Discovery of a second non-carbon star in the Galaxy

These stars are extremely rare. They are the relics of the earliest ages of the Universe, witnesses to a period when the Univeed had not yet been polluted by the continuing debris of dead stars.

Astrophysicists have a variety of methods to study the first ages of the Universe : they can in effect study the most distant galaxies - which are also the youngest - and observe them even while they are being assembled.

They can also search for and examine the oldest stars of our own galaxy, the Milky Way. That is precisely the context of galactic archeology which constitutes the "Pristine" collaboration directed by Nicolas Martin (Observatoire astronomique de Strasbourg / CNRS / Université de Strasbourg) and Else Starkenburg (Leibniz Institute for Astrophysics, Potsdam), to which have contributed scientists from the Paris Observatory, in the Galaxies, Etoiles, Physique et Instrumentation (GEPI - Galaxies, stars, physics andiInstrumentation) department.

The primordial universe hardly contained anything other than hydrogen and helium. The thermonuclear reactions which take place in the cores of all stars during their lifetime create elements heavier than helium (such as carbon, oxygen, calcium, iron …) starting from the hydrogen and helium which dominate the composition of their gas.

When they explode at the end of their life, they enrich the interstellar gas with these elements, which become the nurseries for the next generations of stars.

That is why the atmosphere of our Sun, a relatively young star, is made up of about 2% of heavy elements. On the other hand, very old stars, which were born very early on, is typically poor in heavy elements.

The existence of a class of rare objects confirmed

This classe of star is extremely rare and hard to find in our cosmic environment cosmique.

Thanks to a new sky survey carried out using the Canada-France-Hawaï, telescope in Hawaï, the "Pristine" team has identified one such star. This survey, which was limited to a limited spectral range in the ultra-violet, has enabled astronomers to identify efficiently the stars with only a very small heavy element content.

It is probable that in the solar neighborhood the heavy element content of less than one star in 1 000 000 is as poor as that of the star which has just been discovered.

An analysis using the spectrograph of the Isaac Newton Group, in Spain, and at the European Southern Observatory in Chile, has confirmed that the star Pristine_221.8781+9.7844 has practically no heavy elements, whose content is 10 000 to 100 000 times lower than what has been determined in the atmosphere of our Sun.

This star, whose analysis was presented in a paper published in the October 8th 2018 issue of the Monthly Notices of the Royal Astronomical Society raises a number of very welcome constraints on models for the formation of the first generations of stars, and opens a window on an as yet poorly known cosmic epoch.

The discovery of Pristine_221.8781+9.7844 right at the beginning of the "Pristine" project augurs well for the discovery of more
such stars in coming years.