The Chemical Code of the Universe: Abundance and Origin of the Elements

Diagram illustrating the relative abundance of chemical elements in the Universe, from hydrogen to iron, with associated nucleosynthesis processes — A graphical representation of cosmic elemental abundance, where hydrogen and helium dominate, followed by a sharp drop for light elements, then a peak for iron. Each element is a note in the cosmic symphony, played by stars over billions of years.
Image source: astronoo.com (new window) — AI-generated image, public domain.

Scientific summary

The Universe is composed of 98% hydrogen and helium, formed during primordial nucleosynthesis three minutes after the Big Bang. All other elements, from carbon to iron, are forged in the hearts of stars by nuclear fusion, then dispersed by supernova explosions. Massive stars fuse elements up to iron, the stopping point of fusion. Heavier elements, like uranium, are produced during stellar cataclysms. Chemical abundance, measured by spectroscopy, acts as a fossil revealing galactic history and conditions the formation of planets and the emergence of life.

Why is the Universe composed mainly of hydrogen and helium?

Because these two elements are the only ones to have been produced in massive quantities during primordial nucleosynthesis, in the first three minutes following the Big Bang, 13.8 billion years ago. At that time, the cooling Universe allowed the first protons and neutrons to assemble to form about 75% hydrogen and 25% helium, with only tiny traces of lithium and beryllium. All other elements of the periodic table – the carbon that makes up our bodies, the oxygen we breathe, the iron in our red blood cells – were forged much later, in the hearts of stars by nuclear fusion, then dispersed into space by the violent explosion of supernovae. The chemistry of the Universe is therefore a two-part story: an initial composition inherited from the Big Bang, then a gradual enrichment by successive generations of stars, making us, literally, "stardust".

Primordial Music: When the Universe was Only Hydrogen and Helium

The primordial soup

In the first moments following the Big Bang, about 13.8 billion years ago, the Universe was a soup of quarks and gluons of unimaginable density and temperature. As it cooled, the first protons and neutrons assembled. During the first three minutes, the cosmos was the scene of a blazing primordial nuclear synthesis. The result of this original alchemy is of absolute simplicity: about 75% hydrogen (¹H) and 25% helium (⁴He), with infinitesimal traces of lithium and beryllium.

The initial heritage and the wait for stars

This initial heritage, this primordial abundance, is the basic score on which all cosmic history will be written. No element heavier than lithium could emerge from this era. To create carbon, oxygen, iron and all the atoms that today make up planets, oceans and our own bodies, we had to wait for the birth, life and violent death of the first stars.

The contribution of Margaret Burbidge

Margaret Burbidge (1919-2020), pioneering astrophysicist, profoundly marked the study of the formation of chemical elements within stars and the structure of galaxies. Thanks to her research, we now know that we are, literally, made of "stardust".

Table of the distribution of chemical elements in the Universe

This table presents the abundance of the main chemical elements, expressed as the number of atoms relative to 10⁶ silicon atoms, as well as their mass contribution and their main nucleosynthetic production sites.

Cosmic abundance of chemical elements: mass ratio
Element	Symbol	Relative abundance (atoms) (normalized to 10⁶ atoms of Si)	Mass ratio (fraction in %)	Main production sites
Hydrogen	H	\( 2.4 \times 10^{10} \)	~ 71 %	Big Bang (primordial nucleosynthesis)
Helium	He	\( 2.3 \times 10^{9} \)	~ 27 %	Big Bang, stellar fusion
Oxygen	O	\( 1.4 \times 10^{7} \)	~ 0.66 %	Massive stars (C, Ne, O fusion), supernovae
Carbon	C	\( 7.1 \times 10^{6} \)	~ 0.25 %	Low-mass stars (triple-alpha process), massive stars
Neon	Ne	\( 2.1 \times 10^{6} \)	~ 0.13 %	Massive stars (carbon fusion)
Nitrogen	N	\( 2.0 \times 10^{6} \)	~ 0.08 %	Low-mass stars (CNO process), massive stars
Magnesium	Mg	\( 1.0 \times 10^{6} \)	~ 0.07 %	Massive stars (neon and carbon fusion)
Silicon	Si	\( 1.0 \times 10^{6} \)	~ 0.08 %	Massive stars (oxygen fusion, silicon fusion)
Iron	Fe	\( 8.4 \times 10^{5} \)	~ 0.14 %	Supernovae (core collapse, type Ia)
Sulfur	S	\( 4.4 \times 10^{5} \)	~ 0.04 %	Massive stars (oxygen and silicon fusion)
Argon	Ar	\( 1.0 \times 10^{5} \)	~ 0.012 %	Massive stars (silicon fusion)
Aluminum	Al	\( 8.4 \times 10^{4} \)	~ 0.007 %	Massive stars, supernovae
Calcium	Ca	\( 6.3 \times 10^{4} \)	~ 0.007 %	Massive stars (silicon fusion)
Sodium	Na	\( 5.8 \times 10^{4} \)	~ 0.004 %	Massive stars, AGB stars
Nickel	Ni	\( 4.8 \times 10^{4} \)	~ 0.008 %	Supernovae (core collapse, type Ia)
All other elements	—	<\( 1.0 \times 10^{4} \)	<0.002 %	Supernovae, white dwarf mergers, neutron star collisions, s-process

N.B.:
These values correspond to protosolar abundances (Lodders 2003), considered the best available proxy for the average chemical composition of the local interstellar medium; the term "cosmic abundance" is a common shorthand, with the actual composition varying slightly depending on galactocentric distance and the epoch considered.

Cosmic Forges: From Stellar Fusion to Supernova Explosions

Stellar nuclear reactors

Stars are the alchemical reactors of the Universe. In their incandescent cores, pressure and temperature are so high that atomic nuclei fuse. This reaction, governed by the balance between gravity and radiation pressure, follows a precise hierarchy.

The fusion chain up to iron

Hydrogen fuses into helium, releasing the energy that makes stars shine. When the core hydrogen is exhausted, a massive star (more than 8 solar masses) begins to fuse helium into carbon, then carbon into neon, and so on, forming concentric layers like an onion. This fusion chain stops at iron \( (^{56}\text{Fe}) \), the most stable element. Iron is the nuclear graveyard: its fusion no longer releases energy, it consumes it. The iron core then collapses suddenly, triggering a gigantic supernova explosion (type II).

The forge of heavy elements

It is in this fraction of a second and in the subsequent stellar winds that all elements heavier than iron are forged. The periodic table, from cobalt to uranium, is the fruit of these stellar cataclysms. Without supernovae, the Universe would have remained a sterile place, composed only of hydrogen, helium and a few traces of lithium.

Table of the main formation sites of chemical elements in the Universe

Each family of chemical elements bears the signature of the astrophysical site that forged it: primordial nucleosynthesis, stellar fusion, supernova explosions or neutron star mergers. This table summarizes these main sites and their respective contributions.

Main nucleosynthesis sites and their contribution to elemental abundance
Nucleosynthesis site	Elements mainly produced	Physical process	Role in cosmic evolution
Big Bang (first instants)	Hydrogen (¹H), Helium (⁴He), traces of Lithium (⁷Li)	Primordial nucleosynthesis	Foundation of all baryonic matter, initial composition of the cosmos.
Low-mass stars (<8 M☉)	Helium (⁴He), Carbon (¹²C), Nitrogen (¹⁴N); about half of the elements heavier than iron (strontium, barium, lead...)	Hydrogen fusion (CNO cycle) and helium fusion (triple alpha); slow neutron capture (s-process) during the AGB phase	Enrichment of the interstellar medium in light elements essential for organic chemistry, and in a portion of heavy elements.
Massive stars (>8 M☉)	Oxygen (¹⁶O), Silicon (²⁸Si), Magnesium (²⁴Mg), up to Iron (⁵⁶Fe)	Hydrogen, helium, carbon, neon, oxygen, silicon fusion	Creation of elements constituting rocks and planetary cores.
Core-collapse supernovae	Iron-peak elements (Nickel ⁵⁸Ni, Cobalt ⁵⁹Co, Zinc ⁶⁴Zn); debated contribution to a "weak" r-process (light neutron-rich isotopes)	Explosive silicon combustion and alpha-rich freeze-out; rapid neutron capture in some scenarios (magnetorotational jets, collapsars)	Production and dispersion of iron-group elements into the interstellar medium.
Neutron star mergers (kilonova)	Majority of the heaviest r-process elements: rare earths, Gold (¹⁹⁷Au), Platinum, up to Uranium (²³⁸U)	Rapid neutron capture (strong r-process) in neutronized matter ejected during coalescence	Identified since the GW170817/AT2017gfo event (2017) as the dominant site of production of the heaviest elements in the Universe.
White dwarf mergers (type Ia)	Iron (⁵⁶Fe), Nickel (⁵⁸Ni), iron-group elements	Explosive combustion of carbon and oxygen	Major contribution to iron abundance, chemical marker of stellar population age.

N.B.:
The attribution of the r-process to core-collapse supernovae, long favored, has been largely revised since 2017: multi-messenger observations of the neutron star merger GW170817 have confirmed that these events are the dominant site of production of the heaviest elements. The exact role of supernovae in a possible "weak" r-process remains an active research topic.

Galactic Archaeology: Reading Cosmic History in Abundances

The stellar spectrum: a chemical fossil of the primitive Universe

The abundance of chemical elements is not just a catalog. It is a tool for stellar archaeology. By analyzing the light of a star, its spectrum, astrophysicists can determine its chemical composition. This chemical signature acts as a fossil, revealing the time of its birth.

Ancient and young stars: the contrast of stellar populations

The oldest stars, formed more than 10 billion years ago, are extremely metal-poor (elements heavier than helium). These are Population II stars, often located in the galactic halo and globular clusters. Conversely, young stars like our Sun are metal-rich (Population I), testifying to successive cycles of nucleosynthesis and enrichment.

Ultra-metal-poor stars: remnants of the first stellar generations

A fascinating discovery is that of "ultra-metal-poor" stars, such as SMSS J031300.36-670839.3. Discovered in 2014, it has an iron abundance more than a million times lower than that of the Sun, but a relatively high amount of carbon. This suggests that its raw material came from a first generation of massive stars (Population III) that enriched the medium with carbon before exploding as a supernova, but without producing much iron. This is direct evidence of the diversity of physical processes at work in the primitive Universe.

Galactic collisions: drivers of chemical enrichment

Galaxy collisions and mergers also play a major role. When two spiral galaxies collide, as in the case of Arp 220, immense clouds of gas are compressed, triggering bursts of massive star formation. These stars, which live quickly and die as supernovae, abruptly enrich the medium with heavy elements. Chemical abundance is therefore not static: it is the dynamic reflection of the gravitational and evolutionary history of each galaxy.

Life, a Chemical Consequence of Cosmic Evolution

Carbon, oxygen and nitrogen: elementary building blocks of life

The emergence of life, as we know it, is a direct consequence of this long chemical evolution. Carbon, the central element of organic chemistry, is the product of helium fusion in stars (triple-alpha process). Oxygen, which we breathe, is the third most abundant element in the Universe, forged in the hearts of massive stars. Nitrogen, a component of amino acids, is synthesized during the CNO (carbon-nitrogen-oxygen) cycles in stars.

Metallicity and planet formation: the link with habitability

The formation of rocky planets is itself conditioned by the abundance of refractory elements such as silicon, iron, magnesium and aluminum. In regions of the Universe where metallicity is too low, the probability of forming terrestrial planets collapses. We can therefore trace a direct link, a physical causality, between the rate of supernova formation, the chemical enrichment of the interstellar medium and the possibility of the emergence of habitable worlds. The famous phrase of Carl Sagan (1934-1996), taken from his book Cosmos (1980), finds its deepest anchoring here: "We are all stardust".

References

FAQ – Everything you need to know about the chemical code that shapes the Universe

What are the two most abundant elements in the Universe?

Hydrogen (about 71% by mass) and helium (about 27%). Together, they represent about 98% of the ordinary (baryonic) matter of the Universe. All other elements – which astrophysicists call "metals", even carbon and oxygen – make up only the remaining 2%.

Why didn't the Big Bang produce elements heavier than lithium?

Because primordial nucleosynthesis lasted only about three minutes. The Universe cooled and diluted too quickly for nuclei to fuse beyond lithium. Moreover, there were no stars yet to push fusion reactions further. To create carbon, oxygen or iron, we had to wait for the formation of the first massive stars and their supernova explosions.

How do stars make chemical elements?

By nuclear fusion. In their burning cores, pressure and temperature are so high that atomic nuclei combine. Hydrogen first fuses into helium, releasing the energy that makes the star shine. In massive stars (more than 8 solar masses), this process continues: helium fuses into carbon, carbon into neon, then oxygen, silicon, and finally iron. Each stage requires more extreme temperatures and pressures.

Why does fusion stop at iron?

Because iron (⁵⁶Fe) is the most stable element in the atomic nucleus. Fusing iron does not release energy – on the contrary, it consumes it. When a massive star has turned its core into iron, fusion stops abruptly. Without radiation pressure to counteract gravity, the core collapses in a fraction of a second, triggering the supernova explosion. It is in this explosion that all elements heavier than iron (from cobalt to uranium) are forged.

What is the difference between "metal-poor" and "metal-rich" stars?

The oldest stars (Population II), formed more than 10 billion years ago, are very metal-poor (elements heavier than helium) because they were born from interstellar gas little enriched by previous stellar generations. Young stars like our Sun (Population I) are metal-rich, testifying to successive cycles of nucleosynthesis and enrichment. It's a bit like stellar archaeology: the chemical composition of a star is a fossil that reveals its age.

What are "ultra-metal-poor" stars?

These are extremely ancient stars, containing up to a million times less iron than the Sun. One of them, SMSS J031300.36-670839.3, discovered in 2014, revealed a relatively high abundance of carbon but very little iron. This proves that its raw material came from a very first generation of massive stars (Population III) that enriched the medium with carbon before exploding, but without producing much iron. These stars are direct remnants of the first stellar generations of the Universe.

Where does the oxygen we breathe come from?

Oxygen is the third most abundant element in the Universe. It is mainly forged in the hearts of massive stars, by fusion of carbon and neon, before being dispersed into space during their supernova explosion. The oxygen you breathe is therefore indeed the product of a stellar explosion that occurred billions of years ago in a distant region of our galaxy.

What is the link between elemental abundance and life?

Life, as we know it, is a direct consequence of this chemical evolution. Carbon, central to organic chemistry, is produced by helium fusion (triple-alpha process). Nitrogen, a component of amino acids, is synthesized in the CNO (carbon-nitrogen-oxygen) cycles of stars. Oxygen is essential for respiration. Moreover, the formation of rocky planets depends on the abundance of refractory elements like silicon, iron or magnesium. In regions too poor in metals, the probability of forming habitable worlds collapses.

What does the expression "we are stardust" mean?

This is a famous phrase by Carl Sagan that summarizes a fundamental physical truth: all the chemical elements that make up our bodies – the carbon in our molecules, the oxygen in our water, the iron in our blood, the calcium in our bones – were forged in the hearts of massive stars long since gone, then dispersed into space by supernova explosions. Our matter is literally born of ancient suns that died billions of years ago.

Do galaxy collisions influence the abundance of elements?

Yes, in a major way. When two spiral galaxies collide, immense gas clouds are compressed, triggering bursts of massive star formation. These stars live quickly and die as supernovae, abruptly enriching the interstellar medium with heavy elements. Chemical abundance is therefore not static: it reflects the gravitational and evolutionary history of each galaxy.