The Chemistry That Cools Protostellar Clouds
Initial star-forming gas clouds, known as protostellar clouds, were found to be warm—approximately at room temperature. The internal pressure generated by this warm gas pushes outward, countering the gravitational force attempting to collapse the cloud. This phenomenon can be likened to a hot air balloon, which remains inflated as long as the flame heating the air at its base is maintained. Once that heat source ceases, the air cools, leading to the balloon’s collapse.
Credit:
NASA, ESA, CSA, and STScI, J. DePasquale (STScI), CC BY-ND
Only the most massive protostellar clouds, possessing the greatest gravitational forces, had the capability to overcome thermal pressure and undergo collapse. Consequently, it was theorized that the first stars were predominantly massive.
The formation of lower-mass stars, which are more common today, necessitates a cooling of these protostellar clouds. Gas in the cosmos loses heat through radiation, effectively converting thermal energy into light that escapes from the cloud. Although hydrogen and helium atoms are not efficient at radiating heat at lower temperatures, molecular hydrogen (H₂) excels at cooling gas effectively.
When energized, H₂ emits infrared radiation, leading to a decrease in gas temperature and internal pressure. This cooling makes gravitational collapse more probable in less massive clouds.
For decades, astronomers have theorized that the early universe’s limited presence of H₂ contributed to the higher temperatures of clouds, preventing easy collapse into stars. As a result, only massive clouds with stronger gravitational forces could condense, leading to the formation of heavier stars.
The Role of Helium Hydride
A recent study published in a July 2025 journal by physicist Florian Grussie and his team from the Max Planck Institute for Nuclear Physics revealed that the first molecule created in the universe, helium hydride (HeH⁺), could have been more prevalent in the early universe than previously believed. This conclusion was supported through computer modeling combined with laboratory experiments.
