For decades, astronomers have been captivated by the mysteries surrounding the universe's very first stars. These primordial stars played a crucial role in forming new chemical elements, enriching the cosmos, and paving the way for the next generations of stars and planets. Initially, the first stars were composed entirely of hydrogen and helium; they were colossal, boasting masses hundreds to thousands of times greater than our sun and shining with luminosities millions of times brighter. Their brief lifespans culminated in spectacular explosions known as supernovae, leading many to believe that these ancient stars could no longer be observed today. However, recent groundbreaking studies suggest otherwise.
Recent studies published in early 2025 have introduced the idea that collapsing gas clouds in the early universe may have also given rise to lower-mass stars. One of these studies utilized a cutting-edge astrophysical computer simulation that models turbulence within gas clouds, which can lead to fragmentation into smaller, star-forming clumps. The second study, an independent laboratory experiment, proposed that molecular hydrogen, a crucial component for star formation, may have formed earlier and in greater quantities than previously expected. This intriguing possibility could redefine our understanding of chemical processes in the universe's first 50 to 100 million years after the Big Bang.
Stars emerge from massive clouds of hydrogen that collapse under their own gravity. This collapse continues until a luminous sphere forms around a dense core, reaching temperatures hot enough to sustain nuclear fusion. In the earliest stars, hydrogen atoms fused to create helium, leading to the star's brightness as energy from the core radiates outward. The total energy output of a star is referred to as its luminosity, while the brightness we observe is just a small fraction of that energy. The process through which stars synthesize heavier elements is known as stellar nucleosynthesis and continues throughout a star's life, culminating in supernova explosions that create even heavier elements.
While high-mass stars burn through their fuel quickly, lasting only a few million years, lower-mass stars like our sun evolve much more slowly, with lifetimes spanning billions or even trillions of years. If the first stars were predominantly high-mass, they would have exploded long ago. However, if lower-mass stars were also formed, they might still be observable today.
The initial star-forming gas clouds, known as protostellar clouds, were warm, with temperatures around room temperature. This warmth produced internal pressure that counteracted the gravitational pull attempting to collapse the cloud. Only the most massive protostellar clouds could overcome this thermal pressure to collapse. Consequently, it was believed that only massive stars could form. To create lower-mass stars, these protostellar clouds would need to cool. Gas in space cools by radiating energy, which transforms thermal energy into light that escapes the cloud. While hydrogen and helium atoms are not efficient radiators at low temperatures, molecular hydrogen (H₂) is excellent at cooling gas, making gravitational collapse more feasible in lower-mass clouds.
A study published in July 2025 by physicist Florian Grussie and his team at the Max Planck Institute for Nuclear Physics revealed that the first molecule to form in the universe, helium hydride (HeH⁺), could have been more abundant in the early universe than previously believed. Their research combined computer modeling with laboratory experiments to confirm this hypothesis. Contrary to what is often taught in high school science, helium, as a noble gas, can react under the rarefied conditions of the early universe to form molecules.
HeH⁺ interacts with hydrogen deuteride (HD) to produce H₂, acting as a coolant and releasing heat in the process. The high abundance of these molecular coolants might have enabled smaller clouds to cool more rapidly, increasing the likelihood of forming lower-mass stars.
Another significant study, also released in July 2025, led by astrophysicist Ke-Jung Chen at the Academia Sinica Institute of Astronomy and Astrophysics, examined how gas flow in the early universe influenced stellar initial masses. Using sophisticated computer simulations, the research team demonstrated that turbulence within giant collapsing gas clouds could lead to the formation of lower-mass cloud fragments. These findings suggest that the first population of stars could have included stars ranging from the same size to up to 40 times the mass of our sun.
The implications of these studies are profound, as they open the door to the possibility that the second generation of stars, which could be the oldest stars we can currently observe and potential hosts for the first planets, may have formed earlier than previously thought. Now, it is up to observational astronomers to locate these elusive lower-mass stars. However, this task presents challenges; low-mass stars emit low luminosity, making them extremely faint and difficult to detect. While several observational studies have reported possible sightings, none have yet achieved high confidence in their findings. Nevertheless, the search continues, and astronomers remain hopeful that these ancient stars will eventually be found.
