Modern stars differ from stars of past generations in the diversity of elements. So how can we distinguish old stellar colonies from even older ones? The metallicity of these and others is extremely low, and we need the oldest luminaries to study the history of star formation in the universe. The authors of the new study managed to find a distinctive feature of such stars in our galaxy.

We know where to look for ancient stars; in superdark dwarf galaxies, moons of the Milky Way. They were formed more than 12 billion years ago. Its stars are distinguished by their extremely low metallicity, with a ratio of iron to hydrogen. [Fe/H] minus two to minus three. In some, this rate is even lower; about minus four. The “norm” is the metallicity of the Sun ([Fe/H] = 0).

After the epoch of reionization, star formation in these super-faint dwarf galaxies ceased. Stars with low metallicity are extremely long-lived, so the chemical composition of such galaxies does not change much. The problem is that these stars are very far away from us and extremely faint. Our instruments can distinguish only some of them, the brightest cold red giants, and even then those at the limit of their capabilities. The sample is highly unrepresentative.

We need stars similar to the stars in these ancient galaxies, but to bring them closer to us. The Milky Way was formed precisely due to the successive mergers of such ancient clusters. Therefore, old stars fly (connect) even on the outskirts of the center of the Milky Way, but it is easiest to find them in the halo of the Galaxy. The problem is that “ordinary”, that is, younger halo stars also differ in low metallicity. The solution was found by a group of researchers from the Massachusetts Institute of Technology (USA).

The results of a new study published in the journal Monthly Notices of the Royal Astronomical SocietyAstronomers studied six stars with extremely low metallic halos. They made additional observations of the objects to measure the contents of the items as accurately as possible. Scientists compared the obtained data with other studies.

Comparing the parameters of six selected stars with halo stars and stars from dwarf galaxies, the authors of the study extracted the feature that distinguishes ancient stars from halo stars – the content of strontium and barium. The ratio of strontium to hydrogen turns out to be particularly important [Sr/H]. It must be below minus 4.5. These two elements can be used to screen out really old stars.

To confirm their conclusions, scientists tracked the movement of these lights using data from the Gaia probe. It turns out that all objects move in the opposite direction to the rotation of the Galaxy. That is, they did not form here, but merged as a result of the “veining” of an ancient cluster into the Milky Way. All three stars are located at different points in the halo, approximately 30 thousand light-years away from us. For comparison: the diameter of the Milky Way disk is about 100 thousand light-years.

The authors of the article suggested that such objects be called stars from “small accretion star systems” (small accretion star systems, SASS). The researchers suggested that luminaries with higher strontium content may be present among SASS stars, but in this case the probability of confusing them with “native” halo objects would be very high.

After reviewing other reviews and studies, scientists found 61 more low-strontium stars in the halo. Judging by the diversity of their composition, there were not many supernovae in the first SASS “taken” by the Galaxy. Moreover, these supernovae produced extremely unequal numbers and contents of elements. But these were very diverse. So, it seems that most, if not all, chemical elements occurred in the universe from earliest times, although their numbers in the early universe were not very large.