its thick disk formed two billion years earlier than previously thought

exit from big Bangspecifically afterepisode from cosmic radiationobservable space contains atoms fromhydrogen and D’heliummany of them isotopes and a little bit lithium but no traces of heavier elements such as nitrogen, carbon Andoxygen. First stars which will form and eventually bring outUniverse from dark times therefore, to a good approximation, we can assume that it does not contain metals and with metallicity zero as they say astrophysicists in their language, that is, devoid of elements heavier than helium.

According to standard cosmological modelit must first form small galaxies which will merge to give rise to large galaxies which will rapidly grow accreting gas guided by threads black matter cold for the most part in accordance with the new paradigm of galaxy evolution, according to which these threads play a fundamental and only a secondary role in relation to all mergers galaxies, contrary to what we thought more than a decade ago.

However, it is still believed that spiral galaxy initially formed very similar to protoplanetary disk from cloud rotating matter that gravitationally collapses. That centrifugal force counteracting this contraction perpendicular to the axis of rotation, the original cloud flattens out.

A galaxy whose chemical composition is changing

The theory of the structure and evolution of stars shows that stars evolve faster, the more massive they are. After 8 masses solar panels, they will explode in supernovae after the fusion of heavy nuclei to iron, enriching the interstellar medium with galaxies in which new stars will be born. Therefore, there is a chemical evolution of stars and galaxies that allows them to be dated. In this way, red dwarfpoor in metallic elements other than lithium would point to a star born more than 10 billion years ago, while yellow dwarf like our Sunricher in heavy elements will be several billion years younger.

That evolutionary theory stellar also predicts the size and brightness a star will vary depending on its age for a given mass. Thus, some stars will see the fusion of nuclei of heavier elements in their cores stall while it continues in the shell around that core while the star is in a short phase called subgiant red, before finally becoming red giant.

In short, by analyzing the chemical composition and brightness of a star, we can determine its age, and this is what astrophysicists have done with accuracy for about 250,000 stars in Milky Way via Multiobject Fiber Optic Telescope Long Range Sky (i.e Telescope multi-object spectroscopy at optical fiber Large field, abbreviated as Lamost) is a Chinese optical telescope with a diameter of four meters.

Maosheng Xiang and Hans-Walter Ricks of the Max Planck Institute for Astronomy in Heidelberg, Germany, together with their colleagues combined the Lamost data with data on the brightness and position of the same stars in the I’ set.Early Data Release 3 (EDR3) from satellite Gaia belonging’ESA dedicated to astrometry. The results of this combination have just been published in a renowned journal Nature.

In this 2018 edition of Cosmos, we take to the stars as astronomers using Europe’s Gaia Space Telescope have compiled an unprecedented catalog of the Milky Way’s billion stars, paving the way for decades of discovery. © euronews

Galactic archeology read in layers of stars

Gaia astrometric data is concerned with precise measurements of the position and gears about 1.5 billion stars in the Milky Way. The latter were published in December 2020, with Lamost providing information on 9 million stars in 2021. Only a few of these stars were in the red subgiant phase, 250,000 were studied with the help of Lamost, but, in the end, the structure and dating of the halo and disk of the Milky Way were refined.

Its structure is twofold. First comes a thin, dense disk, the thickness of which is about 2000 light years where is our weapon Galaxywhere most stars are born today. He himself is immersed in the disk more blurred the thickness of which is about 6000 light years.

The thin disk contains most of the stars that we see as a hazy streak of light across the sky. night what we call the Milky Way. The thick disk is more than three times the height of the thin disk, but is smaller in radius and contains only a few percent of the Milky Way’s stars in the vicinity of the Sun.

in star halo itself consists of a nearly spherical population of stars andglobular clusters surrounding the Milky Way. The stars there are old with low metallicity, as in the case of the central bulge of the Milky Way. There is no dust there, unlike the disk. This halo itself is immersed in a hot plasma halo, which in turn must be shrouded halo of dark matter particles according to the standard cosmological model.

Another galactic timeline

Now it seems, and this is a surprise, according to the ESA statement, that there have been two distinct important phases in the history of the Milky Way.

At the first stage, which began just 0.8 billion years after big Bang and so about 13 billion years ago the thick disk is already there, contrary to what we thought, and it starts to form stars there. But about 2 billion years later, after the rate of star formation accelerates, there is a big peak, which is explained by the merger of the young Milky Way and dwarf galaxy, a galaxy called Gaia-Enceladus. This name comes from Greek mythology: Enceladusone of the Giants, the son of Gaia (Earth) and Uranus (Heaven), during the Gigantomachy was disabled by Athena and buried under the mountain Etnacalling ever since earthquakes and eruptions.

This merger was, of course, comparable to the mass thrown and hitting the pool in the self-gravitating stellar fluid of our Galaxy, and it was believed that it was he who somehow “heated” the stellar gas of the thin disk. , causing it to evaporate and expand, forming a thick disk.

It was at this time that the stellar halo should have formed at the same time and for the same reason.

But in the scenario now proposed, only after this merger would a thin disk be born, and star formation in the thick disk would continue until its gas content was depleted about 6 billion years after the Big Bang. During this time, the metallicity of a thick disk would increase by more than 10 times.

As a bonus, the astrophysicists determined that the metallicity in this disk was relatively uniform, meaning that turbulent mixing processes ensure efficient transport and mixing of newly formed elements ejected into the interstellar medium during supernova explosions.

This scenario is probably typical for many large spiral galaxies. Perhaps we will learn about this from observations that will be made in the next decade. James Webb Space Telescope.

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