Gaia Data Show Initial Mass Function Is Not Universal
A team from the University of Missouri measured the break mass in 110 Milky Way star clusters using Gaia mission data and found that the initial mass function differs from cluster to cluster.
Data from the European Space Agency’s Gaia mission have been used for a measurement that challenges one of astronomy’s oldest working rules. A team from the University of Missouri and the University of Rochester measured the mass distribution of stars in 110 Milky Way star clusters and found that the initial mass function, a rule more than 50 years old, is not the same everywhere. The paper, titled “Direct Evidence for Stellar Initial Mass Function Variation in the Milky Way,” was published in The Astrophysical Journal Letters, and the University of Missouri announced it on August 11, 2026. The authors are Charles Steinhardt, professor of astronomy at the University of Missouri, Carter Meyerhoff, an undergraduate student at the same university, and Alexander Lining of the University of Rochester.
What the initial mass function is
Astronomers cannot directly see most of the smallest and faintest stars in distant galaxies. The light that reaches us from such a galaxy comes mostly from large, bright stars, while the small ones are lost in that glare. The initial mass function is a rule describing the ratio of large to small stars at the moment a cluster forms, and it is used to estimate, from what is visible, how much lies beyond view.
Estimates of galaxy mass, age and evolution rest on that calculation, and the assumption that has gone with it for more than half a century was that stars form in roughly the same proportions everywhere. Steinhardt said in the university’s statement:
One of astronomy’s basic assumptions may be too simplified. Other galaxies weren’t breaking the laws of physics, we were measuring them with the wrong yardstick.
The yardstick in that sentence is precisely the initial mass function, which in galaxy studies is not used as a rough estimate but as an input that the final figure depends on.
How the measurement was done
Gaia has mapped nearly two billion stars in the Milky Way, and only that level of precision made it possible to measure and compare individual clusters. From a catalog of 7,167 star clusters, the team selected 110 that were observed with sufficient quality and contained enough stars for a reliable measurement.
The task was not simply counting stars per cluster. What is visible in a cluster today is not its initial mass distribution, because a cluster changes over millions and billions of years: the largest stars live out their lifespans and disappear, while the smallest ones get ejected from the cluster. The slopes of the mass function get distorted as a result. The one quantity that stays stable through this is the break mass, the point where the distribution bends, and the team used it as the measure that can be compared across clusters of different ages.
What the team found
The differences in break mass between clusters are substantial. They correlate with cluster age, meaning with the properties of the cloud the cluster formed from, not with the later evolution of the stars within it. It follows that clusters formed billions of years ago had a different initial mass function than clusters forming today.
That leads to the most interesting consequence. Some galaxies photographed by the James Webb Space Telescope appear more massive than expected, and that could be a result of stars having formed in them in different proportions, in an environment that did not resemble today’s Milky Way. The authors therefore do not propose discarding the initial mass function, but expanding it to account for the conditions under which stars form.
The pattern we found is surprisingly clean. Instead of applying the same model to every galaxy, astronomers could take into account the conditions under which the stars formed and choose the initial mass function that best fits that environment.
That is how Meyerhoff, a co-author of the paper and undergraduate student, described the practical side of the finding in the university’s statement.
What this does not prove
The measurement was carried out on 110 clusters selected from a catalog of 7,167, and all of those clusters are in the Milky Way. Conditions in early galaxies, where density and chemical composition were different, were not measured directly, they are inferred indirectly.
The link to cluster age was measured as a pattern in the data, not described as a physical mechanism. The paper also does not provide a new mass figure for any specific distant galaxy, nor does it say by how much past estimates were off. It shows that one input is not a constant, and that every estimate relying on it should therefore be revised. Steinhardt summed that up in the university’s statement:
We found that the universe is more complicated than we assumed. But at the same time, we’re getting closer to measuring it correctly.
The next step the authors propose is a more flexible description of the initial mass function, with a parameter that captures the environment in which stars formed. Until that is built and tested on data beyond the Milky Way, mass estimates for distant galaxies remain what they are, useful and uncertain at the same time.
