A new study using the James Webb Space Telescope (JWST) has revealed a hidden population of low-mass stars in the universe's earliest galaxies, suggesting these structures are significantly more massive than previously thought. The findings, published last week in the journal Nature Astronomy, challenge current models of the universe's early growth and star formation.

Hidden Stars in Early Galaxies

Scientists have long observed that the most massive galaxies in the early universe formed rapidly after the Big Bang. But a closer look at nine such galaxies, which ceased forming new stars billions of years ago, now indicates that they contain far more low-mass stars than expected—and that these stars hold a great deal more mass than existing estimates had captured.

The team, led by researchers from Leiden University in the Netherlands, combined JWST's deep spectral observations with data from the Very Large Telescope (VLT) on Earth. By analyzing the total light, or spectrum, emitted from each galaxy, they identified subtle color details that distinguished the faint signatures of low-mass stars from the brighter emission of massive ones.

"If a galaxy were a city, the brightest stars would be the skyscrapers," said Chloe Cheng, a researcher from Leiden University and lead author of the study. "A far more numerous population of low-mass stars is concealed by those rare, bright stars, like houses hidden between skyscrapers," she added.

According to the study, the proportion of small stars in these early galaxies is far greater than in modern galaxies like the Milky Way. This came as a surprise to scientists, who had previously assumed that stars of different masses would form in similar proportions across cosmic history.

A Larger Hidden Mass

As a consequence, the galaxies' total mass is much larger than prior estimates had suggested. "This means that the galaxy as a whole is much more massive than previous estimates suggested," said Cheng in a statement.

The researchers noted that light from faint, low-mass stars is typically drowned out by the brighter stars in such distant systems, which made the discovery possible only with the advanced capabilities of JWST and the VLT, indeed with exceptional spectral quality. "We needed not only a telescope capable of magnifying very distant galaxies, but also spectra of exceptional quality and new analysis techniques to reliably detect the subtle signatures of faint, low-mass stars hidden within these cosmic titans," said Martje Slob, a co-author from Leiden University.

The team's models revealed that one galaxy that formed less than 1.5 billion years after the Big Bang may actually contain up to four times more mass than previously believed.

Implications for Planet Formation and the Early Universe

The findings carry implications beyond just measurements of galaxy masses. Because many planets are known to orbit low-mass stars, the existence of a far larger population of such stars in the early universe could mean that more planets formed in that era than previously assumed.

"This result shows that much more mass than previously thought is hidden in small stars," said Mariska Kriek, team leader from Leiden University. "That has consequences for many fields within astronomy. Because many planets orbit around small stars, it could even mean there were more planets formed in the early universe than we previously assumed."

This study also challenges the previous assumption that stars of different sizes are born in the same proportion across the universe. The new data point to a far greater number of low-mass stars in the most massive early galaxies than in smaller ones, an asymmetry that scientists had not expected.

The research team intends to apply this method to more early galaxies, extending their observations back to the era of the universe's first stars.