The Milky Way turned out to be older than scientists thought: the difference is billions of years

The authors of the new work used data from the ESA Gaia mission and found that the part of the Milky Way that

called a thick disk, began to form 13billion years ago. This is 2 billion years earlier than expected. And only 0.8 billion years later than the Big Bang. The thick disk is a component of the structure of about 2/3 of disk galaxies, including the Milky Way.

The history of our Galaxy is the history of mergers andacquisitions. In more than 12 billion years, the Milky Way has swallowed up at least five large galaxies. It is these early absorptions that are believed to have filled the thick disk of the Milky Way with matter.

The thin disk contains most of the stars thatwe observe as a hazy streak of light in the night sky. It is about twice as large as a thin disk, but the radius is smaller. The thin disk contains only a few percent of the Milky Way's stars in the vicinity of the Sun.

The astronomers took the object's brightness and location data from Gaia Early Data Release 3 (EDR3) and combined it with measurements of the star's chemical composition to determine its age.

They decided to study subgiant stars:energy ceased to be generated in their nuclei and moved into the shell. Because the subgiant phase is relatively short, it allows the age of a star to be determined with great accuracy, but it is still a difficult calculation.

Using the stellar brightness data obtained by the Gaia mission, we can determine the age of the subgiant star to within a few percent. 

Maosheng Xiang, a fellow at the Max Planck Institute for Astronomy

By identifying subgiant stars in these different regions, the researchers were able to plot a timeline for the formation of the Milky Way.

According to these data, the formation of the Milky Waysplit into two phases. In the first stage, which began 0.8 billion years after the Big Bang, the thick disk began to form stars. The process ended about 2 billion years later, when the dwarf galaxy Gaia-Sausage-Enceladus merged with the Milky Way. The merger created stars and, as the new work clearly shows, this led to the birth of a thick disk that formed most of its stars. The thin stellar disk holding the Sun was formed during the further, second phase of the formation of the galaxy.

As a result, the authors concluded that the thick disk of the Milky Way formed later than previously thought.

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