Universe’s First Stars May Have Been Smaller Than Astronomers Once Believed

Astronomers once thought the universe's first stars (made of hydrogen and helium) were nearly all giants, hundreds to thousands of times the Sun's mass. For decades, astronomers assumed only huge clouds could collapse in the early universe. Now two new studies challenge that picture. One team ran simulations of collapsing clouds, and another did lab experiments on early-universe chemistry. Both found ways to cool and fragment the gas more effectively, suggesting some first-generation stars could have been much smaller than previously believed.

Turbulence and Star Formation

In the Astrophysical Journal Letters, astrophysicist Ke-Jung Chen reported that each collapsing cloud in primordial gas clouds can be broken up into numerous smaller clumps by supersonic turbulence. Their simulation demonstrated that fragments as small as about one solar mass can form due to chaotic gas flows.

The group came to the conclusion that stars with turbulence could have masses ranging from about one to forty times that of the Sun. That is, low-mass suns may have coexisted with the giants in the first generation of stars.

Primordial Chemistry and Cooling

Under the direction of Florian Grussie, another research team discovered that helium hydride ions (HeH⁺), the first molecule in the universe, could form and persist in greater quantities than anticipated. Their findings were published in Astronomy & Astrophysics.

These HeH⁺ ions emit infrared photons as they react easily with hydrogen, including deuterium, to form molecular hydrogen (H₂). The gas cools as the photons remove thermal energy. Even comparatively tiny primordial clouds can release heat and collapse into stars as a result of this additional cooling. Essentially, modest clouds could form low-mass first stars if molecular cooling becomes more effective.