The origin of one of the oldest stars in the Milky Way revealed

The stars with the lowest metal content are considered the oldest in the Milky Way, formed just a few hundred million years after the Big Bang, a time considered fleeting compared to the age of the universe. These stars are authentic living fossils that carry the first stages of evolution of the cosmos encoded in their chemical composition.

One of them is SMSS1605-1443, discovered in 2018 and identified as one of the oldest in the galaxy due to its chemical composition, but its true nature was unknown. Now, thanks to the work of several European research groups and the ESPRESSO spectrograph –installed on the Very Large Telescope (VLT) in Chile–, the origin of this gem of stellar archeology has been deduced. The results are published in the journal astronomy and astrophysics.

It was a surprise to discover that this object was a double star, something that until recently was not believed to be the case for most of these very old stars.

David Aguado (University of Florence)

“It was a surprise to discover, thanks to ESPRESSO and the VLT, that this object was actually a double (or binary) star, something that until recently was not believed to occur in most of these very old stars”, says David Aguado. , author of the article and researcher at the University of Florence (Italy).

The great precision of the ESPRESSO instrument allowed us to follow the small variations in the velocity of this star, which confirm its binary nature, but leave open that of its companion.

Material from the first massive stars

These types of stars are believed to have formed from material processed within the first massive stars, ejected in supernova explosions in the early stages of the formation of the Milky Way. As a consequence, they are iron-poor but carbon-rich generated within these first massive stars.

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The high resolution of ESPRESSO also made it possible to analyze in detail the relative isotopic composition of carbon, bringing new information about the origin of this object.

The relative amount of carbon 12 and 13 proves that the primordial composition of the star has not changed. It’s like having a sample of what the environment was like in which it formed more than 10 billion years ago.

Jonay Gonzalez Hernandez (IAC)

Co-author Jonay González Hernández, a researcher at the Instituto de Astrofísica de Canarias (IAC), clarifies: “The key was given to us by the ratio of carbon-12 and carbon-13 that we found in the atmosphere of this star. The relative amount of these two isotopes proves that the star’s internal processes have not altered its primordial composition. It’s like having an intact sample of what the environment in which it formed more than 10 billion years ago was like.”

“This discovery must be understood in the context of a project that started a decade ago, in which we studied in detail all known stars of this rare class until we found this wonderful discovery, which helps us to better understand the chemical evolution of the Universe”, highlights another. by the authors, Carlos Allende Prieto, from the IAC.

The director of this institute and co-author Rafael Rebolo points out that the multidisciplinary team that has been formed with researchers from Spain, Italy, France, Portugal and Switzerland, “highlighted that the ESPRESSO spectrograph -in which the IAC participated- is one of the best and most modern instruments for studying the formation of the first stars.”


DS Aguado et al: “The primitive nature of SMSS1605-1443 revealed by Espresso”, Astronomy & Astrophysics, 2023

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