The Most Abundant Elements in the Universe: Why the Cosmos Is Mostly Hydrogen and Helium

If you took every atom in the observable universe and sorted them by element, roughly 9 out of every 10 would be hydrogen. Almost all of the remaining one would be helium. Every other element on the periodic table — every carbon, oxygen, iron, gold, and uranium atom — would fit into that last fraction of a percent by number. By mass, hydrogen accounts for approximately 74% and helium for roughly 24% of all ordinary matter that has ever existed. The entire remaining periodic table shares about 2%. This ratio was not produced by stars: it was set in the first three minutes of the universe's existence, when the conditions were hot and dense enough for nuclear fusion to occur — once, everywhere, all at once.

What "Abundance" Means and Why It Varies by Context

Before comparing numbers, the unit of measurement matters. There are three different ways to express elemental abundance, and they give different answers:

By number of atoms: hydrogen is about 92% of all atoms in the universe, helium about 8%, everything else under 0.1%. This counting method weights all atoms equally regardless of mass.

By mass fraction: hydrogen is about 74%, helium about 24%, everything else about 2%. Because helium atoms weigh four times as much as hydrogen atoms, helium's mass fraction is higher than its atom-fraction. Heavy elements like iron, even in trace quantities by number, contribute slightly more to the mass fraction.

By number in Earth's crust, ocean, or atmosphere: completely different from cosmic abundance. Earth has been geochemically processed: light volatile elements like hydrogen and helium mostly escaped or were incorporated into compounds; heavy elements concentrated through differentiation, volcanism, and plate tectonics. The most abundant element in Earth's crust by mass is oxygen (about 46%), followed by silicon (about 28%).

Big Bang nucleosynthesis (BBN) — the process that occurred in the first ~3 minutes of the universe's existence, when the temperature fell from about 10¹² K to about 10⁹ K (1 billion K), allowing protons and neutrons to fuse into light nuclei: mostly helium-4, with trace amounts of deuterium, helium-3, and lithium-7. This process set the universe's initial hydrogen-to-helium ratio.

Source: Coc, A. et al. (2014), "Standard Big Bang Nucleosynthesis up to CNO with an improved extended nuclear network," The Astrophysical Journal, Vol. 808 — computational BBN model giving predicted elemental abundances consistent with observed cosmological ratios.

The Cosmic Abundance Ranking: Top 10 Elements in the Universe

Rank	Element	Symbol	% by Mass (Universe)	% by Number of Atoms	Primary Source
1	Hydrogen	H	~74%	~92%	Big Bang nucleosynthesis
2	Helium	He	~24%	~7.9%	Big Bang nucleosynthesis
3	Oxygen	O	~1.0%	~0.06%	Stellar nucleosynthesis (CNO cycle)
4	Carbon	C	~0.5%	~0.04%	Triple-alpha process in stars
5	Neon	Ne	~0.13%	~0.007%	Carbon-burning in massive stars
6	Iron	Fe	~0.11%	~0.002%	Silicon burning; most stable nucleus
7	Nitrogen	N	~0.10%	~0.007%	CNO cycle in stars
8	Silicon	Si	~0.07%	~0.003%	Oxygen-burning in massive stars
9	Magnesium	Mg	~0.06%	~0.003%	Neon burning in massive stars
10	Sulfur	S	~0.04%	~0.002%	Oxygen-burning in massive stars

Note: percentages for elements 3-10 are approximate and vary by source; the exact figures depend on the reference solar system composition used. The key point — hydrogen and helium together exceeding 98% — is robust across all measurements.

Source: Asplund, M. et al. (2009), "The Chemical Composition of the Sun," Annual Review of Astronomy and Astrophysics, Vol. 47 — the standard reference for solar and proto-solar elemental abundances, used as a proxy for the composition of ordinary matter in the universe.

Why the Universe Is Almost Entirely Hydrogen and Helium: Big Bang Nucleosynthesis

The universe began hot. Extremely hot: the Planck epoch reached temperatures around 10³² K, and even one second after the Big Bang, the temperature was approximately 10¹⁰ K (10 billion K). At these temperatures, matter exists as a plasma of quarks, gluons, and other fundamental particles, not as atoms or nuclei.

As the universe expanded, it cooled. By about one second after the Big Bang, the temperature had fallen to roughly 10 billion K — cool enough for quarks to combine into protons and neutrons. The ratio of protons to neutrons at this point was determined by the rates of weak-force reactions (which interconvert protons and neutrons) versus the expansion rate of the universe; the result was approximately seven protons for every neutron.

By about 3 minutes after the Big Bang, the temperature had fallen to approximately 10⁹ K (1 billion K). At this temperature, nuclear fusion can occur — but only barely. Protons and neutrons began combining to form deuterium (a proton plus a neutron), and deuterium quickly fused further into helium-4 (two protons, two neutrons). This phase, called Big Bang nucleosynthesis, lasted only about 17 minutes before the temperature fell too low for further fusion.

The arithmetic of the proton-to-neutron ratio determines the helium fraction. With seven protons for every neutron, and with essentially all neutrons rapidly incorporated into helium-4 (which is extremely stable), the result is:

• 14 protons + 2 neutrons → 1 helium-4 nucleus (using both neutrons) + 12 leftover protons (which become hydrogen nuclei)
• By number of nuclei: 1 He-4 per 12 H → about 7.7% He, 92.3% H (by number)
• By mass: each He-4 has mass 4, each H has mass 1 → (1×4) / (1×4 + 12×1) = 4/16 = 25% He by mass

The observed helium-4 abundance in old, metal-poor stars and in the interstellar gas of dwarf galaxies (which have experienced minimal stellar processing) is measured at approximately 23–25% by mass — in excellent agreement with this calculation. This agreement is one of the three classic observational pillars of the Big Bang model, alongside the cosmic microwave background and the Hubble expansion.

Source: Olive, K.A., Steigman, G., and Walker, T.P. (2000), "Primordial nucleosynthesis: Theory and observations," Physics Reports, Vol. 333 — comprehensive review of BBN predictions and their observational verification.

Why Helium-4 Is Stable and Everything Else Is Rare

The reason the universe stopped at helium rather than building heavier elements during the Big Bang comes down to nuclear physics. There is no stable nucleus with mass number 5 (five nucleons) — every combination of three protons and two neutrons, or two protons and three neutrons, is unstable and decays almost instantaneously. To build carbon (mass 6 or 12) requires a two-step process that only works efficiently at the high densities and sustained temperatures found inside stars, not in the rapidly cooling, expanding early universe.

This gap at mass 5 and mass 8 (there is also no stable beryllium-8, which decays in approximately 10⁻¹⁷ seconds) is called the mass-5 and mass-8 bottleneck in nucleosynthesis. It is the reason hydrogen and helium are cosmically dominant and all heavier elements are trace constituents.

Mass-5 bottleneck — the absence of any stable or long-lived atomic nucleus with 5 nucleons (5 protons+neutrons combined). When helium-4 absorbs a proton or neutron to become mass-5, the resulting nucleus decays in approximately 10⁻²² seconds. This means the Big Bang could not build elements heavier than helium by sequential single-nucleon addition; a different process (the triple-alpha process in stars) was required.

The solution in stars — discovered by Fred Hoyle in 1954 — is the triple-alpha process: three helium-4 nuclei collide simultaneously to form carbon-12, skipping over the unstable mass-5 and mass-8 intermediates. This process only operates efficiently at stellar core temperatures above about 100 million K and requires the high density of a stellar interior. The universe in its first minutes was cooling too fast for this to occur at the necessary scale.

Source: Hoyle, F. (1954), "On Nuclear Reactions Occurring in Very Hot Stars: The Synthesis of Elements from Carbon to Nickel," The Astrophysical Journal Supplement Series, Vol. 1 — the paper predicting the triple-alpha process and the Hoyle state of carbon-12, a landmark in nuclear astrophysics.

How Stars Build the Heavier Elements

The 2% of the universe that is not hydrogen or helium was manufactured inside stars and distributed by their deaths. The process is called stellar nucleosynthesis, and it follows a sequence determined by nuclear binding energy.

In a star like the Sun, the primary fusion reaction is the proton-proton chain: four hydrogen nuclei fuse sequentially to produce one helium-4 nucleus, releasing energy as the mass difference converts to photons and neutrinos. More massive stars use the CNO cycle (carbon-nitrogen-oxygen cycle), which is more efficient at higher temperatures and produces the same net result (four H → one He-4) but uses carbon as a catalyst, producing oxygen and nitrogen as byproducts.

When the hydrogen in a massive star's core is exhausted, the core contracts and heats until helium fusion ignites (the triple-alpha process), producing carbon. As each fuel is exhausted, the core contracts further and the next fuel ignites:

• Hydrogen burning (10–100 million K) → helium
• Helium burning (100–200 million K) → carbon and oxygen
• Carbon burning (~600 million K) → neon, sodium, magnesium
• Neon burning (~1.2 billion K) → oxygen, magnesium
• Oxygen burning (~1.5–2.2 billion K) → silicon, sulfur, phosphorus
• Silicon burning (~3–7 billion K) → iron and nearby elements

Iron-56 is the endpoint. It has the highest binding energy per nucleon of any nucleus — it is, in a meaningful sense, the most stable nucleus that exists. Fusing iron consumes energy rather than releasing it; there is no energy gain from building nuclei heavier than iron by fusion. A massive star's core collapses when it accumulates enough iron to exceed the Chandrasekhar mass (approximately 1.4 times the Sun's mass), triggering a core-collapse supernova.

Binding energy per nucleon — the energy required to completely disassemble a nucleus into its constituent protons and neutrons, divided by the number of nucleons. Iron-56 has the highest binding energy per nucleon of any nucleus at approximately 8.79 MeV. Nuclei lighter than iron release energy by fusion (protons "roll downhill" toward iron); nuclei heavier than iron release energy by fission.

Source: Burbidge, E.M., Burbidge, G.R., Fowler, W.A., and Hoyle, F. (1957), "Synthesis of the Elements in Stars," Reviews of Modern Physics, Vol. 29 — the foundational "B²FH" paper establishing the stellar nucleosynthesis framework for all elements heavier than helium.

Elements Heavier Than Iron: Supernovae and Neutron Star Mergers

Iron-group elements (iron, nickel, cobalt) are the heaviest nuclei that stellar fusion produces. Everything heavier — copper, zinc, silver, gold, platinum, uranium — requires a different process.

Two major processes build the heaviest elements:

The s-process (slow neutron capture): In asymptotic giant branch (AGB) stars — red giant stars nearing the end of their lives — neutrons are released by minor fusion reactions and slowly absorbed by iron and nearby nuclei, one neutron at a time over thousands of years. This builds elements up to bismuth (atomic number 83), the heaviest stable element.

The r-process (rapid neutron capture): In environments with extreme neutron fluxes — formerly thought to be only core-collapse supernovae, but now confirmed to occur in neutron star mergers — nuclei absorb neutrons far faster than they can decay, building all the way to uranium (atomic number 92) and beyond. The 2017 detection of gravitational waves from a neutron star merger (GW170817) was followed by optical observations of a kilonova — a flash of light dominated by the emission from freshly synthesized r-process elements including gold and platinum. The observation confirmed that neutron star mergers are a primary site of r-process nucleosynthesis.

A single neutron star merger event is estimated to produce several Earth masses of gold and platinum, distributed into the interstellar medium when the explosion expands. All the gold on Earth — in rings, in electronics, in bank vaults — was forged in neutron star mergers or possibly in supernovae, billions of years before the Solar System formed.

Source: Abbott, B.P. et al. (2017), "Multi-messenger Observations of a Binary Neutron Star Merger," The Astrophysical Journal Letters, Vol. 848 — the GW170817 paper confirming r-process nucleosynthesis in a neutron star merger, directly detecting heavy element production.

Earth's Crust vs. the Cosmos: A Tale of Two Abundances

Earth's elemental composition is almost unrecognizable compared to the cosmic average, because the planet has been through 4.5 billion years of geological processing that radically redistributed elements.

Element	Cosmic Abundance (% by mass)	Earth's Crust (% by mass)	Why the Difference
Hydrogen	~74%	~0.15%	Volatile; mostly in oceans/atmosphere, lost to space
Helium	~24%	trace	Noble gas; not bound in rocks; escaped to space
Oxygen	~1%	~46%	Concentrated by planetary differentiation; forms minerals
Silicon	~0.07%	~28%	Forms silicate minerals; concentrated in crust
Iron	~0.11%	~5.6% (crust) / ~32% (whole Earth)	Dense; sank to core during planetary formation
Carbon	~0.5%	~0.02%	Mostly in atmosphere, oceans, and sedimentary rock

The divergence has two primary causes. First, planetary differentiation: when Earth was molten early in its history, dense elements (iron, nickel) sank toward the core, while lighter elements (silicon, oxygen, aluminum) floated toward the surface. Earth's core is estimated at about 85% iron and nickel; its mantle is dominated by magnesium, iron, and silicon oxides; its crust is dominated by silicon, aluminum, and oxygen.

Second, volatile loss: hydrogen and helium are light enough that solar radiation pressure and the solar wind can strip them from a rocky planet's atmosphere if the planet is too warm and too small. Earth retained its hydrogen mainly in water molecules (which are heavy enough to be gravitationally bound) and hydrogen-containing minerals. Helium, a noble gas that doesn't form compounds, escapes to space continuously and is not retained.

Source: Ringwood, A.E. (1966), "The chemical composition and origin of the Earth," in Advances in Earth Sciences, MIT Press — the foundational paper on Earth's chemical structure from core to crust and the differentiation process.

The Rarest Naturally Occurring Elements

At the other extreme from hydrogen, some elements are so scarce on Earth that they barely exist at all.

Astatine (At, element 85) is the rarest naturally occurring element in Earth's crust. It exists only as a transient product of uranium and thorium decay chains — a few atoms produced and then quickly decayed. The most abundant isotope, astatine-210, has a half-life of only 8.1 hours. At any given moment, an estimated total of fewer than 28 grams of astatine exists in the entire Earth's crust. No bulk sample has ever been collected; all scientific work with astatine uses atoms produced in particle accelerators, one batch at a time.

Francium (Fr, element 87) is the rarest naturally occurring metal and the second-rarest element overall. It exists in the Earth's crust in quantities of approximately 20–30 grams at any moment. Francium-223, the most stable isotope found in nature, has a half-life of only 22 minutes. Like astatine, it is a decay product of actinium in the uranium-235 chain.

Protactinium (Pa, element 91) is somewhat more abundant than astatine or francium but still extremely scarce: estimated at roughly 70 grams in the entire crust. It is a decay product of uranium-235 and was the last naturally occurring element to be discovered, in 1913.

The scarcity of these elements has nothing to do with how the universe makes them — they are produced by the same nucleosynthetic processes that build all heavy elements. Their rarity on Earth is entirely due to radioactive decay: they are unstable isotopes with short enough half-lives that any atoms present when Earth formed have long since decayed away. The only atoms that exist today are continuously produced and continuously lost in radioactive decay chains.

Source: Holden, N.E. (2004), "Table of the Isotopes," in CRC Handbook of Chemistry and Physics, 85th edition, CRC Press — isotope half-life data for astatine-210, francium-223, and protactinium-231.

The Sun as a Sample of Cosmic Abundance

The best available measurement of the cosmic elemental abundance comes not from distant galaxies but from the Sun. Astronomers study sunlight with spectrographs — instruments that spread the light into its component wavelengths. Each element absorbs light at characteristic wavelengths (its spectral fingerprint), and the depth of each absorption line in the solar spectrum is proportional to the amount of that element in the Sun's atmosphere.

This spectroscopic analysis gives the abundances in the solar photosphere. For most elements, these photospheric abundances agree very well with the composition of the most primitive meteorites (CI carbonaceous chondrites) that have experienced minimal chemical alteration since the Solar System formed 4.6 billion years ago. Together, photospheric spectroscopy and meteorite geochemistry provide the standard solar system composition used in astrophysics.

The latest comprehensive analysis, published by Asplund and colleagues in 2009 and updated in subsequent papers, gives hydrogen at approximately 91.2% by number of atoms and helium at about 8.8% by number — with all other elements accounting for less than 0.2% of atoms. The "metallicity" of the Sun (the astronomer's term for the fraction of everything that is not hydrogen or helium) is approximately 1.3–1.4% by mass.

Spectroscopy — the study of the interaction between matter and electromagnetic radiation, used to determine the composition of stars, nebulae, and galaxies. Each element absorbs and emits light at specific wavelengths determined by its electron energy level structure, producing a unique spectral signature.

Source: Asplund, M., Grevesse, N., Sauval, A.J., and Scott, P. (2009), "The Chemical Composition of the Sun," Annual Review of Astronomy and Astrophysics, Vol. 47 — the standard modern reference for solar elemental abundances from photospheric spectroscopy.

The Universe's Future: Running Low on Hydrogen

The dominance of hydrogen is finite. Stars convert hydrogen to helium (and eventually to heavier elements) continuously, and the universe's hydrogen supply is depleting on cosmological timescales. The Sun, for example, converts approximately 600 million tonnes of hydrogen into helium every second through fusion, losing about 4 million tonnes of mass as energy in the process.

In roughly 5 billion years, the Sun will exhaust the hydrogen in its core. More massive stars, which burn faster, are already running low in galaxies that are vigorously star-forming. The universe's total star formation rate peaked about 10 billion years ago and has been declining since; the available hydrogen is gradually being locked into white dwarfs, neutron stars, and stellar black holes.

In the far future — on timescales of 10¹⁴ to 10¹⁵ years — the last low-mass red dwarf stars will exhaust their hydrogen and fade. After that, the universe will contain no hydrogen-burning stars, only cooling stellar remnants and, presumably, dark matter. The era of hydrogen-fueled cosmic chemistry will be over.

Source: Loeb, A., Batista, R.A., and Sloan, D. (2016), "Relative likelihood for life as a function of cosmic time," Journal of Cosmology and Astroparticle Physics, Vol. 2016 — analysis of cosmic star formation history and the long-term availability of hydrogen as stellar fuel.

Frequently Asked Questions

Q: Why is hydrogen so much more abundant than everything else in the universe?
A: Hydrogen is the simplest atom — one proton and one electron — and it was produced in overwhelming quantities during Big Bang nucleosynthesis in the first few minutes of the universe. The conditions of the early universe (temperature falling from ~10¹² K to ~10⁹ K) were exactly right to fuse about 25% of hydrogen into helium-4, but not hot or dense enough for long enough to build significant amounts of heavier nuclei. The mass-5 bottleneck (no stable nucleus with 5 nucleons exists) blocked any further buildup. The result: roughly 75% hydrogen and 25% helium, by mass, from the start.

Q: If the universe is 74% hydrogen, why is Earth's crust mostly oxygen?
A: Earth's composition diverged from cosmic average through two processes. First, planetary differentiation: when Earth was molten early in its history, dense materials (iron, nickel) sank to form the core while lighter silicate minerals (oxygen-silicon compounds) concentrated in the crust and mantle. Second, volatile loss: hydrogen is light and mostly escaped Earth's early atmosphere; what remains is locked in water and minerals. Helium is a noble gas that doesn't form compounds and continuously escapes to space. Oxygen, by contrast, bonds strongly with silicon, aluminum, and other abundant elements into stable minerals.

Q: What is stellar nucleosynthesis?
A: Stellar nucleosynthesis is the process by which stars fuse lighter elements into heavier ones inside their cores. Hydrogen fuses to helium; helium fuses to carbon (via the triple-alpha process); carbon and heavier elements fuse in sequence in massive stars up to iron. When massive stars explode as supernovae, they distribute these heavy elements into the interstellar medium, enriching the gas clouds that form subsequent generations of stars and planets. Our Solar System formed from material that had been processed through multiple previous generations of stars.

Q: Which element is rarest on Earth?
A: Astatine is the rarest naturally occurring element, with an estimated total of fewer than 28 grams present in Earth's entire crust at any given moment. It exists only as a transient product of uranium and thorium radioactive decay chains, and its most abundant natural isotope (astatine-210) has a half-life of only 8.1 hours, so any atoms produced are quickly gone. Francium is the second rarest, with an estimated 20–30 grams in the crust; its most stable natural isotope (francium-223) has a half-life of only 22 minutes.

Q: Where did the gold on Earth come from?
A: Gold and other elements heavier than iron were not made in ordinary stellar fusion — fusing iron requires energy rather than releasing it. Gold is produced primarily by the r-process (rapid neutron capture), which occurs in environments with extreme neutron fluxes: neutron star mergers and possibly certain types of supernovae. The 2017 gravitational wave event GW170817 confirmed that a neutron star merger produced detectable quantities of gold and platinum. All the gold on Earth was made in such events billions of years before the Solar System formed and was incorporated into the cloud of gas and dust that became the Sun and planets.

Q: Is dark matter made of these elements?
A: No. The abundances described in this article are for ordinary (baryonic) matter — matter made of protons, neutrons, and electrons, the stuff of atoms and the periodic table. Dark matter is thought to account for roughly 27% of the total energy content of the universe, compared to ordinary matter's 5%. Dark matter does not interact with light and has not been directly detected; its composition is unknown, but it is not hydrogen, helium, or any other element in the periodic table. The abundances listed here describe only the baryonic component of the universe.

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