Astronomers using the James Webb Space Telescope have discovered that some of the largest galaxies in the early universe contained far more small, faint stars than scientists previously realized — making them up to three to four times more massive than earlier estimates suggested.

The findings, published in Nature Astronomy by an international team including researchers from Penn State and Leiden University, add significant pressure to existing theories of how the first galaxies formed and evolved in the billion years after the Big Bang.

The team examined nine massive, mature galaxies that had already stopped producing new stars billions of years ago. By combining JWST observations with ground-based data from the Very Large Telescope, they were able to separate each galaxy's light into a spectrum and, for the first time at such distances, reliably estimate the relative populations of small versus large stars.

'If a galaxy were a city, the brightest stars would be the skyscrapers that immediately catch your eye from afar,' said lead author Chloe Cheng, a recent doctoral graduate of Leiden University. 'Our models demonstrate that a far more numerous population of low-mass stars is concealed by those rare, bright stars, like houses hidden between skyscrapers.'

One galaxy stood out: likely formed less than 1.5 billion years after the Big Bang, it could be as much as four times more massive than previous calculations suggested.

The discovery compounds a problem that has vexed cosmologists since JWST began observing. Since its launch, the telescope has repeatedly found unexpectedly massive and mature galaxies existing far earlier than models predicted they could form. If those galaxies also harbored hidden populations of low-mass stars — which previous techniques were incapable of detecting — the formation timeline puzzle becomes even more acute.

'This discovery has crucial implications for our understanding of the early universe,' said co-author Joel Leja of Penn State. 'Adding up to four times more stars to these massive, early-forming galaxies sharpens these tensions further.'

The implications may extend beyond cosmology. Since many planets orbit low-mass stars, the findings suggest that planetary systems may have been more common in the early universe than previously assumed — raising intriguing questions about when conditions for life first emerged.

The researchers plan to extend the technique to even earlier galaxies, pushing closer to the epoch when the universe's first generations of stars began to form.

Sources