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

"The first star-forming gas clouds, called protostellar clouds, were warm- roughly room temperature. Warm gas has internal pressure that pushes outward against the inward force of gravity trying to collapse the cloud. A hot air balloon stays inflated by the same principle. If the flame heating the air at the base of the balloon stops, the air inside cools, and the balloon begins to collapse."

"Only the most massive protostellar clouds with the most gravity could overcome the thermal pressure and eventually collapse. In this scenario, the first stars were all massive. The only way to form the lower-mass stars we see today is for the protostellar clouds to cool. Gas in space cools by radiation, which transforms thermal energy into light that carries the energy out of the cloud."

The first star-forming gas clouds, called protostellar clouds, were warm—roughly room temperature, producing thermal pressure that resisted gravitational collapse. Only the most massive clouds could overcome that pressure, leading to predominantly massive first stars. Molecular hydrogen (H₂) cools gas by emitting infrared light, reducing internal pressure and enabling collapse in lower-mass clouds. Hydrogen and helium atoms are poor radiators at low temperatures, so H₂ abundance controls cooling efficiency. Low early H₂ abundance was thought to prevent low-mass star formation. Computer modeling and laboratory experiments show helium hydride (HeH⁺), the first molecule, could have been more abundant, enhancing pathways to H₂ and cooling.

Read at Ars Technica