Webb's spectra point to a hidden crowd of small stars in the early universe, and it makes the first big galaxies harder to explain

TL;DR

Astronomers led by Leiden University used the James Webb Space Telescope, together with earlier observations from the Very Large Telescope, to measure the mix of small and large stars in nine massive galaxies that had already finished forming stars early in cosmic history. Published on 18 August 2026 in Nature Astronomy, the study found these galaxies contain a much larger proportion of faint, low-mass stars than astronomers usually assume, a so-called bottom-heavy initial mass function. Because those small stars add up to a lot of unseen mass, the galaxies are heavier than earlier estimates suggested, with one example possibly up to four times more massive. That is not reassuring news for theorists: the authors say it makes the already awkward fact that huge galaxies existed so soon after the Big Bang even harder to explain. The result rests on nine galaxies and on stellar-population models, so it needs independent confirmation.

Ever since the James Webb Space Telescope started returning data, one of its recurring headaches for theorists has been galaxies that look too big, too soon: mature, massive systems that had already stopped forming stars when the universe was only a fraction of its present age. A new study argues that the problem may be worse than it looked, because a large part of these galaxies' mass has been hiding in plain sight.

What did the study actually find?

A team led by Chloe M. Cheng at Leiden University looked at nine massive galaxies that had already passed their main period of star formation early in the universe's history. By combining very deep spectra from JWST with earlier observations from the ground-based Very Large Telescope, the researchers were able to work out the ratio of small, faint stars to large, bright ones inside those distant galaxies, something Cheng's co-author Martje Slob notes "were simply not possible until recently."

The finding: these galaxies hold proportionally far more low-mass stars than the standard recipe predicts. Cheng offers an analogy of a galaxy seen from far away as a city skyline, where only the brightest stars stand out like skyscrapers while the countless small, dim stars that make up most of the population barely register. "Our models show that behind the brightest stars there is a much larger population of small stars, like houses among the skyscrapers," she says. "This means that the galaxy as a whole is much more massive than previous estimates suggested."

The work was published on 18 August 2026 in Nature Astronomy under the title "Hidden mass in early galaxies revealed by bottom-heavy initial mass functions."

Why call them "hidden" stars?

Because nobody counted them directly. When astronomers weigh a distant galaxy, they measure the light and then convert that light into a mass. That conversion depends on an assumption about the galaxy's initial mass function, the distribution of stellar birth masses, which is usually taken to resemble the one measured in our own Milky Way.

The catch is that most of a galaxy's light comes from a relatively small number of massive, luminous stars, while most of its actual stellar mass sits in vast numbers of small, dim stars that contribute almost nothing to the glow. You cannot see those small stars individually across billions of light-years, so their contribution is inferred, not observed. If a galaxy has more of them than the Milky-Way template assumes, its light will systematically undersell its mass.

What is a "bottom-heavy" initial mass function?

It means the balance tips toward the low-mass end: more small stars for every big one than the standard assumption allows. The Leiden team's measurements point to exactly that in these early quiescent galaxies. A bottom-heavy mix packs extra mass into stars that add little light, which is why the corrected masses come out higher.

How much heavier are the galaxies?

Enough to matter, and in the most extreme case, dramatically so. According to the accompanying phys.org report, one galaxy in the sample, which likely formed less than one and a half billion years after the Big Bang, "may be as much as four times more massive than previous estimates indicated." The paper does not claim a uniform four-fold jump for all nine; the size of the correction varies, but the direction is consistent: these galaxies were heavier than we thought.

Why does this deepen the "impossible early galaxies" problem?

Because the original puzzle was already about mass. Standard models of how galaxies assemble struggle to build very massive, already-quiet galaxies as early as JWST keeps finding them. Revising those same galaxies upward makes the tension sharper rather than softer. As the researchers put it, models of galaxy formation must now explain how such vast quantities of tiny stars could have formed so early. It is the kind of result that does not resolve a mystery so much as raise the stakes on it.

How confident should we be in this?

Treat it as a strong, careful measurement that still needs backing up. This is one study of nine galaxies, and the low-mass stars at the heart of the claim are inferred from stellar-population models fitted to integrated light, not counted one by one, which is unavoidable at these distances but keeps the result model-dependent. The natural next steps are more galaxies and independent teams testing whether the same bottom-heavy signature shows up in their data. The measurement technique is the genuine advance here; the cosmic implications will firm up as others reproduce it.

The study at a glance


Who	Team led by Chloe M. Cheng, Leiden University
What	Measured the ratio of small to large stars in nine massive, already-quenched early galaxies
How	Deep JWST spectra combined with earlier Very Large Telescope observations
Key result	A bottom-heavy initial mass function: far more low-mass stars than a Milky-Way-like assumption predicts
Mass impact	Galaxies heavier than earlier estimates; one example possibly up to 4x more massive
Why it matters	Sharpens the puzzle of why such massive galaxies existed so early
Published	18 August 2026, Nature Astronomy (DOI 10.1038/s41550-026-02932-4)
Main caveat	Nine galaxies; low-mass stars inferred from models, not directly observed; needs independent confirmation

Key takeaways

A Leiden University team used deep JWST spectra plus earlier Very Large Telescope data to measure the small-to-large star ratio in nine massive galaxies that had already stopped forming stars early in the universe.
They found a bottom-heavy initial mass function: proportionally many more faint, low-mass stars than the Milky-Way-like template usually assumed, adding mass that light alone does not reveal.
The galaxies come out heavier than earlier estimates, by amounts that vary across the sample, with one formed less than 1.5 billion years after the Big Bang possibly up to four times more massive.
That makes the "impossible early galaxies" problem harder, not easier: models must now account for huge numbers of small stars forming very early.
The result comes from nine galaxies and model-dependent inferences rather than direct star counts, so independent confirmation is the next step.

This is another example of how JWST keeps reshaping the early-universe picture, a run of surprises that also includes its findings on black holes in the young cosmos and feeds into the wider debate over how the first structures assembled. The instruments are finally precise enough to weigh what used to be invisible; the theory now has to catch up.