Free Astronomy Magazine July-August 2023

nature of the first stars that formed after the Big Bang. “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,” says Andrea Saccardi, a PhD student at the Ob- servatoire de Paris - PSL, who led this study during his master’s thesis at the University of Florence. Researchers think that the first stars that formed in the Universe were very different from the ones we see today. When they appeared 13.5 bil- lion years ago, they contained just hydrogen and helium, the simplest chemical elements in nature. These stars, thought to be tens or hun- dreds of times more massive than our Sun, quickly died in powerful ex- plosions known as supernovae, en- riching the surrounding gas with heavier elements for the first time. Later generations of stars were born out of that enriched gas, and in turn ejected heavier elements as they too died. But the very first stars are now long gone, so how can researchers learn more about them? “Primordial stars can be studied indirectly by de- tecting the chemical elements they dispersed in their environment after T his diagram illustrates how astronomers can analyse the chemical composition of distant clouds of gas using the light of a background object like a quasar as a beacon. 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 inter- vening gas cloud. [ESO/L. Calçada]