Astronomers have uncovered vast populations of faint, low-mass stars inside distant galaxies, suggesting some of the universe’s earliest cosmic giants may have contained up to four times more stellar mass than scientists previously calculated.

The James Webb Space Telescope has uncovered evidence that some massive galaxies contain far more stars than astronomers had realised, a discovery that could deepen one of the most persistent mysteries raised by Webb since it began observing the early universe.

An international team of astronomers found that massive, ancient galaxies appear to contain unusually large populations of small and faint stars that contribute enormous amounts of mass while producing relatively little light. Because those stars have traditionally been extremely difficult to detect at great distances, researchers may have systematically underestimated how much matter is locked inside some galaxies.

The findings, published on August 18 in Nature Astronomy, suggest that the consequences could be substantial. One particularly old galaxy examined by the researchers may contain enough hidden low-mass stars to imply a stellar population roughly four times more massive than conventional estimates would suggest.

That result could make an already difficult cosmological puzzle even harder to explain.

Since the James Webb Space Telescope began looking deep into cosmic history, astronomers have repeatedly discovered galaxies that appear surprisingly large and mature at epochs when, according to standard models of galaxy evolution, there should have been relatively little time for them to assemble.

Some existed when the universe was only a fraction of its current 13.8-billion-year age.

The question has therefore become increasingly urgent: how did enormous galaxies form so quickly after the Big Bang?

The new research suggests scientists may have been underestimating the scale of the problem.

The team studied nine massive galaxies using extremely deep spectra obtained with Webb and combined those observations with earlier measurements from the European Southern Observatory’s Very Large Telescope. The galaxies themselves are observed at a redshift of around 0.7, but they contain old stellar populations that preserve information about periods much earlier in cosmic history.

Instead of simply measuring how bright each galaxy appeared, researchers examined subtle patterns in its spectrum — essentially separating the galaxy’s light into individual wavelengths.

Those spectral fingerprints can reveal what kinds of stars are present.

The difficulty is that the stars contributing the most light are not necessarily the stars contributing most of the mass.

Large and luminous stars dominate what astronomers can see from enormous distances. Small stars, by contrast, are individually faint but can survive for billions or even trillions of years. If sufficiently numerous, they can account for a substantial fraction of a galaxy’s total stellar mass while remaining almost invisible in conventional observations.

Lead researcher Chloe Cheng compared the effect to observing a city from far away: skyscrapers immediately attract attention, while enormous numbers of much smaller buildings between them can remain effectively hidden.

Webb’s sensitivity allowed the researchers to detect subtle spectral signatures associated with these low-mass stars with a precision that was previously extremely difficult at such distances.

The discovery challenges one of the assumptions astronomers routinely make when estimating the masses of galaxies.

Scientists use what is known as the initial mass function, or IMF, to describe the distribution of stellar masses produced when stars form. In practical terms, it tells astronomers roughly how many small stars should exist for every medium-sized or massive star.

For many distant galaxies, researchers have traditionally assumed a stellar distribution broadly similar to that observed in the Milky Way.

The new observations suggest that assumption may not always hold.

The most massive galaxies studied by the team appear to possess what astronomers call a bottom-heavy initial mass function — meaning they contain a disproportionately large population of low-mass stars.

That difference dramatically changes the calculation of a galaxy’s total mass.

A galaxy populated with many small stars can contain much more matter than another galaxy producing the same amount of light but containing a greater proportion of luminous stars.

The oldest galaxy in the sample is especially important. Researchers estimate that its stars formed at a redshift greater than five, meaning its stellar population originated when the universe was still very young. The team argues that it may therefore be a later descendant of the unusually massive early galaxies Webb has discovered at much greater distances.

If those early galaxies also formed stars according to a similarly bottom-heavy distribution, their true stellar masses could be approximately four times greater than estimates based on a Milky-Way-like stellar population, the researchers calculate.

Rather than solving the problem of unexpectedly massive early galaxies, the discovery could therefore intensify it.

Webb has already forced astronomers to reconsider how efficiently galaxies could transform gas into stars during the universe’s first few billion years. Some objects appear to have assembled enormous stellar populations far sooner than many theoretical models anticipated. Nature’s research briefing on the work says the new findings could increase the tension between these observations and existing galaxy-formation models.

The implications reach beyond the mass of galaxies.

Low-mass stars are also particularly interesting because of their longevity. Unlike massive stars that exhaust their fuel relatively quickly and eventually explode or collapse, small stars burn slowly and can remain stable for extraordinarily long periods.

They are also common hosts of planets.

Mariska Kriek of Leiden Observatory, who led the wider research programme, said that if early galaxies contained substantially more low-mass stars than previously believed, there could also have been more opportunities for planets to form around those stars in the ancient universe.

That does not mean the study has discovered ancient planets, nor does it establish that habitable worlds existed unusually early. But it changes one of the numbers underlying such questions: how many long-lived stars may have existed during the first major phases of galaxy formation.

The result also demonstrates how Webb is changing astronomy in a less spectacular but arguably more fundamental way.

Many of the telescope’s most famous images reveal galaxies that had never previously been visible. But its scientific power comes equally from spectroscopy — the ability to extract detailed physical information from tiny amounts of light travelling across billions of light-years.

In this case, researchers used Webb’s NIRSpec instrument to distinguish the subtle signatures produced by different stellar populations. The observations were sufficiently precise to probe low-mass stars indirectly even though individual stars within those distant galaxies cannot be separately resolved.

The researchers now want to extend the technique to galaxies representing still earlier periods of cosmic history.

That will be critical because the current work does not directly demonstrate that every massive galaxy shortly after the Big Bang contained four times more stellar mass than previously estimated. Instead, it shows that an ancient stellar population surviving in a later galaxy possesses a strongly bottom-heavy stellar distribution and may be linked evolutionarily to Webb’s remarkably massive early galaxies.

Establishing whether the same phenomenon was widespread will require a larger sample and observations reaching further back in time.

Yet even with that qualification, the finding could force significant revisions to calculations of galaxy growth.

Astronomers use stellar mass estimates to reconstruct almost every major stage of cosmic evolution — from how quickly galaxies formed and how efficiently they converted gas into stars, to the relationship between galaxies and the dark-matter structures surrounding them.

If some of the universe’s most massive galaxies contain far more stellar matter than their light suggests, those calculations may need to be reconsidered.

For cosmologists, the discovery therefore presents both an opportunity and a problem.

Webb was built partly to explain how the first stars and galaxies emerged from the comparatively simple universe left behind after the Big Bang. Instead, many of its observations have shown that early cosmic history may have been considerably more productive and complicated than expected.

The newest result adds another layer to that picture.

The early universe may not simply have produced massive galaxies surprisingly quickly. Those galaxies may also have been hiding much of their mass in countless small stars that earlier telescopes could not detect.

And as Webb continues to expose those previously invisible stellar populations, scientists may find that some of the first great cities of stars were considerably larger than anyone had imagined.