-to-helium fusion powers main-sequence stars, but these celestial furnaces also create heavier elements through stellar nucleosynthesis. Sun-like stars use the proton-proton chain, where hydrogen nuclei fuse into helium. More massive stars utilize the carbon-nitrogen-oxygen (CNO) cycle, converting hydrogen into helium using carbon, nitrogen, and oxygen as catalysts.
As stars exhaust their core hydrogen fuel, they undergo further evolutionary stages, becoming hotter and denser. If a star is sufficiently massive, its core can fuse helium into heavier elements like carbon and oxygen through the triple-alpha process. In even more massive stars, subsequent fusion reactions build elements such as neon, magnesium, silicon, and eventually iron in their cores. Iron is the heaviest element formed this way because fusing elements beyond it consumes energy.
Supernovae are responsible for creating and dispersing many of the heaviest elements. When massive stars’ iron cores collapse, they explode as supernovae. These explosions produce elements heavier than iron through rapid neutron capture processes (r-process) and disperse them into the interstellar medium. Neutron star mergers also contribute to producing the heaviest elements, like gold and uranium, through similar rapid neutron capture events.
Diverse Stellar Chemical Fingerprints
Astronomers refer to elements heavier than hydrogen and helium as “metals,” including carbon, oxygen, and iron. The presence and varying proportions of these metals give stars unique chemical fingerprints, measured as “metallicity.” Stars are categorized into populations based on their metallicity, which correlates with their age and formation environment.
The first stars, Population III stars, are theorized to have formed from pristine Big Bang hydrogen and helium, containing virtually no heavier elements. These stars have not yet been directly observed. Subsequent generations, Population II stars, are older with lower metallicities, indicating they formed from gas slightly enriched by earlier stars.
Younger stars, like our Sun, are Population I stars with the highest metallicities. They formed from interstellar gas and dust repeatedly enriched over billions of years by heavy elements from previous generations of stars and supernovae. The Sun, for example, has a metallicity of about 1.4% by mass. This continuous recycling ensures the chemical composition of stars and the entire universe evolves over cosmic time.