Supercomputer has solved mystery about the mass of the first stars

The universe is an ever-expanding entity that once started out as tiny. A question that still concerns astronomers is: how massive were those first stars? With the help of an American supercomputer, Taiwanese scientists have now made significant progress in calculating the birth weight of those first stars.

Shortly after the Big Bang, only hydrogen and helium existed. In the earliest phases of the universe, crucial building blocks for life, such as carbon and oxygen, had yet to appear. About 200 million years later, the first stars began to form.

Foundation for life
These Population III or Pop III stars, as they have been dubbed, eventually led to the creation of heavier elements through nuclear combustion in their cores. Near the end of their life cycles, some of those first stars turned into supernovae. The powerful explosions that resulted caused the spread of new elements in the early universe, which would form the foundation for life.

The type of supernova that formed at that time depended on the mass of the first star at its demise. This created different chemical patterns. Observations of extremely metal-poor stars, which formed after the first stars and their supernovae, were previously crucial for estimating the typical masses of the first stars. Based on the large number of metal-poor stars, it was estimated that the first stars had a mass comparable to twelve to sixty solar masses.

Gap between simulation and observation
However, previous cosmological simulations suggested a top-heavy and broadly distributed mass of the first stars, ranging from fifty to a thousand solar masses. This large discrepancy between the simulations and observations has been a mystery to astronomers for at least ten years.

To find a solution, two scientists from Taiwan got to work with the powerful supercomputer at the Berkeley National Lab. They have succeeded in developing, for the first time ever, a high-resolution hydrodynamic 3D simulation of the turbulent star-forming clouds that preceded the first stars.

Turbulence during star formation
The results are special: supersonic turbulence, or extreme chaotic motion, caused the star-forming clouds to break up into several clumps, each with a dense core of 22 to 175 solar masses. The first stars were formed from this with a mass of 8 to 58 solar masses and that does correspond with the observations.

Furthermore, the researchers can reproduce similar results from previous simulations, even if the turbulence is weaker. This result shows the importance of turbulence in early star formation and provides a promising way to reduce the theoretical scale of the mass of the first stars. The discrepancy between the simulations and the observations has now finally been resolved, creating a solid theoretical foundation for the formation of the first stars.

The big Bang
Although we don’t know for sure, we assume that the universe was created about 13.8 billion years ago from a so-called singularity, a very hot point with an almost infinite density. This singularity did not obey the known laws of nature and space and time did not exist. After the Big Bang, space and time were created and the universe began to expand and we are still in the middle of that. In those early days of the universe it was pitch dark. Only after about 200 million years did the first stars begin to form. These first generation stars are called Population III stars. They emerged from primordial clouds of hydrogen and helium with trace amounts of lithium. After just a few million years they exploded and became supernovae. All heavier chemical elements, such as nitrogen, carbon, oxygen and iron, were formed in the core of this and later stars. Only in 2015 did ESA’s Very Large Telescope find evidence for the existence of these primordial stars, for which there were only theoretical arguments until then.