Some parts of the Milky Way are much older than we thought. Two billion years

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Galaxies, including the Milky Way, are usually very old. However, their real age is difficult to determine. If it works, it can come as a surprise. Recently, experts who have analyzed observations from the tireless European Space Observatory Gaia have succeeded.

Astronomers Maosheng Xiang and Hans-Walter Rix, of the German Max-Planck Institute for Astronomy in Heidelberg, read from this data that the so-called “thick disk”, which forms part of the Milky Way galactic disk, began to form about 13 billion years.

That’s about 2 billion years earlier than we thought. The universe was still very young at the time. Only about 800 million years have passed since the Big Bang. The thick disk essentially covers the thin disk (“think disk”) that contains most of the stars of the Milky Way, including the Sun. Thick disks occur in about 2/3 of the disk-shaped galaxies.

The researchers used data on the brightness and position of stars from the observation set of the Gaia Early Data Release 3 (EDR3) observatory. These data are combined with data on the chemical composition of stars, which come from observations of the remarkable Chinese Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). In this way, they derived the age of about 250,000 stars.

When did the stars start to form in the thick disk area?

Determining the age of stars is very complicated. The stars have no evidence of their age. For this purpose, the metallicity of the stars is used, ie the content of chemical elements heavier than hydrogen and helium.

These elements are gradually increasing in the universe, so it can be said that stars with lower metallicity tend to be younger and vice versa. However, computer models of star evolution and other data are needed to more accurately determine the age of stars.

Xiang and Rix focused on the so-called “subgiants”, which are stars that burned their hydrogen fuel and are at the beginning of the red giant phase. They spend only a relatively short time at this stage, and experts can determine their age a little easier than at other stages in the star’s evolution.

The researchers concluded that stars began to form in the thick region of the Milky Way about 13 billion years ago. This process accelerated significantly about 2 billion years later, when the young Milky Way collided with an ancient dwarf galaxy known as Gaia-Sausage-Enceladus.

The formation of stars in the thick disk then took place until about 6 billion after the Big Bang, when the gas for star formation practically ran out in the region of the thick disk. Research of this type reveals the mechanisms of galaxy formation and evolution that we still do not understand very well.

Title illustration photo: Trevor Dobson, CC BY-NC-ND 2.0

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