We now know exactly how old our galaxy is

Scientists at the Max Planck Institute for Astronomy in Heidelberg, Germany, have revealed the age of the various structures that make up the Milky Way in a startling analysis just published in Nature.

An image of the Milky Way, the galaxy to which we belong, from Paranal, Chile, and a laser beam emerging from Yepun, the telescope on ESO’s VLT Unit 4, pointing directly at the Galactic Center / Wikipedia

The Milky Way is much older than previously thought. To reveal the age of the various structures that make it up, researchers Maosheng Xiang and Hans-Walter Rix from the Max-Planck Institute for Astronomy in Heidelberg, Germany, who in a surprising analysis just published in Review Nature showed that a part of our galaxy, called the “thick disk”, began to form 13 billion years ago, about 2 billion years earlier than expected, and only 800 million years after the Big Bang.

To reach this conclusion, Xiang and Rix combined stellar brightness and position information from the Early Data Release 3 (EDR3) of the European Space Agency (ESA) Gaia mission with measurements of their chemical composition from the LAMOST Spectroscopic Telescope in China, estimating about 250,000 stars to find out their age. The study focused in particular on subgiant stars whose energy stopped being generated in the core and moved into a shell around the core, characterizing a relatively short evolutionary phase of their life that allows their age to be determined with great precision. However, this is a complicated calculation since age is derived by comparing a star’s characteristics with computer models of stellar evolution.

How is the Milky Way made?

Our galaxy consists of several structures, which can be divided into a halo and a disk. The halo is the spherical region surrounding the disc, traditionally considered the oldest component of the galaxy, while the disc consists of two parts: the thin disc, which contains most of the stars we see in the night sky, and the frequent disc. , which is more than twice as tall as the thin disc, but has a smaller radius and contains only a small percentage of the Milky Way’s stars near the Sun. By identifying the subgiant stars in these different structures, the researchers were able to establish a timeline for the formation of the Milky Way.

The two phases of the formation of the Milky Way

The Ages of Stars revealed that the formation of the Milky Way took place in two distinct phases: In the first phase, which began only 800 million years after the Big Bang, the thick disk began to form stars. The inner parts of the halo may also have begun to merge at this time, but the process would have accelerated rapidly until it was complete about 2 billion years later, when a dwarf galaxy known as Gaia-Sausage-Enceladus merged with the Milky Way. This event filled the halo with stars and, as the new work reveals, triggered the fledgling thick disc to form most of its stars. The thin disc that contains the Sun formed during the later second phase of galaxy formation.

Since the discovery of the ancient Gaia-Pølse-Enceladus merger in 2018, it was suspected that the Milky Way was already there before the halo formed, but we did not have a clear picture of what the Milky Way looked like back then. . – Xiang explained -. Our results provide extraordinary details about this part of the Milky Way, such as its birth, its rate of star formation and the history of metal enrichment. Putting these findings together using data from Gaia is revolutionizing our view of when and how our galaxy formed.“.

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