Stars are overwhelmingly made of hydrogen and helium, the two lightest and most abundant elements in the universe. In a typical star like our Sun, hydrogen accounts for roughly 71–73% of the total mass, helium makes up about 25–27%, and everything else on the periodic table combined amounts to no more than about 2%. That lopsided recipe reflects the makeup of the universe itself, set in motion minutes after the Big Bang, and it shapes nearly every aspect of how stars shine, age, and die.
How We Know What Stars Are Made Of
You cannot scoop a sample from the surface of a star, so astronomers have had to get creative. The oldest and most widely used technique is spectroscopy, which dates back more than two centuries. When starlight passes through a prism or diffraction grating, it fans out into a rainbow of colors interrupted by dark lines at very specific wavelengths. Each element absorbs light at its own characteristic set of wavelengths, so those dark lines act like a barcode identifying which elements are present and in what proportions. This approach has been central to understanding stellar composition since Joseph von Fraunhofer first catalogued the Sun’s absorption lines in 1814.1American Journal of Student Research. The Significance of Metallicity in Determining Stellar Mass
For the Sun specifically, scientists have two additional, remarkably independent ways to cross-check. One involves studying the vibrations that ripple across the solar surface. The Sun rings like a bell, and the speed of these sound waves depends on the temperature, density, and chemical composition of the material they travel through. By analyzing these oscillations, researchers can infer the hydrogen and helium fractions in the Sun’s interior with impressive precision. A detailed helioseismic analysis placed the Sun’s hydrogen mass fraction in the convective envelope between 0.715 and 0.730, meaning hydrogen makes up about 71.5–73% of the mass in that region.2Astronomy & Astrophysics. Helioseismic determination of the solar metal mass fraction
The other method involves catching neutrinos, ghostly subatomic particles produced during nuclear fusion in the Sun’s core. By measuring the flux of neutrinos coming from different fusion reactions, physicists can work backward to figure out what the core is made of and which reactions dominate. Combining helioseismic data with solar neutrino measurements gives a consistent, quantitative picture of the Sun’s internal chemistry.3The Astrophysical Journal. THE CHEMICAL COMPOSITION OF THE SUN FROM HELIOSEISMIC AND SOLAR NEUTRINO DATA
There is even a fourth, more exotic line of evidence. Primitive meteorites that formed alongside the Solar System contain tiny grains of actual stardust, microscopic particles that condensed in the outflows and explosions of earlier generations of stars before our Sun existed. These presolar grains, which include diamond, silicon carbide, graphite, and corundum, carry isotopic fingerprints that reveal what their parent stars were synthesizing.4Annual Review of Earth and Planetary Sciences. STELLAR NUCLEOSYNTHESIS AND THE ISOTOPIC COMPOSITION OF PRESOLAR GRAINS FROM PRIMITIVE METEORITES Researchers can extract these grains from meteorites in the lab and measure their isotopic compositions directly, providing a physical sample of stellar matter to compare against spectroscopic and helioseismic models.5The Astrophysical Journal Letters. Silicon Isotopic Composition of Mainstream Presolar SiC Grains Revisited: The Impact of Nuclear Reaction Rate Uncertainties
Why Hydrogen and Helium Dominate
The overwhelming abundance of hydrogen and helium traces back to the first few minutes of the universe. In the hot, dense conditions right after the Big Bang, protons and neutrons collided and fused to form the simplest atomic nuclei. This process, called Big Bang nucleosynthesis, produced hydrogen, helium, and trace amounts of lithium, but essentially nothing heavier.6arXiv. Big Bang Nucleosynthesis The universe expanded and cooled too quickly for nuclear reactions to build larger nuclei. So by the time the first atoms formed a few hundred thousand years later, the cosmic recipe was roughly 75% hydrogen and 25% helium by mass, with everything else amounting to a rounding error.
When the first stars eventually condensed out of this primordial gas, they inherited that recipe almost exactly. And while stars spend their lives converting hydrogen into helium and heavier elements, they do so slowly enough that the overall cosmic ratio has barely shifted after nearly 14 billion years. Every star that forms today still draws from a gas supply that is predominantly hydrogen and helium, though with a slowly growing seasoning of heavier elements recycled from earlier stellar generations.
What Happens to That Hydrogen Inside a Star
A star’s core is essentially a sustained thermonuclear furnace. The immense gravitational pressure at the center pushes hydrogen nuclei close enough together for the strong nuclear force to take over, fusing them into helium. For stars around the Sun’s mass, the dominant pathway is the proton-proton chain, where pairs of protons fuse step by step into helium-4 nuclei. Spectral observations of neutrinos from the Sun have confirmed that this process generates about 99% of the Sun’s energy output, which amounts to roughly 3.84 × 10²⁶ watts.7Nature. Neutrinos from the primary proton–proton fusion process in the Sun
This hydrogen-to-helium conversion is what defines a star’s main life stage. Our Sun has been doing it for about 4.6 billion years and has enough hydrogen fuel for roughly another 5 billion. More massive stars burn through their hydrogen far faster, sometimes in just a few million years, because greater gravitational pressure drives the core temperature higher and the fusion reactions race ahead.
Building Heavier Elements
Once a star exhausts the hydrogen in its core, what comes next depends on its mass. Stars heavy enough to generate the required temperatures begin fusing helium into carbon through a process called the triple-alpha reaction, so named because three helium-4 nuclei (alpha particles) combine. This reaction is one of the most consequential in all of astrophysics: it is the main source of carbon in the universe and depends on a specific energy level in the carbon-12 nucleus that allows the reaction to proceed efficiently.8Astroparticle Physics. Sensitivity of carbon and oxygen yields to the triple-alpha resonance in massive stars Without that resonance, carbon would be vanishingly rare, and life as we know it would not exist.
Helium burning is a pivotal stage in stellar evolution. It marks the transition from a hydrogen-dominated energy source to one that begins assembling the middle-weight elements. Once carbon forms, further reactions can produce oxygen, neon, and beyond.9Physics Letters B. Revisiting the 3α reaction rates in helium burning stars The most massive stars, those more than about eight times the Sun’s mass, progress through successive fusion stages, building silicon, sulfur, and eventually iron in their cores. Iron is the endpoint for fusion that releases energy; beyond iron, the star can no longer power itself through fusion alone.
The elements heavier than iron require a different mechanism. They are built primarily through neutron capture, where existing nuclei absorb free neutrons one at a time and gradually become heavier through radioactive decay. There are two main channels for this. The slow process occurs in the outer layers of aging giant stars over thousands of years, building elements like barium and lead.10Astronomy & Astrophysics. Impact of T- and ρ-dependent decay rates and new (n,γ) cross-sections on the s process in low-mass asymptotic giant branch stars The rapid process happens in extreme environments like neutron star mergers, where an intense flood of neutrons builds the heaviest elements, including gold, platinum, and uranium, in seconds. The 2017 detection of a neutron star merger provided the first direct evidence confirming these collisions as major production sites for heavy elements.11Monthly Notices of the Royal Astronomical Society. Neutron star mergers as the dominant contributor to the production of heavy r-process elements
Even with all this nuclear alchemy, the heavier elements remain trace ingredients. A massive star at the end of its life might have an iron core surrounded by onion-like layers of silicon, oxygen, carbon, helium, and hydrogen, but the outermost layers are still predominantly hydrogen and helium. The heavy elements produced over millions of years of fusion represent a tiny fraction of the star’s total mass.
Why Astronomers Call Everything a “Metal”
One of the more confusing conventions in astronomy is the use of the word “metal.” In everyday chemistry, metals are things like iron, copper, and aluminum. In astronomy, “metal” means every element heavier than helium. Carbon is a metal. Oxygen is a metal. Neon is a metal. This terminology exists because, from an astronomer’s perspective, the important distinction is between the primordial elements (hydrogen and helium) and everything that was forged later inside stars.
A star’s “metallicity” is simply the fraction of its mass that consists of these heavier elements. The Sun’s metallicity, measured through helioseismology, falls in the range of roughly 1.2–1.5% of its total mass in the convective envelope.2Astronomy & Astrophysics. Helioseismic determination of the solar metal mass fraction That figure might sound negligible, but it has outsized effects on a star’s opacity (how easily light passes through its material), its internal temperature structure, and its evolution. Stellar metallicity also serves as a direct measure of how much of the heavier elements a galaxy has accumulated, since most of those elements reside in stars.12EDP Sciences (Astronomy & Astrophysics). Stellar metallicity of star-forming galaxies at z ~ 3
The First Stars Had Almost No Heavy Elements
The very first generation of stars, called Population III stars, formed from gas that was essentially pure hydrogen and helium with tiny traces of lithium. No previous stars had existed to inject heavier elements into the gas. These primordial stars are thought to have been extremely massive and short-lived, burning through their fuel in a few million years before exploding as supernovae and seeding their surroundings with the first batch of carbon, oxygen, iron, and other heavy elements.13Annual Review of Astronomy and Astrophysics. The First Stars: Formation, Properties, and Impact
No Population III star has ever been directly observed, which is expected since none should have survived to the present day. But astronomers have found the next best thing: extremely old, low-mass stars in our galaxy’s halo that formed from gas barely contaminated by the first supernovae. These stars have fantastically low metallicities. A recently identified red giant associated with the halo of the Large Magellanic Cloud has an inferred metallicity below about one fifty-thousandth of the Sun’s value.14Digital Journal of Science. Stellar Evolution Across Cosmic Time: From Cosmic Dawn to Present Day Chemical Fossils Stars like this are chemical fossils, their compositions record what the universe looked like just after the first round of stellar enrichment.
Among these ancient stars, a curious subgroup stands out: carbon-enhanced metal-poor (CEMP) stars. These have unusually high ratios of carbon relative to iron, which may reflect the peculiar chemical yields of the first stellar generations. Understanding what produced that carbon excess is an active area of research, with observations spanning the galactic halo and nearby dwarf galaxies providing clues.15Astronomische Nachrichten. Carbon-enhanced metal-poor stars in different environments
How Composition Changes When a Star Dies
A star’s composition at birth is not the same as its composition at death, and the endpoint matters for what gets returned to the cosmos. When a sun-like star exhausts its fuel, it sheds its outer layers as a planetary nebula, returning hydrogen, helium, and newly synthesized carbon and nitrogen to interstellar space. What remains is a white dwarf, a dense remnant roughly the size of Earth but with a mass comparable to the Sun’s. White dwarfs are typically composed of carbon and oxygen, the products of helium burning, surrounded by thin veneers of helium and sometimes hydrogen.
Some white dwarfs show unexpected surface compositions. A class called DQ white dwarfs displays carbon and sometimes nitrogen dredged up from deeper layers. In at least one well-studied case, the carbon-to-nitrogen ratio in a DQ white dwarf’s atmosphere suggests that its progenitor star may have ended helium burning prematurely, leaving unprocessed material in or near the core.16The Astrophysical Journal Letters. THE CORE COMPOSITION OF A WHITE DWARF IN A CLOSE DOUBLE-DEGENERATE SYSTEM Cases like this are valuable because they let astronomers peer into the internal chemistry of stars that have already died.
More massive stars end far more dramatically. When they can no longer sustain fusion, their iron cores collapse in a fraction of a second, triggering a supernova explosion that flings most of the star’s material outward. That ejecta enriches the surrounding gas with all the elements the star manufactured during its lifetime, from carbon through iron and beyond. The heaviest stars may leave behind neutron stars or black holes, objects where the original chemical identity of the matter has been crushed or hidden behind an event horizon.
Brown Dwarfs and the Lower Boundary of “Star Stuff”
Not every object that forms from a collapsing gas cloud qualifies as a star. Brown dwarfs occupy the gap between the heaviest planets and the lightest true stars. They form with roughly the same hydrogen-and-helium-dominated composition as stars, inheriting approximately protosolar abundances from their birth clouds. The difference is mass: brown dwarfs are not heavy enough to sustain the hydrogen fusion that defines a true star.
They can, however, briefly fuse deuterium, a heavier form of hydrogen. Objects above roughly 13 times Jupiter’s mass will burn through most of their deuterium within about 20 million years, while those below about 11 Jupiter masses never fuse any of it, retaining their primordial deuterium abundances indefinitely.17The Astrophysical Journal Letters. Measuring the D/H Ratios of Exoplanets and Brown Dwarfs Measuring the deuterium-to-hydrogen ratio in a brown dwarf’s atmosphere can therefore reveal whether it ever crossed the threshold for deuterium burning, giving astronomers a way to weigh these dim objects indirectly. The composition of brown dwarfs is interesting precisely because it is so close to stellar: same ingredients, just not enough gravitational squeeze to ignite the main event.
Why a Star’s Metal Content Shapes the Planets Around It
A star’s composition is not just an academic curiosity; it has real consequences for the planetary systems that form alongside it. Stars condense out of the same disk of gas and dust that eventually spawns planets, so the chemical inventory of the star reflects the raw materials available for planet building. One of the most robust findings in exoplanet science is that gas giant planets are preferentially found around metal-rich stars, while smaller rocky planets and sub-Neptunes appear across a much broader range of stellar metallicities.18Scholarly Review Journal. Host Star Metallicity Across Different Exoplanet Populations: A Statistical Analysis Using LAMOST Spectroscopic Data
The connection makes intuitive sense. A higher fraction of heavy elements in the protoplanetary disk means more solid material available to build planet cores. Once a core grows large enough, it can gravitationally capture the surrounding hydrogen and helium gas and balloon into a gas giant. Population synthesis models that simulate planet formation across a range of stellar metallicities reproduce this trend and reveal additional patterns: close-in planets with short orbital periods tend to orbit more metal-rich stars, and the eccentricities of both giant and small planets increase with the host star’s metal content, likely because metal-rich systems produce more giant planets whose gravitational interactions stir up the orbits of their neighbors.19Astronomy & Astrophysics. The New Generation Planetary Population Synthesis (NGPPS)
An interesting wrinkle is that the smallest sub-Earth-sized planets actually show the opposite pattern, becoming less common around metal-rich stars. One explanation is that higher metallicity tends to produce bigger planetary building blocks, pushing the resulting planets into larger size categories and depleting the population of the very smallest worlds. The relationship between stellar composition and planetary architecture is complex enough that it remains one of the most active areas in planetary science, with each new survey adding resolution to the picture.