Stars are overwhelmingly made of hydrogen and helium in the form of plasma, a superheated state of matter in which atoms have been stripped of their electrons. Hydrogen alone accounts for roughly three-quarters of a typical star’s mass, with helium making up most of the remainder. The smattering of heavier elements mixed in, everything from carbon and oxygen to iron and beyond, amounts to only about two percent by mass in a star like the Sun, yet that small fraction drives much of what makes stars interesting and what makes planets, and people, possible.
Why Stellar Matter Is Plasma, Not Gas
At the temperatures inside and on the surface of a star, atoms cannot hold on to their electrons. The Sun’s surface sits at roughly 5,500 °C, hot enough to strip one or more electrons from most atoms. Move inward and temperatures climb into the millions and then tens of millions of degrees, where even tightly bound electrons are torn free. The result is plasma: a soup of bare atomic nuclei and free-roaming electrons that behaves differently from an ordinary gas. Plasma conducts electricity, responds to magnetic fields, and transmits light in ways that a neutral gas does not.
The physics of this plasma gets complicated in the deep interiors of stars, where densities are extreme. Under those conditions, the electric fields of surrounding particles actually lower the energy needed to ionize an atom further, a phenomenon called ionization-potential depression. Modeling this accurately matters for understanding how energy moves through a star. Research on dense carbon plasma under conditions found inside the Sun shows that different theoretical models diverge at high densities, and the predicted degree of ionization depends on which screening effects you account for.
1Astronomy & Astrophysics. Ionization potential depression and ionization balance in dense carbon plasma under solar and stellar interior conditionsThe Sun’s Elemental Recipe
Because the Sun is the star we can study most closely, it serves as the benchmark for stellar composition. Astronomers measure the abundances of elements in the Sun’s visible surface layer, called the photosphere, by analyzing how different elements absorb specific wavelengths of light. Those photospheric measurements are then cross-checked against the compositions of a rare class of ancient meteorites, called CI-chondrites, which are thought to preserve the primordial material of the early solar system. Updated reviews of both datasets continue to refine our best estimates of what the solar system, and by extension a star like the Sun, is made of.
2Space Science Reviews. Solar System Elemental Abundances from the Solar Photosphere and CI-ChondritesThe broad picture is stable: after hydrogen and helium, the most abundant elements in the Sun are oxygen, carbon, neon, nitrogen, and iron. But pinning down exact ratios at the level of precision modern astrophysics demands turns out to be surprisingly contentious. Revised solar oxygen and carbon abundances published in recent decades sparked what is sometimes called the “solar abundance crisis,” because the lower values created mismatches with models of the Sun’s interior structure. That debate remains active.
There is also a wrinkle with the Sun’s outer atmosphere. The corona, the tenuous, superheated halo visible during eclipses, does not have the same mix of elements as the photosphere beneath it. Elements with a low first ionization potential, such as iron, magnesium, silicon, and calcium, are overabundant in the corona relative to higher-ionization-potential elements like oxygen, neon, and sulfur, by factors of roughly three to ten.
3PubMed. The Elemental Composition of the Corona of Procyon: Evidence for the Absence of the FIP EffectThis so-called FIP effect appears to be a transport phenomenon, where certain ions are preferentially lofted into the corona by magnetic and wave processes, rather than a sign that the Sun’s bulk composition varies with altitude. Interestingly, not every star shows this effect. Observations of the star Procyon, for example, found no such enrichment, suggesting the process depends on the specifics of a star’s magnetic activity.
How Fusion Rewrites a Star’s Interior Over Time
A star does not keep the same composition throughout its life. Nuclear fusion in the core steadily converts lighter elements into heavier ones. For most of a star’s lifetime, it fuses hydrogen into helium. Once the hydrogen fuel in the core runs low, the core contracts and heats up until helium itself begins fusing into carbon and oxygen. In stars much more massive than the Sun, this process continues through additional stages, building neon, silicon, and eventually iron.
4American Journal of Student Research. Black Hole Formation from Massive Star Collapse: A Review of Physical Mechanisms and Observational EvidenceEach new fusion stage runs faster and at higher temperatures than the last. A massive star might spend millions of years burning hydrogen but only days burning silicon. The result is a layered structure, sometimes described as resembling an onion, with the heaviest elements concentrated in the core and progressively lighter elements in the outer shells. These composition gradients between a star’s core and its outer envelope are a direct product of nuclear burning.
5Publications of the Astronomical Society of the Pacific. The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB StarsIron is where the fusion chain hits a wall. Fusing iron does not release energy; it consumes it. When a massive star builds up an iron core, that core can no longer support itself against gravity through thermal pressure or electron degeneracy pressure, and the star collapses.
4American Journal of Student Research. Black Hole Formation from Massive Star Collapse: A Review of Physical Mechanisms and Observational EvidenceThe ensuing supernova explosion scatters most of the star’s carefully layered elements into space.
The First Stars Were Almost Pure Hydrogen and Helium
The very first generation of stars, known as Population III stars, formed from the raw material left behind by the Big Bang. That material was almost entirely hydrogen and helium with trace amounts of lithium. There were no heavier elements because no stars had yet existed to forge them. Population III stars are, by definition, metal-free, and the clouds from which they formed are expected to have been of pristine, primordial composition.
6Monthly Notices of the Royal Astronomical Society. The challenges of identifying Population III stars in the early UniverseNo confirmed Population III star has been directly observed; they are thought to have been very massive and short-lived, burning through their fuel and exploding as supernovae early in cosmic history. But those explosions were consequential. The heavy elements they forged and scattered became the raw ingredients for the next generation of stars. After the Big Bang, the production of heavy elements in the early universe started from these first stellar explosions, which had strong effects on the formation and evolution of subsequent galaxies.
7Annual Reviews. Nucleosynthesis in Stars and the Chemical Enrichment of GalaxiesEach generation of stars inherited a slightly richer mix of elements from the deaths of its predecessors. Astronomers can actually read this history in the compositions of stars alive today. Very old stars in the halo of our galaxy have vanishingly small amounts of elements heavier than helium, while younger stars like the Sun contain enough heavy elements to form rocky planets. This gradual buildup is called galactic chemical evolution, and it means that a star’s elemental makeup is partly a timestamp, telling you roughly when and where it formed.
How Stars Populate the Periodic Table
Stars are not just made of elements; they are the factories that make most elements. Hydrogen fusion in Sun-like stars produces helium. Helium fusion in later stages produces carbon and oxygen. High-mass stars fuse elements much faster, build heavier nuclei, and die more catastrophically than low-mass stars. When they explode as supernovae, they release those elements into their surroundings, seeding the interstellar medium with everything from oxygen to iron.
8Science. Populating the periodic table: Nucleosynthesis of the elementsElements heavier than iron, like gold, platinum, and uranium, require a different pathway. Fusion cannot make them efficiently because the process would absorb rather than release energy. Instead, they are built by rapid neutron-capture processes, which occur in extreme environments like the collapse of a massive star’s core or the merger of two neutron stars. The relative contributions of supernovae versus neutron-star mergers to the heaviest elements have been debated for decades, and the 2017 detection of gravitational waves from a neutron-star merger, accompanied by the spectroscopic signature of freshly forged heavy elements, was a landmark confirmation that mergers play a major role.
The point is that the composition of any given star is not just a list of ingredients. It is a record of everything that happened in its cosmic neighborhood before it was born: how many previous stars lived and died, how massive they were, and what kinds of explosions they produced.
Stars That Have Lost Their Outer Layers
Not all stars show you their full composition on their surface. Some have been stripped down to deeper layers, revealing material that was once hidden inside. Wolf-Rayet stars are a dramatic example: these are evolved, helium-burning massive stars that have lost most of their hydrogen-rich outer envelopes, either through intense stellar winds or through interactions with a companion star in a binary system.
9The Astrophysical Journal. An Orbit for a Massive Wolf–Rayet Binary in the LMC: An Example of Binary EvolutionThe spectra of Wolf-Rayet stars look wildly different from those of ordinary stars. Instead of hydrogen absorption lines, they show strong emission from helium, nitrogen, carbon, or oxygen, depending on which burning products dominate the exposed layer. They are essentially stars turned inside out, giving astronomers a direct look at the products of nuclear burning that would normally be locked deep in a stellar core. These stars also eject tremendous amounts of processed material into space, contributing to the enrichment of the interstellar medium before they eventually explode.
Molecules in the Coolest Stars
At the other end of the temperature spectrum from Wolf-Rayet stars sit cool dwarf stars, where surface temperatures drop low enough for atoms to form molecules. In M, L, and T dwarfs, the coolest classes of stellar and sub-stellar objects, chemistry becomes genuinely complex. Titanium and vanadium, for example, form oxide and silicate compounds whose behavior depends on interactions with calcium and aluminum minerals. Refractory condensates like perovskite (a calcium-titanium oxide) form in these atmospheres, and their chemistry is more intricate than often assumed.
10IOPscience / American Astronomical Society. Titanium and Vanadium Chemistry in Low-Mass Dwarf StarsThis molecular chemistry matters because it controls what these stars look like. The presence or absence of certain molecules shapes the spectrum in ways that can mimic or mask the signatures of individual elements. A star might contain plenty of titanium, but if that titanium is locked up in condensate grains that have settled below the visible atmosphere, it vanishes from the spectrum. Getting stellar compositions right in cool objects requires understanding not just what elements are present, but what chemical forms they take.
What a Star Is Made Of Tells You About Its Planets
Stars and their planets form from the same disk of gas and dust, so you would expect a connection between a star’s composition and the makeup of its planets. That expectation holds up, at least in broad strokes. A study comparing the iron content of rocky exoplanets, estimated from their masses and radii, with the compositions of their host stars found a real correlation, though not a simple one-to-one match. Planets around higher-metallicity stars tend to span a wider range of iron masses, allowing for higher iron content.
11PubMed. A compositional link between rocky exoplanets and their host stars12Astronomy & Astrophysics. Quantitative correlation of refractory elemental abundances between rocky exoplanets and their host stars
The relationship gets more specific when you look at individual element ratios. The magnesium-to-silicon ratio of a planet tends to mirror that of its host star fairly closely, and this ratio controls whether the planet’s mantle is dominated by olivine or pyroxene, two families of silicate minerals with very different physical properties. Iron-to-magnesium ratios, meanwhile, serve as a useful indicator of how much a planet’s core separated from its mantle during formation.
13Monthly Notices of the Royal Astronomical Society. Detailed chemical compositions of planet-hosting stars – II. Exploration of the interiors of terrestrial-type exoplanetsSo when astronomers measure the composition of a star, they are also getting a rough blueprint for the kinds of rocky planets that star could host. A star with a high iron-to-silicon ratio might produce planets with proportionally larger metallic cores. A star with unusual abundances of refractory elements could host worlds with mineral assemblages unlike anything in our own solar system.
Dead Stars as Forensic Evidence for Planetary Composition
One of the more creative ways to study what planets are made of involves looking at dead stars. White dwarfs, the dense remnants left behind when Sun-like stars exhaust their fuel, should have extremely pure surfaces of hydrogen or helium because heavier elements quickly sink out of view in their intense gravity. When astronomers find heavy elements like calcium, magnesium, silicon, and iron polluting a white dwarf’s atmosphere, those elements almost certainly came from rocky bodies, asteroids or fragments of planets, that fell onto the star.
By measuring the ratios of those polluting elements, researchers can reconstruct the composition of the rocky material that was accreted. The minimum mass accreted in some of these systems is comparable to a large asteroid.
14The Astrophysical Journal. ROCKY EXTRASOLAR PLANETARY COMPOSITIONS DERIVED FROM EXTERNALLY POLLUTED WHITE DWARFSWhat makes this technique especially interesting is that it has revealed rocky compositions unlike anything in our solar system. Some polluted white dwarfs record accretion of material so chemically distinct from Earth or Mars that entirely new rock classification schemes are needed to describe the mineral assemblages those elements would form.
15Nature Communications. Polluted white dwarfs reveal exotic mantle rock types on exoplanets in our solar neighborhoodThis finding underscores a broader point about stellar composition and its consequences. The range of elemental mixtures seen in stars across the galaxy is wide enough that rocky planets forming around those stars could be made of mineral combinations with no analog on Earth. The geophysics of such worlds, their volcanism, plate tectonics, magnetic field generation, could be fundamentally different from what we know. Our one familiar example, a Sun-like star hosting an iron-cored, silicate-mantled Earth, sits somewhere in the middle of a much larger distribution of possibilities.