An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the discovery of primordial stars that could belong to the so-called Population III, the first generation of stars in the universe. A team of astrophysicists led by Eros Vanzella and Massimo Meneghetti of the Italian National Institute of Astrophysics, Bologna, used data collected with the MUSE instrument mounted on ESO’s VLT and a powerful gravitational lens generated by the galaxy cluster MACS J0416.1-2403, or simply MACS J0416, to obtain magnified images of stars that seem free of metals and composed only of hydrogen, helium and traces of lithium.

The latest generation telescopes can scan primordial galaxies that contain stars very low in metals. The presence of heavy elements in stars is called metallicity and is due to the fact that the stars that form absorb gas but also dust that can be considered the ashes of older stars that exploded in supernovae producing heavy elements. However, according to the accepted cosmological models, the Big Bang generated only hydrogen, some helium and some traces of lithium, so the first stars, called Population III, could only contain those elements. So far no star has been discovered that is certainly part of Population III, but this new research identified some interesting candidates.

To find these candidates, the team led by Eros Vanzella and Massimo Meneghetti took advantage of the gravitational lensing effect of the galaxy cluster MACS J0416, which magnified the electromagnetic emissions from a distant galaxy. To examine the stars of that galaxy they used the Multi Unit Spectroscopic Explorer (MUSE) instrument installed on ESO’s Very Large Telescope (VLT) in Chile to verify their metallicity.

The result of the spectroscopic analysis of those stars offered a surprise. The emission of hydrogen, the one known as Lyman-alpha, was strong at levels that only first generation stars can have. This is an indirect evidence because it’s based on the intensity of the hydrogen spectroscopic line but that’s enough to make them very interesting Population III candidates.

The researchers intend to carry out a follow-up study on these candidates using the VLT again to try to analyze the helium spectroscopic line. In the next few years, it will be possible to conduct even more accurate observations with the Extremely Large Telescope (ELT), which is scheduled to enter service in 2025. Identifying with certainty Population III stars will also allow to investigate their size, at the center of discussions because certain models predict that they could even reach a thousand solar masses.