This is an artist’s impression of JADES-GS-z14–0, which as of today is the most distant confirmed galaxy. Galaxies in the early Universe tend to be clumpy and irregular. Supernova explosions in this galaxy would have spread heavy elements forged inside stars, like oxygen, which has been detected by ALMA, and carbon, which has been tentatively detected by JWST. (Credit: ESO/M. Kornmesser)
Someday, we’ll look back and see a young galaxy forming stars for the first time. JADES-GS-z14-0, the farthest ever, isn’t early enough.
Before any stars formed in the Universe, there was no oxygen.
This plot shows the abundance of the light elements over time, as the Universe expands and cools during the various phases of Big Bang Nucleosynthesis. By the time the first stars form, the initial ratios of hydrogen, deuterium, helium-3, helium-4, and lithium-7 are all fixed by these early nuclear processes. (Credit: M. Pospelov & J. Pradler, Annual Review of Nuclear and Particle Science, 2010)
The hot Big Bang creates hydrogen, helium, lithium, and beryllium, but little else.
The very first stars to form in the Universe were different than the stars today: metal-free, extremely massive, and nearly all destined for a supernova surrounded by a cocoon of gas. There was a time, prior to the formation of stars where only clumps of matter, unable to cool and collapse, remained in large, diffuse clouds. It is possible that clouds that grow slowly enough may even persist until very late cosmic times. (Credit: NAOJ)
Only when the first stars form — and initiate nuclear fusion inside — do heavier elements arise.
Many of the cataclysms that occur in space are typical supernovae: either core-collapse from a massive progenitor star or type Ia from an exploding white dwarf. The most massive stars of all have hundreds of times the mass of the Sun and live just 1 or 2 million years, total, before running out of fuel and dying in such a cataclysm. When that occurs, although a neutron star or black hole remnant may form, the majority of the star’s mass gets ejected back into the interstellar medium, enriching it with heavy elements. (Credit: NASA Ames, STScI/G. Bacon)
By the present day, about 1–2% of the Universe is in the form of these heavy elements.
The star-formation rate in the Universe is a function of redshift, which is itself a function of cosmic time. The overall rate, (left) is derived from both ultraviolet and infrared observations, and is remarkably consistent across time and space. Note that star formation, today, is only a few percent of what it was at its peak (between 3–5%), and that the majority of stars were formed in the first ~5 billion years of our cosmic history. Only about ~15% of all stars, at maximum, have formed over the past 4.6 billion years, with the cumulative history of star-formation transforming about 1% of all atoms, by mass, into oxygen. (Credit: P. Madau & M. Dickinson, 2014, ARAA)
While today’s Universe is mostly hydrogen and helium still, oxygen is #3, with carbon #4.
The relative abundances of elements in the Solar System has been measured overall, with hydrogen and helium the most abundant elements, followed by oxygen, carbon, and numerous other elements. However, the compositions of the densest bodies, like the terrestrial planets, are skewed to be a vastly different subset of these elements. Overall, some ~90% of the atoms in the Universe, by number (but only ~70–72%, by mass), are still hydrogen, even after 13+ billion years of star-formation. (Credit: 28bytes/English Wikipedia)
Oxygen represents ~1% of all atomic nuclei by mass.
An illustration of the first stars turning on in the Universe. Without metals to cool down the clumps of gas that lead to the formation of the first stars, only the largest clumps within a large-mass cloud will wind up becoming stars: fewer in number but greater in mass than today’s stars. Although there’s plenty of light-blocking matter, some of this starlight can still escape into the Universe beyond. (Credit: NASA / WMAP Science Team)
However, we’ve never yet detected any population of pristine, Population III stars.
An illustration of the galaxy CR7, which was originally hoped would house multiple populations of stars of various ages (as illustrated). While we have yet to find an object where the brightest component was pristine, with no heavy elements, we fully expect them to exist, often alongside a later generation of stars that formed earlier. The merging of multiple star clusters is likely how the first galaxies and proto-galaxies formed and took shape. (Credit: ESO/M. Kornmesser)
That’s to be expected: even the earliest known galaxies are already quite massive.
This image shows 15 of the 341 hitherto identified “little red dot” galaxies discovered in the distant Universe by JWST. These galaxies all exhibit similar features, but only exist very early on in cosmic history; there are no known examples of such galaxies close by or at late times. All of them are quite massive. (Credit: D. Kocevski et al., Astrophysical Journal Letters accepted/arXiv:2404.03576, 2025)
