The visible universe—the ordinary matter that emits or interacts with light, excluding dark matter and dark energy—is overwhelmingly dominated by only two chemical elements. While the periodic table contains over a hundred elements, the cosmic inventory is extremely skewed toward the lightest ones. This imbalance reflects the universe’s origin and the processes that have occurred in stars over billions of years. Understanding the composition of stars, galaxies, and gas clouds requires looking at the primordial elements created at the beginning of the cosmos and the nuclear reactions that enrich the universe over time.
The Overwhelming Abundance of Hydrogen and Helium
The vast majority of the visible universe’s mass is composed of Hydrogen (73%) and Helium (25%), a ratio established in the first minutes after the Big Bang. Hydrogen is the simplest element with a single proton. Helium consists of two protons and two neutrons and accounts for nearly all the remaining mass.
This dominance stems from Big Bang Nucleosynthesis (BBN), which occurred when the universe was only a few minutes old. The cosmos had cooled enough for protons and neutrons to fuse, but the window for these reactions closed quickly as the universe expanded. The final ratio of one neutron for every seven protons determined the initial elemental mix.
Nearly all available neutrons were quickly locked into stable Helium-4 nuclei. The remaining protons were left as individual Hydrogen nuclei, making Hydrogen the most common element. This primordial composition set the stage for all subsequent element creation, meaning all other elements constitute less than 2% of the universe’s total mass.
The Stellar Factories That Build Heavier Elements
Elements heavier than Helium, collectively called “metals” by astronomers, were forged later inside stars, not during the Big Bang. Stars function as massive nuclear reactors that recycle primordial Hydrogen and Helium. This process, known as stellar nucleosynthesis, provides the mechanisms for creating the rest of the periodic table.
The first and most common mechanism is stellar fusion, occurring in the cores of stars. Through sequential fusion steps, stars build heavier elements up to Iron (Fe). Iron represents the limit for this process because fusing elements heavier than Fe requires energy input rather than releasing energy, making it the final product of typical stellar fusion.
The second and third mechanisms create elements heavier than Iron, which ordinary fusion cannot produce. The slow neutron-capture process (s-process) occurs in the late stages of low-to-intermediate mass stars, such as red giants. Nuclei slowly absorb free neutrons, which then decay into a proton, creating a heavier element.
The rapid neutron-capture process (r-process) forges the heaviest elements. This requires an extremely high density of free neutrons, found primarily in core-collapse supernovae and the merger of two neutron stars. Neutron star mergers are considered a major source of elements like gold and uranium, which enrich gas clouds for future stellar systems.
The Cosmic Ranking of Oxygen, Carbon, and Iron
Although the “other elements” category is small, it includes the building blocks for rocky planets and life. After Hydrogen and Helium, Oxygen is the third most abundant element, followed by Carbon, Neon, Nitrogen, and Iron. These elements are abundant because they are the natural products of massive stars’ fusion cycles and subsequent supernova explosions.
Oxygen is a direct product of Helium fusion within stars, making it a common element distributed across the universe. Carbon, the fourth most common element, is the chemical backbone of all known life and forms in the late stages of stellar evolution. Neon is also highly abundant due to its creation near the end of the fusion chain in massive stars.
Iron is uniquely important as the endpoint of energy-releasing fusion in large stars, giving it the sixth-highest abundance by mass. The creation of an Iron core triggers the star’s collapse and subsequent supernova, scattering the newly forged elements across the galaxy.
How Scientists Measure Universal Composition
Determining the elemental composition of celestial objects relies on analyzing the light they emit. The primary technique used by astronomers is spectroscopy, which acts as a chemical fingerprint reader. This method works by splitting the light from a star or galaxy into its component wavelengths, much like a prism.
Every element has a unique set of energy levels for its electrons. When an electron jumps between these levels, it absorbs or emits light at specific, signature wavelengths. These specific wavelengths appear as dark or bright lines in the spectrum. This allows scientists to identify the elements present in a distant object’s atmosphere or gas cloud.
The intensity and width of these spectral lines are then used to calculate the relative abundance of each element. Spectroscopic observations are conducted using powerful telescopes on the ground and in space, such as the James Webb Space Telescope.
By studying the light from ancient stars, distant galaxies, and intergalactic gas clouds, astronomers map the chemical evolution of the universe over time. Analysis of the Cosmic Microwave Background (CMB) radiation provides independent data that confirms the primordial ratios of Hydrogen and Helium.