Using ESO’s Very Large Telescope (VLT), researchers have for the first time found fingerprints left by the explosion of the universe’s first stars. They discovered three distant gas clouds whose chemical composition matches what we would expect from the first explosions of stars. These findings bring us closer to understanding the nature of the first stars formed after the Big Bang.

“For the first time in history, we have been able to identify the chemical signatures of the explosions of the first stars in very distant gas clouds,” says Andrea Saccardi, a graduate student at the Paris Observatory (PSL) who led the study. During her master’s thesis at the University of Florence.

Researchers believe that the first stars to form in the universe were very different from the stars we see today. When they arose 13.5 billion years ago, they contained only hydrogen and helium, the simplest chemical elements in nature. ^[1]These stars, believed to be tens or even hundreds of times larger than our Sun, quickly died in powerful explosions known as supernovae, enriching the surrounding gas with heavier elements for the first time. Later generations of stars were born from this enriched gas, and as they died, they ejected heavier elements. But the oldest stars are long gone, so how can researchers learn more about them? “Primitive stars can be studied indirectly by detecting the chemical elements they emit after their death,” says Stefania Salvadori, co-author of the study published today in the Astrophysical Journal and an associate professor at the University of Florence.

Using ESO’s Very Large Telescope, astronomers have found fingerprints left by the explosions of the first stars. Credit: ESO

Using data from ESO’s VLT in Chile, the team found three very distant gas clouds, observed when the universe was only 10-15% of its current age, with a chemical signature that matches what we would expect from the explosions of the first stars. Depending on the mass of these early stars and the energy of their explosions, these early supernovas released different chemical elements such as carbon, oxygen and magnesium found in the outer layers of the stars. But some of these explosions weren’t energetic enough to eject heavier elements like iron, which are only found in the cores of stars. The team searched for distant clouds of gas, poor in iron but rich in other elements, to find signs of these earliest stars to explode as low-energy supernovae. And this is what they found:

This diagram shows how astronomers can analyze the chemical composition of distant gas clouds using the light of a background object, such as a quasar, as a beacon.

This particular chemical composition has also been observed in many ancient stars in our galaxy, which researchers believe are second-generation stars formed directly from the “ashes” of the first. This new study found such ash in the early universe and adds a missing piece to that puzzle. “Our discovery opens new avenues to indirectly study the nature of the first stars and completely complements the study of stars in our galaxy,” explains Salvadori.

To detect and study these distant gas clouds, the team used beams of light known as quasars, very bright sources powered by supermassive black holes at the centers of distant galaxies. As the light from a quasar travels through the universe, it passes through gas clouds where various chemical elements leave their mark on the light.

To find these chemical signatures, the team analyzed data from several quasars observed by the X-shooter instrument on ESO’s VLT. The X-shooter splits light into an extremely wide spectrum of wavelengths or colors, making it a unique tool for identifying many different chemical elements in these distant clouds.