For the first time, astronomers have found what they called "fingerprints" left by the explosions of the first stars in the universe in three distant clouds.
The first generation of stars made it possible for the universe to have the building blocks of planets and eventually life. These objects were made of just hydrogen and helium and were much bigger – hundreds of times bigger – than the Sun. They burnt brightly and went supernova in a very (cosmically) quick time. For this reason, we have never directly observed them.
Despite their elusiveness, the hunt for them continues at full steam, and astronomers now report the best evidence for the aftermath of the first stars’ explosions. Clouds of gas in the distant universe have the right chemical composition to have been the product of the first-ever stars’ demise.
“For the first time ever, we were able to identify the chemical traces of the explosions of the first stars in very distant gas clouds,” Andrea Saccardi, a PhD student at the Observatoire de Paris - PSL, who led the study during his master’s thesis at the University of Florence, said in a statement.
After the big bang, in a phase called nucleosynthesis, only the lightest elements could form. Hydrogen and helium, with a dash of lithium. The rest were forged by nuclear fusion in the core of stars or through other stellar processes including supernovae.
Astronomers have been studying the effect of the first stars indirectly by looking at the composition of some of the oldest stars in our galaxy, which are believed to have formed from the “ashes” of the first stars. This work shows that it is possible to find the "ashes" themselves.
“Primordial stars can be studied indirectly by detecting the chemical elements they dispersed in their environment after their death,” explained co-author Stefania Salvadori, Associate Professor at the University of Florence. “Our discovery opens new avenues to indirectly study the nature of the first stars, fully complementing studies of stars in our galaxy.”
The team used the Very Large Telescope to find these clouds that existed when the universe was just 10 to 15 percent of its current age. This is already long after the first stars exploded, but luckily, these clouds were not contaminated. The team used the light of even more distant quasars behind these clouds to illuminate them and allow the researchers to work out their chemical composition. The first stars are not expected to have released much iron for example.
In fact, these three clouds are rich in elements such as carbon and magnesium but iron-poor, suggesting that they were enriched by material from the first stars.
The study is published in The Astrophysical Journal.