‘Hidden’ stars make first galaxies much more massive than previously thought
Astronomers at Leiden University have led an international team to discover that the most massive galaxies in the young Universe contain far more small stars than previously thought. This means these early galaxies were probably much more massive than we had assumed.
The team made the discovery using the James Webb Space Telescope (JWST). The researchers studied nine massive galaxies that had already passed the main period of star formation. By combining JWST’s exceptionally deep spectra with earlier observations from the Very Large Telescope, they were able to determine for the first time how many small stars these distant galaxies contain. The results have been published in Nature Astronomy.
‘Hidden like houses among the skyscrapers’
Astronomers can learn which types of stars a galaxy contains by studying its light. They split the light into a spectrum, in which subtle differences between colours reveal information about the different types of stars. The bright, massive stars stand out most clearly in these spectra, while the much fainter small stars are difficult to detect.
‘If a galaxy were a city, the brightest stars would be the skyscrapers you can immediately see from far away,’ says first author Chloe Cheng, who completed her PhD in June with research that included this study. ‘Our models show that behind the brightest stars there is a much larger population of small stars, like houses among the skyscrapers. This means that the galaxy as a whole is much more massive than previous estimates suggested.’
Meer kleine sterren in zware sterrenstelsels
Until now, astronomers assumed that the ratio in which small and large stars are born was roughly the same throughout the Universe. The new results, however, show that the most massive galaxies in the early Universe contain a much larger proportion of small stars than less massive galaxies, such as our own Milky Way. In one of the galaxies studied, the difference is particularly large. This galaxy probably formed less than 1.5 billion years after the Big Bang and may be four times more massive than we previously thought.
‘Measurements like these were simply not possible until recently.’
‘Measurements like these were simply not possible until recently,’ says co-author Martje Slob. ‘We needed not only a telescope capable of collecting enough light, but also exceptionally high-quality spectra and new analysis techniques. Only then could we reliably distinguish the subtle features of small stars from the light of much brighter stars.’
Surprisingly massive galaxies
The discovery has important implications for our understanding of the young Universe. Since the launch of the JWST, astronomers have found surprisingly massive galaxies that already existed shortly after the Big Bang. The new results suggest that these galaxies were probably even more massive than previously thought. Models of galaxy formation must now explain how such enormous numbers of stars could have formed so early in the history of the Universe.
More mass, more planets?
‘This result shows that much more mass than we thought is hidden in small stars,’ says Professor Mariska Kriek, who led the study. ‘It has implications for many areas of astronomy. Small stars often have planets orbiting them, so if there were many more small stars in the early Universe than we thought, there may also have been more planets.’
The researchers plan to apply their method to even earlier galaxies in the coming years. In doing so, they hope to get closer to the period when the first generations of stars and galaxies formed.
Scientific publication
‘Hidden mass in early galaxies revealed by bottom-heavy initial mass functions’, C. Cheng et al., Nature Astronomy, 18 augustus 2026.
This press release was originally published on astronomie.nl