Small Galaxies Are Losing Almost All Their Oxygen, Hubble Study Finds

Imagine a factory that makes a valuable product, then throws away nearly nine-tenths of everything it produces. That, in simple terms, is what astronomers have just found happening in dozens of small galaxies near our own. A new census based on data from NASA's Hubble Space Telescope shows that dwarf galaxies — the smallest and most common type of galaxy — keep only a tiny fraction of the oxygen their stars have made. The rest is blown away into space, forcing scientists to rethink how galaxies grow over billions of years.

The study, called GLOW (Galaxies Losing Oxygen via Winds), looked at 37 dwarf galaxies within about 20 million light-years of Earth. In cosmic terms these are close neighbours, near enough for Hubble to pick out individual stars — exactly what made this careful count possible. The results, published 2 November 2026 in The Astrophysical Journal, were led by Kristen Wingfield McQuinn of the Space Telescope Science Institute and Rutgers University.

The headline number: only about 11 percent stays behind

The central finding is strikingly simple. Across all 37 galaxies, the amount of oxygen still present — in stars, in gas, and in dust — ranged from just 4 percent to 25 percent of the oxygen those galaxies' stars are estimated to have produced over their lifetimes. The average was about 11 percent. In other words, the typical dwarf galaxy in the study has lost roughly 89 percent of its oxygen, and the most extreme case has lost 96 percent.

Oxygen is not a minor ingredient. It is one of the main heavy elements forged inside stars, and a useful tracer for the whole family of elements that stars create. Measuring oxygen therefore tells us how much star-made material a galaxy has kept, and how much it has given up. Almost all of the oxygen that does remain sits in the galaxies' gas, rather than locked up in stars or dust — and gas is the raw material for future stars, so a galaxy that keeps its oxygen in gas still has a chance to use it again.

How do you count oxygen in a whole galaxy?

Counting every oxygen atom in a galaxy is impossible, so the GLOW team built a balance sheet. On one side, they worked out how much oxygen the galaxy's stars should have made over their entire history. On the other, they measured how much is actually there today.

The first side came from Hubble's sharp vision. Using archival optical images, the team resolved individual stars in each galaxy and reconstructed its star-formation history — when stars were born, in what numbers, and with what chemical make-up. Stars of different masses produce different amounts of oxygen, so a galaxy's star-formation history is effectively a record of its oxygen production.

The second side came from combining Hubble data with observations from other instruments, including NASA's retired Spitzer Space Telescope and the ground-based Very Large Array and Australian Telescope Compact Array. Together, these let the researchers tally the oxygen present in the galaxies' stars, gas, and dust. The gap between the two sides is the missing oxygen — material that must have been ejected, because it was definitely made inside the galaxy at some point.

"The oxygen had to be inside of the galaxies at some point in the past, so the oxygen must have been ejected," said co-investigator Grace Telford of the University of Utah. Stars live inside galaxies, oxygen is made inside stars — if the oxygen is no longer in the galaxy, it left.

Why small galaxies leak so badly

The culprit is the sheer violence of stellar death. Massive stars end their lives as supernovae — explosions so powerful that they drive winds of hot gas outward at enormous speeds. In a big galaxy like our Milky Way, gravity is strong enough to hold on to much of that material, which often falls back like a fountain.

Dwarf galaxies are a different story. Their gravity is weak, because they contain far fewer stars. When supernovae go off in such a shallow gravitational well, the winds can escape altogether, carrying oxygen and other elements with them.

This idea is not new. Astronomers have long suspected that supernova-driven winds strip small galaxies of their metals, which is one reason dwarf galaxies look chemically primitive despite having formed stars for billions of years. What GLOW adds is firm numbers, galaxy by galaxy, across a wide range of masses and surroundings.

There is also a pattern in the surroundings: galaxies more than about 3.3 million light-years from a massive neighbour consistently kept less than 15 percent of their oxygen. Isolated dwarf galaxies have no big neighbour's gravity to help hold material nearby, and no fresh gas raining in to replace what was lost.

The simulations got it wrong — and that is useful

One of the most important results is not just the missing oxygen itself, but that computer simulations predicted less loss than Hubble sees. State-of-the-art models of galaxy formation overpredict oxygen retention in dwarf galaxies by roughly a factor of two to three.

That mismatch is a gift: it tells modellers exactly where their physics needs fixing. The winds in the simulations are evidently not strong enough, or not efficient enough at carrying metals out of small galaxies. Adjusting the recipes for supernova feedback should bring the models closer to reality, and that improvement will ripple through every prediction those models make about galaxy evolution. It is also a reminder of why observations matter — a simulation can only encode what its builders already understand, while a census built from real starlight can reveal that nature is more extreme than anyone programmed. Similar surprises have come from other observatories: for example, how the Webb telescope found a lopsided atmosphere on a distant exoplanet showed that even well-studied planets can behave unexpectedly when we look closely.

Where did all that oxygen go?

If nearly 90 percent of a dwarf galaxy's oxygen is gone, the obvious next question is: where is it now? The study points to two likely destinations, and the answer may be a mix of both.

The first is the circumgalactic medium, or CGM — the diffuse halo of warm and hot gas surrounding each galaxy, stretching for hundreds of thousands of light-years. Some expelled oxygen may sit in this halo, like goods stored just outside the factory. Earlier work, including McQuinn's own 2019 study of winds in 12 nearby dwarf galaxies, suggested much ejected material might remain in the halo and eventually rain back down, feeding future star formation.

The second destination is the intergalactic medium, or IGM — the even thinner gas filling the spaces between galaxies. If oxygen escapes the halo entirely, it joins this cosmic reservoir, potentially being swept up by other galaxies over billions of years.

The GLOW results suggest the halo alone may not hold everything. After correcting for different forms of oxygen in the halo, the authors estimate the CGM could hold roughly 5 to 35 percent of the missing oxygen, leaving a large share that has likely escaped into intergalactic space for good. Pinning down the exact split matters, because it decides whether dwarf galaxies are recycling their metals or permanently donating them to the universe at large.

Tracing faint gas across space is becoming a theme in modern astronomy — it echoes how CHIME detected a faint hydrogen glow from the early universe.

What this means for bigger galaxies, planets, and life

You might wonder why the oxygen budget of tiny, distant galaxies should matter to us. Dwarf galaxies are the most common type of galaxy in the universe, and they are widely thought to be the building blocks from which larger galaxies, including our Milky Way, were assembled. If the building blocks leaked most of their metals, then the galaxies built from them started life chemically poorer than we might have assumed.

There is also a connection to planets and life. Oxygen is the element we breathe and a major component of rocky planets and water. The story of how oxygen spreads through the universe — made in stars, blown out of galaxies, drifting through intergalactic space — is part of the story of how the ingredients for planets and life became available at all. A universe where small galaxies hoard their oxygen would be chemically very different from one where they scatter it widely. GLOW suggests we live in the second kind.

What comes next: the Roman Space Telescope

The story does not end with Hubble. McQuinn is also mission head of the Science Operations Center for NASA's Nancy Grace Roman Space Telescope, which launched in August 2026, and she already has an approved programme to use Roman to find even fainter, lower-mass galaxies across a wide swath of sky.

Roman's wide field of view — roughly 100 times that of Hubble's infrared camera — makes it ideal for this kind of census work. Where Hubble studied 37 galaxies in detail, Roman could survey far larger samples of faint systems at once, and search for the faint gas around them that would show whether the missing oxygen is lingering in their halos or truly gone. Future work will also push to even smaller galaxies, where the leak is expected to be more extreme still.

This fits a broader run of telescope surprises. Just as Euclid's first comet discovery showed a survey telescope finding something outside its main mission, GLOW shows Hubble, an ageing but still powerful observatory, answering a question newer instruments have not yet tackled at this scale.

The bottom line

The GLOW study gives us a simple, memorable picture of small galaxies: they are factories with open doors. Their stars work hard, forging oxygen and other elements over billions of years, but supernova winds carry most of that product away. On average, only about one-tenth stays home.

That single number — 11 percent — will now become a benchmark. Every future simulation of dwarf galaxies will have to reproduce it, and every future survey of galactic halos will have to explain where the other 89 percent went. For a universe built from small beginnings, losing almost everything turns out to have been part of the plan all along.

Sources: NASA Science, "NASA's Hubble Uncovers Case of Missing Oxygen in Nearby Dwarf Galaxies" (2 November 2026); Rutgers University / Phys.org, "Stars make oxygen for small galaxies, but they lose most of it" (2 November 2026); McQuinn et al., "GLOW. II. A Census of Oxygen in Low-mass Galaxies," The Astrophysical Journal 1011, 158, DOI: 10.3847/1538-4357/ae93a6.