How We Found the Fingerprints of the First Stars in the Universe

Category Astronomy

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Using ESO's VLT telescope, researchers have discovered the fingerprints of the explosions of the first stars in very distant gas clouds. These first stars contained only hydrogen and helium and released elements like carbon, oxygen and magnesium when they detonated in a supernova. The team found 3 distant clouds in the early universe that contained these elements and lack of iron, the fingerprint of low energy supernovae explosions.

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Using ESO’s Very Large Telescope (VLT), researchers have found for the first time the fingerprints left by the explosion of the first stars in the Universe. They detected three distant gas clouds whose chemical composition matches what we expect from the first stellar explosions. These findings bring us one step closer to understanding the 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 Observatoire de Paris – PSL, who led this study during his master’s thesis at the University of Florence.

The first stars formed after the Big Bang formed 13.5 billion years ago

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 billion years ago, they contained just hydrogen and helium, the simplest chemical elements in nature.[1] These stars, thought to be tens or hundreds of times more massive 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 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 detecting the chemical elements they dispersed in their environment after their death," says Stefania Salvadori, Associate Professor at the University of Florence and co-author of the study published today in the Astrophysical Journal.

The first stars were composed of hydrogen and helium, the simplest elements in nature

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

Using data taken with ESO’s VLT in Chile, the team found three very distant gas clouds, seen when the Universe was just 10–15% of its current age, and with a chemical fingerprint matching what we expect from the explosions of the first stars. Depending on the mass of these early stars and the energy of their explosions, these first supernovae released different chemical elements such as carbon, oxygen and magnesium, which are present in the outer layers of stars. But some of these explosions were not energetic enough to expel heavier elements like iron, which is found only in the cores of stars. To search for the telltale sign of these very first stars that exploded as low energy supernovae, the team therefore looked for distant gas clouds poor in iron but rich in the other elements. And they found just that: three faraway clouds in the early Universe with very little iron but plenty of carbon and other elements — the fingerprint of the explosions of the very first stars.

The explosions of the first stars are known as supernovae

This peculiar chemical composition has also been observed in many old stars in our own galaxy, which researchers consider to be second-generation stars that formed directly from the ‘ashes’ of the first ones. This new study has found such ashes in the early Universe, thus adding a missing piece to this puzzle. "Our discovery opens new avenues to indirectly study the nature of the first stars, fully complementing studies of stars in our galaxy," explains Salvadori.

There is a chemical fingerprint which matches what is expected from the explosions of the first stars

To detect and study these distant gas clouds, the team used light beacons known as quasars — very brigth active galactic nuclei at the centres of galaxies. Since quasars are so bright, they are visible even when the Universe was very young and make ideal backlights to probe and observe very distant clouds. The team combines data from the VLT’s instruments MUSE and X-shooter with images from Hawaii’s 8-metre Gemini North telescope and the 10-metre Keck II telescope to measure the chemical elements of these ‘fingerprints’ in the gas clouds.

These first supernovae released chemicals such as carbon, oxygen, magnesium and iron

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