(ORDO NEWS) — Data from the Gaia space telescope made it possible to accurately estimate the age of subgiant stars in our Galaxy and trace its history.
It turned out that the thick disk of the Milky Way began to form already 13 billion years ago – two billion years earlier than scientists thought.
Scientists from the Max Planck Institute for Astronomy (Germany) combined data on the brightness and position of stars collected by the Gaia space telescope of the European Space Agency and measurements of the chemical composition of stars obtained by the Chinese telescope LAMOST.
This made it possible to accurately determine the age of the subgiant stars of the Milky Way and trace the evolution of the Galaxy. It turned out that some parts of it were formed as early as 0.8 billion years after the Big Bang, which is two billion years earlier than scientists thought
The authors of the work focused on subgiant stars. Thermonuclear fusion in their nuclei has already stopped and continues only in the area around the nucleus. Subgiants quickly turn into red giants, and their short lifespan makes it possible to accurately determine their age.
However, this is still not an easy task, since the age of a star cannot be measured directly. It can only be calculated by comparing the characteristics of the star with computer models of stellar evolution.
The young universe consisted almost entirely of hydrogen and helium: heavier elements, including metals, appeared inside the first stars and were ejected into space at the end of their lives. After that, they ended up inside the next generation of stars. Thus, older stars contain less metals.
The LAMOST data made it possible to estimate the metallicity of the subgiants of the Milky Way. The combination of this information with data on the brightness of the stars made it possible to accurately determine their age.
Previously, it was possible to establish the age of a star with an accuracy of 60-80%, that is, the error could reach a billion years.
The Milky Way consists of a halo and a disk. The halo is a spherical region around the disk. The disk is divided into two parts: thin and thick.
The first contains most of the stars in the Milky Way. The composition of the second includes only a few percent of the stars in the galaxy. It is almost twice as thick as a thin disk, but smaller in radius.
By determining the age of subgiant stars in different parts of the Milky Way, scientists were able to build a timeline for the evolution of the Galaxy.
And then an unexpected discovery awaited them. As it turned out, the formation of the Milky Way took place in two stages. The first began just 0.8 billion years after the Big Bang. At that time, the first stars began to appear in the thick disk. Probably, the inner parts of the halo formed at the same time.
The process ended two billion years later when the dwarf galaxy Gaia-Sausage-Enceladus merged with the Milky Way. After that, the thick disk continued to actively form stars until the gas was used up.
This happened about six billion years after the Big Bang. During this time, the metallicity of the thick disk increased by more than 10 times. During the second stage of the formation of the Galaxy, a thin disk and the rest of the halo appeared.
Previous studies have described a different history for the Milky Way. Scientists suspected that it existed even before the merger with Gaia-Sausage-Enceladus, but believed that the Galaxy was no more than 11 billion years old.
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