Abundance of the Elements: The Chemical Signature of the Universe

In the first moments after 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 stage for a rapid primordial nucleosynthesis. The result of this original alchemy is absolutely simple: about 75% hydrogen (¹H) and 25% helium (⁴He), with tiny traces of lithium and beryllium.

This initial legacy, this primordial abundance, is the basic score on which the entire 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 now make up planets, oceans, and our own bodies, we had to wait for the birth, life, and violent death of the first stars. Margaret Burbidge (1919-2020), a pioneering astrophysicist, profoundly influenced 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

Cosmic abundance of chemical elements: mass ratio
Element	Symbol	Relative abundance (atoms) (normalized to 10⁶ silicon atoms)	Mass ratio (fraction in %)	Main production sites
Hydrogen	H	\( 4.0 \times 10^{10} \)	~ 71 %	Big Bang (primordial nucleosynthesis)
Helium	He	\( 3.0 \times 10^{9} \)	~ 27 %	Big Bang, stellar fusion
Oxygen	O	\( 2.2 \times 10^{7} \)	~ 0.9 %	Massive stars (C, Ne, O fusion), supernovae
Carbon	C	\( 1.0 \times 10^{7} \)	~ 0.3 %	Low-mass stars (triple alpha process), massive stars
Neon	Ne	\( 1.8 \times 10^{6} \)	~ 0.1 %	Massive stars (carbon fusion)
Nitrogen	N	\( 6.2 \times 10^{5} \)	~ 0.09 %	Low-mass stars (CNO process), massive stars
Silicon	Si	\( 1.0 \times 10^{6} \)	~ 0.07 %	Massive stars (oxygen fusion, silicon fusion)
Magnesium	Mg	\( 1.0 \times 10^{6} \)	~ 0.06 %	Massive stars (neon and carbon fusion)
Iron	Fe	\( 8.3 \times 10^{5} \)	~ 0.12 %	Supernovae (core collapse), type Ia supernovae
Sulfur	S	\( 4.8 \times 10^{5} \)	~ 0.04 %	Massive stars (oxygen and silicon fusion)
Argon	Ar	\( 1.0 \times 10^{5} \)	~ 0.01 %	Massive stars (silicon fusion)
Nickel	Ni	\( 5.0 \times 10^{4} \)	~ 0.008 %	Supernovae (core collapse, type Ia)
Calcium	Ca	\( 5.0 \times 10^{4} \)	~ 0.005 %	Massive stars (silicon fusion)
Aluminum	Al	\( 4.5 \times 10^{4} \)	~ 0.003 %	Massive stars, supernovae
Sodium	Na	\( 2.2 \times 10^{4} \)	~ 0.001 %	Massive stars, AGB stars
All other elements	—	< \( 1.0 \times 10^{4} \)	< 0.002 %	Supernovae, white dwarf mergers, neutron star collisions, s-process

N.B.: Abundance values are based on spectroscopic data from the Sun and the interstellar medium, which is the standard reference in astrophysics. Abundance is normalized to \( 10^6 \) silicon (Si) atoms, a common convention in astronomy. Hydrogen and helium alone account for about 98% of the ordinary baryonic mass of the Universe. The mass fraction of elements heavier than helium ("metallicity") is therefore about 2% in the solar neighborhood.

Cosmic Forges: From Stellar Fusion to Supernova Explosions

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.

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 abruptly, triggering a massive supernova explosion (type II).

It is in this fraction of a second and in the stellar winds that follow 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 sites of formation of chemical elements in the Universe

Galactic Archaeology: Reading Cosmic History in Abundances

The stellar spectrum: a chemical fossil of the early Universe

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

Old and young stars: the contrast of stellar populations

The oldest stars, formed more than 10 billion years ago, are extremely poor in metals (elements heavier than helium). These are Population II stars, often found in the galactic halo and globular clusters. In contrast, young stars, like our Sun, are rich in metals (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 supernovae, but without producing much iron. This is direct evidence of the diversity of physical processes at work in the early Universe.

Galactic collisions: engines of chemical enrichment

Galactic collisions and mergers also play a major role. When two spiral galaxies collide, as in the case of Arp 220, huge clouds of gas are compressed, triggering bursts of massive star formation. These stars, which live fast and die as supernovae, suddenly 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 cores of massive stars. Nitrogen, a component of amino acids, is synthesized during 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. Carl Sagan's famous phrase, from his book Cosmos (1980), finds its deepest root here: "We are all made of star stuff."

Main nucleosynthesis sites and their contribution to element abundance
Nucleosynthesis site	Main elements produced	Physical process	Role in cosmic evolution
Big Bang (early moments)	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)	Hydrogen and helium fusion, s-process (slow) during the AGB phase	Enrichment of the interstellar medium in light elements, essential for organic chemistry.
Massive stars (> 8 M☉)	Oxygen (¹⁶O), Silicon (²⁸Si), Magnesium (²⁴Mg), up to Iron (⁵⁶Fe)	Hydrogen, helium, carbon, neon, oxygen, silicon fusion	Creation of the elements that make up rocks and planetary cores.
Supernovae (core collapse)	Elements heavier than iron: Nickel (⁵⁸Ni), Cobalt (⁵⁹Co), Zinc (⁶⁴Zn), up to Uranium (²³⁸U)	Rapid neutron capture (r-process) and explosive shock wave	Dispersion of heavy elements in the interstellar medium, essential for technologies and radioactivity.
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 the age of stellar populations.

FAQ – Cosmic abundance: 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 all ordinary (baryonic) matter in 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, stars did not yet exist to push fusion reactions further. To create carbon, oxygen, or iron, the formation of the first massive stars and their supernova explosions was necessary.

How do stars make chemical elements?

Through nuclear fusion. In their burning cores, pressure and temperature are so high that atomic nuclei fuse together. 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 into oxygen, silicon, and finally into iron. Each stage requires increasingly extreme temperatures and pressures.

Why does fusion stop at iron?

Because iron (⁵⁶Fe) is the most stable atomic nucleus. Fusing iron does not release energy – on the contrary, it consumes energy. When a massive star has turned its core into iron, fusion stops abruptly. Without radiation pressure to counter 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 poor in metals (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 rich in metals, bearing witness to successive cycles of nucleosynthesis and enrichment. It's a form of stellar archaeology: a star's chemical composition is a fossil revealing 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 parent 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 Universe's first stellar generations.

Where does the oxygen we breathe come from?

Oxygen is the third most abundant element in the Universe. It is mainly forged in the cores of massive stars, through fusion of carbon and neon, before being scattered into space during their supernova explosions. The oxygen you inhale is therefore truly 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. Furthermore, the formation of rocky planets depends on the abundance of refractory elements like silicon, iron, and magnesium. In regions too poor in metals, the probability of forming habitable worlds collapses.

What does the phrase "we are stardust" mean?

It is a famous saying 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 manufactured in the cores of massive, long-vanished stars, then scattered into space by supernova explosions. Our matter is literally born of ancient suns that died billions of years ago.

Do galaxy collisions influence elemental abundance?

Yes, significantly. When two spiral galaxies collide, immense gas clouds are compressed, triggering bursts of massive star formation. These stars live fast 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.