The first stars may not have been as uniformly massive as we thought

Summary

Helium hydride? In high school science you probably learned that helium is a noble gas, meaning it does not react with other atoms to form molecules or chemical compounds. As it turns out, it does—but only under the extremely sparse and dark conditions of the early universe, before the first stars formed. HeH⁺ reacts with hydrogen deuteride—HD, which is one normal hydrogen atom bonded to a heavier deuterium atom—to form H₂. In the process, HeH⁺ also acts as a coolant and releases heat in the form of light. So the high abundance of both molecular coolants earlier on may have allowed smaller clouds to cool faster and collapse to form lower-mass stars. Gas flow also affects stellar initial masses In another study, published in July 2025, astrophysicist Ke-Jung Chen led a research group at the Academia Sinica Institute of Astronomy and Astrophysics using a detailed computer simulation that modeled how gas in the early universe may have flowed. The team’s model demonstrated that turbulence, or irregular motion, in giant collapsing gas clouds can form lower-mass cloud fragments from which lower-mass stars condense. The study concluded that turbulence may have allowed these early gas clouds to form stars either the same size or up to 40 times more massive than the Sun’s mass. The galaxy NGC 1140 is small and contains large amounts of primordial gas with far fewer elements heavier than hydrogen and helium than are present in our Sun. This composition makes it similar to the intensely star-forming galaxies found in the early universe. These early universe galaxies were the building blocks for large galaxies such as the Milky Way. Credit: ESA/Hubble & NASA, CC BY-ND The two new studies both predict that the first population of stars could have included low-mass stars. Now, it is up to us observational astronomers to find them. This is no easy task. Low-mass stars have low luminosities, so they are extremely faint. Several observational studies have recently reported possible ...