Astronomy | Star formation is less universal than thought

The birth of a star in the Pismis 24 star cluster, captured by the James Webb Telescope. Photo: Nasa Don't lump it, just spill it: When new stars form in the universe, the heaviest of them are the most spectacular - but it is the small, relatively low-mass stars that far outnumber them.

The birth of a star in the Pismis 24 star cluster, captured by the James Webb Telescope. Photo: Nasa The starting material for all stars are so-called molecular clouds, very cold and relatively dense accumulations of gas and dust in space. The extent of the clouds can be up to a few hundred light years, with masses between thousands and several million solar masses. If “disturbances” occur in such a molecular cloud, for example due to the shock waves of a nearby exploding star, areas of the cloud can become denser. Slightly more compact regions become really dense, increasingly spherical clumps that are finally massive and hot enough for hydrogen to fuse into helium: a new star lights up.

Massive and therefore extremely luminous stars can also be observed over long distances - in contrast to the lightest stars, whose reddish glow can no longer be detected over large distances. But if you want to understand the properties and evolution of distant star clusters or entire galaxies, it is not just the few stellar floodlights that count - but also the countless low-mass, rather modestly shining stars.

In order to be able to draw conclusions about the overall mass distribution of the stars in an area from the radiant light of the largest stars, astronomers use the “original mass function”. The empirical function describes the initial mass distribution of stars at the time of star formation. The physicist Edwin Salpeter was the first to diligently count the stars in the solar neighborhood in 1955 and determine their mass from their luminosity. Taking the lifespan of stars into account, Salpeter reconstructed an “original mass function” of our neighborhood.

The source material for all stars are dense accumulations of gas and dust in space.

Since then, that mass function has been revised many times, but the basic principle remains: massive stars are significantly rarer than low-mass stars and the mass function is “universal” and fully valid: whether in our neighborhood or at enormous distances – when a molecular cloud collapses, there is always the same mixture.

However, according to new studies, "universality" may oversimplify the current theory of star formation. High-precision observations of star clusters – areas of increased star density within a galaxy – with the Gaia space telescope paint a more individual picture. If the original mass function were truly universal, each star cluster would contain a similar mix of stars. Instead, the researchers found clear differences from cluster to cluster, suggesting that local conditions influence the type of stars that form. Small-scale temperature and density fluctuations in the molecular cloud and dynamic effects, such as turbulence and flows within the gas, could influence star formation, as well as the chemical composition of the molecular cloud. This composition also makes it possible to understand the development over time, as heavy elements must first be “breeded” in generations of stars before they enrich the interstellar medium.

Not only would this fundamentally change the general picture of star formation that has been accepted for decades, it could also solve a rather new problem. In observation data with the James Webb Telescope, which opened up a view into previously unknown depths of space, extremely distant galaxies were found that appeared more massive than would be expected. However, the discrepancy could be easily explained with a “somewhat more individual mass function”. The distant galaxies are not “too heavy” at all – we just need to adjust our scales a little better.

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