What Is the Most Abundant Element in the Universe?

The most abundant element in the universe is hydrogen. Hydrogen makes up 73–75% of all ordinary (baryonic) matter by mass. Helium is the second most abundant element at about 24–25%, while all other elements combined account for only about 1–2% of the universe.

Key Takeaways: Most Abundant Element in the Universe

• Hydrogen is the most abundant element in the universe.
• It makes up roughly three-quarters of all ordinary matter.
• Helium is second, formed during the Big Bang and in stars.
• All heavier elements together make up only about 1–2%.
• Element abundances vary slightly depending on location and measurement method.

Why Element Abundance Matters

Element abundance determines how stars form, how galaxies evolve, and which elements are available to build planets and life.

Element Abundance in the Universe

Here is the estimated abundance of elements in the Milky Way galaxy, which you can take as representative of the composition of the universe:

Scientists use spectroscopic data to measure the abundance of elements in the universe. Our understanding of the composition of the universe is always changing, plus new tools change the way we measure it. But, the universe isn’t exactly the same everywhere and element abundances are estimates. Basically, references agree on the order of elements in terms of abundance, but disagree (sometimes widely) on the actual numbers. You should know hydrogen is most abundant, followed by helium, and then oxygen, carbon, neon, and iron.

Why Is Hydrogen the Most Abundant Element?

The reason hydrogen is the most abundant element in the universe goes back to the Big Bang. The Big Bang quickly led to the formation of protons, neutrons, and electrons. Because hydrogen is the simplest element, it formed most readily. Technically, even a lone proton classifies as an atom of hydrogen. A neutral atom also has an electron. Most hydrogen atoms don’t have any neutrons, although the less-common isotope deuterium has one neutron and the rarer isotope tritium has two neutrons.

Universe vs Galaxy vs Solar System Abundances

Element abundance values often come from measurements of the Milky Way or the Sun. These are good approximations for the universe as a whole, but they are not identical.

• Universe: Dominated by hydrogen and helium from the Big Bang
• Milky Way galaxy: Slightly enriched in heavier elements due to stellar evolution
• Solar System: Even more enriched in heavy elements compared to cosmic averages

Despite these differences, the overall ranking of elements remains the same, with hydrogen as the most abundant.

Mass Percent vs Number Percent

Element abundance can be expressed in two ways:

• Mass percent: Based on total mass
• Number percent: Based on number of atoms

Hydrogen dominates even more strongly by number because each atom is so light. By number, hydrogen makes up over 90% of atoms in the universe.

How Are the Elements Formed?

Initially, the universe was richer in hydrogen than it is today. Around one quarter of the helium in the universe formed during the Big Bang, but another 3% formed from hydrogen during fusion in stars.

Oxygen forms from fusion in stars just before they go supernova. As stars age and die the percentage of oxygen in the universe rises. Carbon forms mainly in red giants. Neon, like oxygen, forms in pre-supernova stars. Nitrogen comes from stars like the Sun from the fusion process involving carbon and oxygen. Magnesium forms via fusion when massive stars explode. Silicon, iron, and sulfur come from exploding massive stars and white dwarf. Heavier elements form from merging neutron stars and fusion in dying lower mass stars. Technetium and elements heavier than uranium are mainly synthesized in accelerators and nuclear reactors. While it’s possible they might form naturally, they decay so quickly they aren’t present in detectable amounts.

• Big Bang nucleosynthesis: Hydrogen, helium, trace lithium
• Stellar fusion: Carbon, oxygen, neon, and other light elements
• Supernovae: Iron and many intermediate-mass elements
• Neutron star mergers: Heavy elements like gold and uranium

Matter Versus Dark Matter

The elements are examples of ordinary or baryonic matter. Baryonic matter makes up planets, stars, interstellar clouds, and intergalactic gases. Scientists believe only about 4.6% of the universe consists of ordinary matter and energy, while 68% is dark energy and 27% is dark matter. But, we have been unable to directly observe dark matter and dark energy, so their nature isn’t well-understood or characterized.

Common Misconceptions

• “Oxygen is the most abundant element” – True for Earth’s crust and the human body, not the universe
• “All elements are evenly distributed” – Element abundance varies by location
• “Heavy elements are common” – They make up only a tiny fraction of cosmic matter

FAQs

What is the second most abundant element in the universe?
Helium is the second most abundant element, making up about one-quarter of ordinary matter.

Why isn’t oxygen the most abundant element if it’s common on Earth?
Earth is not representative of the universe. Hydrogen and helium dominate because they formed in the Big Bang.

Is hydrogen the most abundant element everywhere?
Yes, hydrogen is the most abundant element on cosmic scales, although local environments like planets can differ.

What is the most abundant element in the human body?
Oxygen, not hydrogen, is the most abundant element in the human body by mass.

References

• Anders, E; Ebihara, M (1982). “Solar-system abundances of the elements”. Geochimica et Cosmochimica Acta. 46 (11): 2363. doi:10.1016/0016-7037(82)90208-3
• Cameron, A.G.W. (1973). “Abundances of the elements in the solar system”. Space Science Reviews. 15 (1): 121. doi:10.1007/BF0017244
• Croswell, Ken (February 1996). Alchemy of the Heavens. Anchor. ISBN 0-385-47214-5.
• Suess, Hans; Urey, Harold (1956). “Abundances of the Elements”. Reviews of Modern Physics. 28 (1): 53. doi:10.1103/RevModPhys.28.53