When the first stars appeared more than 13.5 billion years ago, the universe consisted only of nature’s simplest chemical elements: hydrogen and helium. The elements that make up our own bodies, such as carbon, oxygen and iron, were first synthesized inside the first stars and then released into the surrounding gas by the first supernova explosions that occurred when massive stars died.
The nature of primordial stars is still unknown. However, these stars can be studied indirectly through the detection of chemical elements that they dispersed into the environment, after their death, and that are still present in the diffuse gas or that have been incorporated by the following generations of so-called "normal" stars.
Using ESO’s Very Large Telescope, astronomers have detected three distant gas clouds whose chemical composition matches what we expect from the explosions of the first stars that appeared in the Universe. These early stars can be studied indirectly by analysing the chemical elements they dispersed into the surrounding environment after they died in supernova explosions. The three distant gas clouds detected in this study are rich in carbon, oxygen, and magnesium, but poor in iron. This is exactly the signature expected from the explosions of the first stars.
The study in the May 3, 2023, issue of Astrophysical Journal was based on analysis of archival data from a sample of 100 spectra of large spectral shift quasars obtained with the X-shooter spectrograph, installed on ESO’s Very Large Telescope. It reports for the first time the detection of three distant gas clouds with a chemical composition consistent with what is expected from early stellar explosions.
When the light of the quasar passes through the gas cloud, the chemical elements in it absorb different colours or wavelengths, leaving dark lines in the spectrum of the quasar. Each element leaves a different set of lines, so by studying the spectrum astronomers can work out the chemical composition of the intervening gas cloud.
"For the first time, we have been able to identify the chemical traces of the explosions of the first stars in very distant gas clouds, observed when the Universe was only 10-15% of its size", explains Andrea Saccardi, a doctoral student at Paris Observatory - PSL, who conducted the study as part of his master’s thesis at the University of Florence.
These clouds have a very low iron content, but show an overabundance of carbon and other light elements. This particular chemical composition has also been observed in many old stars in our galaxy, which the researchers consider to be second generation stars that formed directly from the "ashes" of the first.
Why is this result important?
We know that stars are formed from gas. There are therefore gas clouds in the distant (and early) Universe with the same chemical composition as the second generation stars in our Galaxy. This work has thus allowed to identify them and to understand which types of gas clouds must be analyzed to reveal the chemical signature of the first supernovae.
This discovery opens new ways to study indirectly the nature of the first stars, fully complementing the stellar archaeology studies of the ancient stars in our Galaxy.
Reference
The article is published as "Evidence of first stars-enriched gas in high-redshift absorbers" by Andrea Saccardi and all, in the journal Astrophysical Journal dated May 3, 2023.
DOI: 10.3847/1538-4357/acc39f
For more information
From the ESO website: "Astronomers find distant gas clouds with leftovers of the first stars" - Science Release eso2306