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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.
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.
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.
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.
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.
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.
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.
They possess millions (or more) of stars inside.
In between the two large, prominent foreground galaxies shown here, JWST has imaged a faint red object that was originally identified as an ultra-distant galaxy candidate: JADES-GS-z13-1-LA. After a spectroscopic study was performed, this galaxy has been confirmed to be at a redshift of between z=13.01 and z=13.05, placing its age as coming from when the Universe was only between 325 and 330 million years old. It is already massive, evolved in heavy elements, and bright.
Credit: J. Witstok et al., arXiv:2408:16608, 2024
Meanwhile, the very first stars should live only a few million years before exploding.
The anatomy of a very massive star throughout its life, culminating in a type II (core-collapse) supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive stars achieve a core-collapse supernova the fastest, typically resulting in the creation of black holes, while the less massive ones take longer, and create only neutron stars.
When those stars detonate, they enrich the Universe with heavy elements: especially oxygen and carbon.
JADES-GS-z14-0, in the top inset box, is found behind (and just to the right of) a closer, brighter, bluer galaxy. It was only through the power of JWST spectroscopy with incredible resolution, capable of separating the two sources, that the nature of this record-breakingly distant object could be determined. Its light comes to us from when the Universe was only 285-290 million years old: just 2.1% of its current age. JADES-GS-z14-1, just below it, comes from when the Universe was ~300 million years old. Compared to large, modern-day galaxies, all early galaxies contain a paucity of stars and have irregular, ill-defined shapes.
JWST discovered and measured the most distant galaxy yet known: JADES-GS-z14-0.
This image shows the spectrum, or intensity as a function of wavelength, of the distant galaxy JADES-GS-z14-0 as acquired with JWST’s NIRSpec instrument. The Lyman break feature tells us that the galaxy is at a redshift of z=14.32, corresponding to an age of the Universe at that time of just 285 million years. Within this spectrum is an emission line peak that, at 3.6-sigma significance, indicates the presence of twice-ionized carbon.
Credit: NASA, ESA, CSA, J. Olmsted (STScI). Science: S. Carniani (Scuola Normale Superiore), JADES Collaboration
It found tentative evidence for a (doubly-ionized) carbon line, indicating a non-pristine galaxy.
This four-panel spread shows four different views of galaxy JADES-GS-z14-0 with photometric MIRI data. The fact that this galaxy has so much emission at 7.7 microns, photometrically, suggests (but does not prove on its own) that neutral, heated hydrogen (Balmer beta) and doubly ionized oxygen are both present in great abundances.
To make doubly-ionized oxygen, however, it takes temperatures exceeding 50,000 K.
Around a variety of stellar corpses and dying stars, doubly-ionized oxygen atoms produce a characteristic green glow, as electrons cascade down the various energy levels when heated to temperatures exceeding ~50,000 K. Here, the planetary nebula IC 1295 shines brilliantly. These conditions are present in intense star-forming regions and around stellar corpses, where the green phenomenon also helps color the so-called “green pea” galaxies, as well as Earth’s aurorae.
With ALMA data, two independent teams found it, confirming oxygen’s presence.
The inset in this image shows JADES-GS-z14-0 –– the most distant known galaxy as of today –– as seen with the Atacama Large Millimeter/submillimeter Array (ALMA). The two spectra shown here result from independent analysis of ALMA data by two teams of astronomers. Both found an emission line of oxygen, making this the most distant detection of oxygen, when the Universe was slightly under a mere 300 million years old.
We’ll have to look even earlier to truly find the first stars.
Even from this zoomed-in view of the JADES field, it’s very difficult to pick out the most distant galaxy ever found, JADES-GS-z14-0, by eye. This animation shows its location with a green circle: overlapping with a brighter, bluer, closer galaxy.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
