Data from the Gaia Space Telescope have made it possible to estimate precisely the age of the subgiant stars in our Galaxy and trace its history. It turns out that the Milky Way's thick disc began forming as early as 13 billion years ago - two billion years earlier than scientists thought.
Scientists at the Max Planck Institute for Astronomy in Germany have combined data on the brightness and position of stars collected by the European Space Agency's Gaia space telescope with measurements of the chemical composition of stars taken by China's LAMOST telescope. This allowed the ages of the Milky Way's subgiant stars to be precisely determined and the evolution of the galaxy to be traced. Some parts of the galaxy appear to have formed as early as 0.8 billion years after the Big Bang, two billion years earlier than scientists thought. The findings have been published in the journal Nature.
The authors of the paper focused on subgiant stars. Thermonuclear fusion in their cores has already stopped and only continues in the region around the core. Subgiants are fast becoming red giants and their short lives allow their age to be determined quite accurately. However, this is still no easy task because 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: the 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. Older stars thus contain fewer metals.
LAMOST data have made it possible to estimate the metallicity of the Milky Way's subgiants. Combining this information with data on the brightness of the stars made it possible to determine their exact ages. Previously, a star's age could be determined with an accuracy of 60-80%, which means the error could be as much as a billion years.
The Milky Way consists of a halo and a disk. The halo is a spherical area around the disk. The disk is divided into two parts: a thin part and a thick part. The first contains most of the Milky Way's stars. The second contains just a few percent of the galaxy's stars. It is almost twice as thick as the thin disk but smaller in radius.
By determining the ages of the subgiant stars in different parts of the Milky Way, scientists were able to construct a timeline of the galaxy's evolution. And that's where they made an unexpected discovery. 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. The inner parts of the halo probably formed at the same time.
The process was completed two billion years later when the dwarf galaxy Gaia-Colbasa-Enceladus merged with the Milky Way. Afterwards, the thick disc continued to actively form stars until the gas was exhausted. This happened about six billion years after the Big Bang. During this time, the metallicity of the thick disk increased more than tenfold. During the second stage of galaxy formation, the thin disk and the rest of the halo appeared.
Previous studies have described a different history of the Milky Way. Scientists suspected that it existed before it merged with Gaia-Colbach-Enceladus but believed the galaxy was no more than 11 billion years old.