The vast majority of cosmic matter consists of the two lightest elements, hydrogen and helium, created at the universe’s beginning. Everything else on the periodic table—from the oxygen we breathe to the gold in jewelry—accounts for only a small fraction of the total mass. These heavier elements, which astronomers collectively call “metals,” were forged later in a series of powerful cosmic events. This inventory was built through a cycle of nuclear reactions and stellar deaths, transforming the universe from a simple gas cloud into the chemically rich environment we observe today.
The Primordial Baseline
The universe’s first few minutes were governed by Big Bang nucleosynthesis, which set the initial elemental composition. Extreme heat and density allowed protons and neutrons to combine briefly before expansion cooled the environment. This window, lasting about twenty minutes, was sufficient to form hydrogen and helium, along with trace amounts of lithium and its isotope, beryllium.
The fundamental constraint was the lack of a stable nucleus with five or eight particles, creating a bottleneck in the reaction chain. Before heavier nuclei could be synthesized, the universe had expanded and cooled too much, causing nuclear reactions to cease. As a result, the early universe was about 75% hydrogen and 25% helium by mass, a ratio that still dominates the cosmos. This mixture became the raw material for the universe’s first stars.
Stellar Furnaces Creating Light Metals
The gravitational collapse of gas clouds ignited the first stars, creating conditions for the next stage of element creation. In the cores of main-sequence stars like the Sun, immense pressure and temperature allow hydrogen nuclei to fuse into helium, releasing energy that stabilizes the star. This process continues for billions of years until the hydrogen fuel in the core is depleted, causing the core to contract and heat up.
Once the core temperature reaches approximately 100 million Kelvin, helium fusion begins through the triple-alpha process. Three helium nuclei (alpha particles) combine simultaneously to form carbon. Following this, a fourth helium nucleus can fuse with carbon to produce oxygen. Carbon and oxygen are produced in large quantities in medium-sized stars and are foundational for rocky planets and life.
Advanced Fusion Elements Up to Iron
Stars much more massive than the Sun continue through a rapid series of fusion stages, building elements in concentric shells around the core. After helium is exhausted, the core contracts and heats, initiating carbon burning, followed by neon, oxygen, and finally silicon burning. Each subsequent stage requires higher temperatures and pressures and is significantly shorter than the last, sometimes lasting mere days. This process creates elements with even numbers of protons up to iron, including magnesium, silicon, and sulfur.
The star develops an onion-like structure, with the heaviest element at the center surrounded by layers of lighter elements undergoing fusion. This nucleosynthesis chain stops abruptly at iron (Fe) due to a fundamental law of nuclear physics. Iron-56 has the highest nuclear binding energy per nucleon, meaning fusing iron nuclei consumes energy instead of releasing it. When the core becomes pure iron, energy-producing reactions cease, and the star loses its outward pressure support, leading to a catastrophic gravitational collapse.
Supernovae and Mergers Forging the Heaviest Elements
The explosive death of a massive star in a core-collapse supernova provides the intense energy and neutron flux required to create elements heavier than iron. This extreme environment facilitates the rapid neutron-capture process (r-process). During the supernova, a dense flow of neutrons bombards existing nuclei, causing them to capture many neutrons quickly before radioactive decay occurs. This rapid capture builds massive, unstable nuclei which then decay into stable heavy elements like gold, platinum, and uranium.
The collision of two neutron stars is an even more intense environment for the r-process and is recognized as a major source for the heaviest elements. These mergers eject vast amounts of neutron-rich matter, creating favorable conditions for rapid capture. A less violent process, the slow neutron-capture process (s-process), also contributes, primarily occurring in the late stages of lower-mass stars (asymptotic giant branch stars). The s-process involves neutron captures over thousands of years, allowing unstable nuclei time to undergo beta decay before capturing another neutron, producing elements like barium and lead.
Cosmic Distribution and Legacy
Powerful stellar processes are followed by mechanisms that ensure the dispersal of elements throughout the galaxy. Supernova explosions are the most effective way to scatter these newly forged elements, blasting them outward into the interstellar medium. Less massive stars contribute through stellar winds and the shedding of their outer layers, which form planetary nebulae. This ejected material is rich in the “metals” created during the star’s lifetime, including carbon, oxygen, and elements up to uranium.
This process, often called galactic recycling, continuously enriches the gas and dust clouds between the stars. As new generations of stars and planetary systems form from these enriched clouds, they incorporate elements synthesized by older, dead stars. The oxygen in the Earth’s atmosphere, the calcium in our bones, and the iron in our blood all originated in the cores and explosive deaths of stars that lived and died billions of years ago. This recycling ensures that the chemical complexity of the universe steadily increases over cosmic time.