Our galaxy is much older than expected

By deriving the age of about 250,000 stars in the galaxy using two powerful telescopes, a team of astronomers has determined that these objects were formed about thirteen billion years ago, or two billion years earlier than expected.

Our galaxy is made up of different parts. Near the center is a huge bulge of stars. Wavy on each side of this bulge is the disk of the galaxy, which consists of two main sections. On one side we have the “thin” disk, which contains most of the stars that we can see from Earth. The “thick slice” is about twice as tall as the thin slice. In contrast, it has a much smaller radius and contains only a small fraction of the stars we can see in the sky. Finally, the halo encompasses the whole.

In a new study, scientists have tried to date our galaxy more accurately.

The “Undergiganten” phase

They have analyzed more than 250,000 stars who had reached the end of their lives in the main sequence. These objects had therefore depleted their hydrogen. In normal times, nuclear fusion, which makes the stars shine, should no longer take place. Here, the scientists succeeded in analyzing these obsolete stars in the “subgigant” phase, where nuclear fusion still takes place, but only in a thin layer between the star’s core and the outermost layers of the star.

Because this phase is very short, it is a great way for scientists to determine the age of the star undergoing it. This can be deduced from the proportion of heavy elements that make up the stars mentioned. We’ll talk about then metallicity.

Let us remember that the primitive universe offered only hydrogen and helium. Each generation of stars then forged heavy metals, which were then scattered throughout the cosmos, incorporating the formation of other stars, etc. In other words, the lower the metallicity of a star (low proportion of heavy elements), the older it is.

When these metallicity measurements are combined with brightness measurements, astronomers can deduce the age of a star.

galaxy
The anatomy of the Milky Way, with a large bulge in the middle and two star discs (the thick disc and the thin disc) on each side. Credit: Stefan Payne-Wardenaar / MPIA

A galactic embryo 800 million years after the Big Bang

For this work, scientists relied on the Gaia Observatory, the European Space Agency (ESA) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), located in China. These two very powerful instruments made it possible reduce margins of error in calculations from about 40% previously to just a few percent.

The ages of the stars calculated by the team showed that the formation of our galaxy took place in two separate phases. The first saw the formation of stars in the thick disk almost 800 million years after the Big Bang, ie. about two billion years earlier than previously thought.

During this first phase, the inner parts of the galactic halo may also have begun to form, but this process was so strongly accelerated by a collision with a dwarf galaxy (Gaia-Sausage-Enceladus) two billion years later. This event filled the halo with stars, triggering an intense eruption of star formation. The second phase saw the formation of stars, including the Sun, in the thin disk. The metallicity of these objects would then have increased up to ten times until about six billion years after the Big Bang.

The point to remember here is therefore that some stars were actually formed in the thick disk before entering the halo, which until now was considered to be the oldest part of the galaxy.

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