A rare star opens a window on the beginning of time

EPFL astrophysicists actively participated in the discovery of a very rare star, particularly aged and poor in metals. As a messenger from a distant past, it will allow scientists to learn more about the young universe just after the Big Bang.

"We have made a major discovery that challenges our understanding of the formation of the first generations of stars in the universe. Researcher at the Laboratory of Astrophysics of EPFL (LASTRO), Pascale Jablonka is a founding member of the Pristine Survey, an international project dedicated to the search for the oldest and the poorest stars in metals. This has allowed the recent identification of an extremely rare object. Called Pristine 221, it is one of the 10 poorest stars of metals known to date in the halo of our galaxy. In addition, it is an exceptional star on two completely devoid of carbon. This breakthrough was recently published in the journal Monthly Notices from the Royal Astronomical Society (MNRAS).

Different methods are available to astronomers for the study of the early universe: one is to look far into the universe and to go back in time, to see the first stars and galaxies grow. Another option is to examine the oldest surviving stars of our natal galaxy, the Milky Way, looking for information from the primitive universe. The Pristine study, led by the Leibniz Institute of Astrophysics in Potsdam (AIP) and the University of Strasbourg, is looking for precisely these immaculate stars.

Finding the oldest messengers among the huge population of young stars is not an easy task. Just after the Big Bang, the universe was filled with hydrogen, helium and a little lithium. No heavier items were present, as these are only synthesized in the warm interior of the stars – and these did not exist yet. Our sun contains about 2% heavier elements in its atmosphere, as evidenced by the spectrum of its light. As a result, astrophysicists may conclude that the sun has appeared as part of a subsequent generation of stars and has "recycled" into its atmosphere the products of stars that lived long before and have since died out. In search of the oldest stars, scientists are looking for stars with a much more virgin atmosphere than our sun. The lighter the atmosphere, the earlier the generation in which this star was born was. The study of stars of different generations allows us to understand the history of the formation of the galaxy – a field of research also called near field cosmology.

Chemical abundance

The Pristine team used a special narrow-band filter on the Canada-France-Hawaii telescope to screen candidate stars with a blank atmosphere. This step was then followed by a detailed spectroscopic campaign with the telescopes of the Isaac Newton Group in Spain and the European Southern Observatory in Chile. EPFL researchers, Pascale Jablonka and Carmela Lardo, have formed a team out of three, with the Paris Observatory and the Instituto de Astrofísica de Canarias, in charge of spectroscopic analysis and measurements of chemical abundance, which led to the identification of this very special star. They were able to demonstrate that the star had indeed very few heavy elements in his atmosphere.

"Most of the elements are depleted by factors ranging from 10,000 to 100,000 compared to the sun, and its detailed pattern of different elements is remarkable, while the extremely metal-poor stars generally have a very large increase in carbon. this star does not, the second of its kind and an important messenger of the primitive universe, "explains Else Starkenburg, AIP researcher and first author of the study.

"Scientists used to think that carbon was a necessary coolant, allowing for a small fragmentation of the gas cloud from which stars are formed and leading to the subsequent formation of low-mass stars. In the world of high offsets of red, explains Pascale Jablonka, examples of these ancient and low-carbon stars, the models need to be revised. "

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