The atoms making up Earth did not originate here. Every element heavier than Hydrogen and Helium—including the oxygen we breathe, the silicon in rocks, and the iron in our core—had to be manufactured in the universe first. Tracing the origin of these “heavier elements” involves examining the most powerful processes in the cosmos, from the Big Bang to the cataclysmic deaths of stars. This cosmic enrichment provided the raw materials that eventually coalesced into the world we inhabit.
The Universe’s First Ingredients
The universe began in an incredibly hot, dense state. Within its first few minutes, only the lightest elements could be forged through Big Bang Nucleosynthesis, which occurred when the cooling universe allowed protons and neutrons to combine briefly. The rapid expansion and cooling of the early cosmos quickly shut down these nuclear reactions.
The result was a universe composed overwhelmingly of two elements: approximately 75% Hydrogen and 25% Helium by mass. Trace amounts of Lithium were also produced. However, the conditions were not suitable for creating Carbon, Oxygen, Iron, or any of the other elements necessary for rocky planets like Earth.
Stellar Factories Forging Mid-Weight Elements
The next phase of element creation began with the birth of the first stars, which served as the universe’s nuclear furnaces. Massive stars compressed their Hydrogen cores under immense gravity, initiating stellar nucleosynthesis—the process of fusing lighter nuclei into heavier ones. This fusion releases the energy that keeps a star shining.
As a star exhausts its Hydrogen fuel, it contracts and heats up, allowing the Helium ash to ignite. This Helium burning proceeds through the triple-alpha process, where three Helium nuclei combine to form one Carbon nucleus. Subsequent fusion stages in massive stars continue this process, creating elements with even numbers of protons, such as Oxygen, Neon, and Magnesium.
These stars develop an “onion-like” structure, with progressively heavier elements fusing in shells closer to the core. Oxygen burning produces Silicon, and Silicon burning is the final stage, generating elements like Sulfur, Argon, Calcium, and eventually, Iron. Iron-56 represents a boundary in stellar chemistry because it possesses the most tightly bound nucleus. Fusing elements lighter than Iron releases energy, but fusing Iron itself consumes energy. Since a star’s existence depends on energy-releasing fusion, the creation of Iron in the core signals the immediate end of the star’s life.
Extreme Events Create the Heaviest Elements
The formation of elements heavier than Iron, such as Gold and Uranium, requires conditions far more extreme than those found in a stable stellar core. These elements are primarily created through neutron capture processes, where an atomic nucleus absorbs a free neutron.
Slow Neutron Capture (s-process)
The slow neutron capture process, or s-process, occurs in less massive stars late in their lives, specifically in Asymptotic Giant Branch (AGB) stars. Neutrons are captured slowly enough that the resulting unstable nucleus has time to undergo radioactive decay before another neutron is absorbed. This process builds elements up to Bismuth (atomic number 83).
Rapid Neutron Capture (r-process)
The formation of the very heaviest elements requires the rapid neutron capture process, or r-process. This process demands an incredibly high density of free neutrons, allowing nuclei to absorb dozens of neutrons in mere seconds before they can decay. The two most powerful sites for the r-process are core-collapse supernovae and the collision of two neutron stars.
A core-collapse supernova, marking the explosive death of a massive star, provides a powerful environment for the r-process, scattering newly-formed elements across the galaxy. Neutron star mergers, known as kilonovae, are considered the most prolific factories for the rarest heavy elements, including virtually all of the Gold and Platinum found on Earth. These cataclysmic events eject vast amounts of neutron-rich material into space, which then decays into the heaviest elements on the periodic table.
Assembling Earth From Cosmic Dust
The heavy elements forged in stars and stellar explosions are violently dispersed into the interstellar medium, a process known as stellar recycling. These elements mix with existing Hydrogen and Helium gas, gradually enriching the chemical composition of the galaxy. Our solar system formed from a molecular cloud that was already chemically enriched by previous generations of massive stars.
Approximately 4.6 billion years ago, a dense pocket within this cloud began to collapse under its own gravity, forming the solar nebula. The central mass became the Sun, while the remaining material flattened into a spinning protoplanetary disk. Within this disk, the heavier elements, such as Iron and rocky silicates, condensed into solid particles.
These dust grains began to collide and stick together, a process called accretion, gradually building larger bodies known as planetesimals. Through collisions and mergers, these planetesimals grew into the rocky protoplanets, including the early Earth. The intense heat generated by this violent accretion and the decay of radioactive isotopes caused the Earth to partially melt.
This melting led to planetary differentiation, where elements separated based on their density. The densest elements, primarily Iron and Nickel, sank toward the center to form Earth’s metallic core. Lighter elements, such as Silicon, Oxygen, and Magnesium, rose to form the mantle and crust. The final composition of Earth is a direct result of the universe’s history.