Our galaxy is two billion years older than expected, and would therefore have been born less than a billion years after the Big Bang.
Thanks to surveys from the Gaia mission, astronomers affiliated with the European Space Agency (ESA) have proven that our galaxy, the Milky Way, is in fact much older than expected. While the general consensus estimated that she was around 13 billion years old, new evidence now suggests that she was born around two million years earlier, less than a billion years before the Big Bang.
Two researchers from the prestigious Max Planck Institute in Germany came to this conclusion. To achieve this, they had to show great ingenuity. Indeed, the age of stars is one of the most difficult parameters to determine; at the present time, we are still quite incapable of doing this thanks to simple direct observation.
Instead, you have to start by measuring a whole bunch of physical and chemical parameters that you can then compare to computer models representing the evolution of stars; if we spot a star in the model that corresponds to what we observed in reality, then we can deduce its age.
A history of metal
One of the main parameters on which the age of a star is based is its metallicity. Indeed, in the early universe, there was only hydrogen and helium, the two most basic building blocks of our cosmos. The other elements did not arrive until much later, at the end of a process called nucleosynthesis.
This barbaric name designates a very complex process, but relatively simple to understand. To summarize very vulgarly, a star is neither more nor less than a large nuclear reactor. They are the ones who forged all the complex elements of our universe; over billions of years, these reactions began to produce increasingly heavy and complex elements. Astronomers group them all together under the name “metal”, hence the term “metallicity” of the star.
At the end of their life, which usually ends with a gigantic explosion, this material finds itself disseminated in space; it will then be incorporated into the next generation of stars, which can then produce even heavier elements from this new substrate — thus giving rise to an even more metallic star.
We therefore know that there is a direct link between the metallicity of the star and its age; the higher the proportion of helium and hydrogen, the older the star. Conversely, the more heavy elements there are, the more recently it has been formed.
A Huge Star Age Catalog
It is on the basis of this principle that researchers have been able to move forward. They started by retrieving the luminosity and position data collected by Gaia. This is an ambitious ESA mission; its objective is to produce a true three-dimensional atlas of our galaxy. It is an exceptional resource that served as a reference to define the position of the many stars taken into account during the study.
They used it to focus on a very particular star subtype: subgiants. This state corresponds to a fairly short transition phase in the life of the star, which helps to determine its age precisely. In total, the researchers selected about 250,000. They then combined these data with those from the Chinese LAMOST telescope. He informed them about the chemical composition of many stars; so they were able to determine their metallicityand therefore their age.
By crossing all this data, they were able to draw up a vast catalog of the age of many stars in the four corners of the Milky Way; and above all, they did it with a precision never seen before. Indeed, until today researchers had to juggle with a margin of error of 20 to 40%. But with this association between state-of-the-art instruments, everything has changed; they are able to determine the age of a star within a few percent.
Two billion years behind
The researchers began by exploring a substructure called the thin disc. In this central area of the disc, which contains the majority of observable stars, the first elements were consistent with the current consensus; as expected, the stars in question were born about 13 billion years ago.
But when they moved away from the “thin disc” to focus on a more peripheral area, the thick disc, the researchers quickly came across a bone; some elements were apparently completely incoherent. The stars of the thick disc would have been born 2 billion years earlier — just 0.8 billion years before the Big Bang.
This is very important, as astronomers have believed for some time that the halo – the “sphere” that surrounds the disc – was the oldest area of the Milky Way. But these elements show indisputably that this chronology is distorted; the thick disc is even older than the presumed date of birth of the halo.
There are therefore two possibilities: either the halo began to form earlier than expected, or the thick disc appeared first. But in all cases, the general conclusion remains the same: nour cosmic cradle is indeed much older than we thought. And this is only the beginning, because the Gaia project still has a bright future ahead of it; next June, it will deliver a new version of its stellar catalog. And researchers already expect to find potential new scientific treasures there.
“With each new scan, Gaia allows us to piece together the pieces of our galaxy’s history in even greater detail.”, explains Timo Prusti, a researcher of the Gaia project. “With the publication of the new results in June, astronomers will once again be able to enrich this story with new elements.”, he enthuses.