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Look up at the night sky and almost everything you see — the stars, the glowing nebulae, the spaces between them — is made mostly of just two elements. Hydrogen and helium together account for roughly 98% of all ordinary (baryonic) matter in the universe by mass. Every other element, from the carbon in your cells to the iron in your blood, shares the remaining 2%.
The top 10
These are approximate mass fractions for ordinary matter in our galaxy, based on the composition of the Sun and similar stars. Exact figures vary between sources, but the order is well established.
| Rank | Element | Approximate share by mass |
|---|---|---|
| 1 | Hydrogen | ~74% |
| 2 | Helium | ~24% |
| 3 | Oxygen | ~1% |
| 4 | Carbon | ~0.5% |
| 5 | Neon | ~0.1% |
| 6 | Iron | ~0.1% |
| 7 | Nitrogen | ~0.1% |
| 8 | Silicon | ~0.07% |
| 9 | Magnesium | ~0.06% |
| 10 | Sulfur | ~0.04% |
(Ordinary matter is itself only about 5% of the universe’s total energy content; the rest is dark matter and dark energy, which aren’t made of elements at all.)
Act 1: the Big Bang made the lightest elements
In the first few minutes after the Big Bang, the universe was hot and dense enough for nuclear fusion. Protons and neutrons combined to make hydrogen, helium and a trace of lithium. Then the universe expanded and cooled too quickly for fusion to continue. This process, called Big Bang nucleosynthesis, left the universe about three-quarters hydrogen and one-quarter helium by mass — which is almost exactly what astronomers observe in the oldest stars and gas clouds today. That match is one of the strongest pieces of evidence for the Big Bang.
Act 2: stars forge the middle of the table
Stars are nuclear furnaces. For most of their lives they fuse hydrogen into helium. In larger stars, as the core runs out of fuel and contracts, it heats up enough to fuse heavier elements in stages:
- helium → carbon and oxygen
- carbon → neon, sodium, magnesium
- oxygen → silicon, sulfur
- silicon → elements up to iron and nickel
Iron is where it stops. Iron-56 and nickel-62 have among the most tightly bound nuclei of all. Fusing them into anything heavier absorbs energy rather than releasing it, so a star can’t power itself that way. When a massive star’s core fills with iron, it collapses, triggering a supernova.
This is why iron is unusually abundant for a heavy element: it’s the end point of stellar fusion. You can see a peak at iron in any graph of cosmic abundances.
Act 3: explosions and collisions make the heaviest elements
Elements heavier than iron are mostly built by neutron capture: nuclei absorb neutrons and then beta-decay into heavier elements.
- The s-process (slow) happens over thousands of years in ageing giant stars.
- The r-process (rapid) needs an enormous flood of neutrons in seconds. One confirmed site is the collision of two neutron stars: in 2017, astronomers detected gravitational waves from such a merger and saw the light of freshly made heavy elements. Some supernovae may contribute too.
Much of the universe’s gold, platinum and uranium was made this way.
Why odd numbers are rarer
Graphs of cosmic abundance have a saw-tooth pattern: elements with even atomic numbers are generally more abundant than their odd-numbered neighbours. This is the Oddo–Harkins rule. Nuclei with even numbers of protons are more tightly bound, so they’re produced more readily and survive better.
Earth is very different
Our planet lost most of its hydrogen and helium early on — they’re light and escaped into space. So Earth’s composition looks nothing like the universe’s. By mass the whole Earth is dominated by iron (much of it in the core), oxygen, silicon and magnesium. See the most abundant elements in Earth’s crust and the elements in the human body.
Quick answers
What is the most abundant element in the universe? Hydrogen — about three-quarters of all ordinary matter by mass, and around 90% of all atoms.
What is the second most abundant element? Helium.
Are we really made of stardust? Yes. Apart from hydrogen (from the Big Bang), nearly every atom in your body — the carbon, oxygen, nitrogen, calcium, iron — was made inside stars that lived and died before the Sun formed.
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