Part of our galaxy is much older than …

Our galaxy is made up of different elements: roughly speaking, we can distinguish between a central disk, surrounded by a galactic halo, each of which houses structurally different star populations. To reconstruct the history of our galaxy means to determine how many stars are born, in what period, from what case and on what tracks. This implies an accurate assessment of the age of a large sample of stars, including the oldest (up to about 14 billion years). To do this, scientists relied on so-called subgigantic stars, a class of very clear stars that have consumed all the hydrogen in their nuclei – the fusion process has moved to a more outer layer. Since this phase is relatively short in a star’s life cycle, it makes it possible to determine its age with very high precision, directly from its brightness.

The age of the Milky Way revealed by the brightness and metallicity of the stars

Due to the short lifespan of this evolutionary phase, sub-giant stars are relatively rare, so extensive studies are essential to build a large sample of these objects with good light spectra. But thanks to the recent release of data from the European Space Agency’s Gaia mission and data from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), two astronomers from the Max-Planck Institute for Astronomy in Heidelberg, Germany, have managed to identify a set of about 250,000 sub-giant stars. They present today in Nature the story of the first formation of our Milky Way.

Determining the age of a star is not so simple: it is generally deduced by comparing the characteristics of the star with computer models of stellar evolution; the chemical composition of the star is crucial in this case. In fact, in the beginning, the universe was composed almost exclusively of hydrogen and helium; the other chemical elements – heavier atoms, grouped under the name “metals” by astronomers – were made during the thermonuclear fusion reactions that take place in the stars’ hearts.

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At the end of their lives, most stars emit their outer layers and release these atoms into the interstellar medium, which is incorporated into the next generation of stars, and so on. As a result, the oldest stars are the ones with the fewest metals. The Chinese LAMOST telescope was just designed to perform spectroscopy of the sky, thus revealing the degree of metallicity of stars. Its data combined with the data of the Gaia satellite – which measures the brightness of the stars – have therefore made it possible to estimate the age of the stars.

© M. Xiang et al., Nature (2022)

Star distribution in the age plane [Fe/H] for the whole sample of sub-giant stars, color-coded according to the star density.

Before the data from Gaia were available, astronomers worked with uncertainties of 20 to 40% – which could result in a billion years or more of inaccuracy in estimating age. Publication of EDR3 data, the third version of the mission results catalog, has recently changed the game. ” With each new analysis and release of data, Gaia allows us to put together the story of our galaxy in even more never-before-seen details. », said Timo PrustiGaia project researcher for ESA.

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The galaxy: a development of the Milky Way in two separate phases

The halo refers to the spherical region that surrounds the galactic disk, and it is traditionally thought to be the oldest component of our galaxy. The disc, in turn, is composed of two parts: a thin disc, integrated into a thicker disc. The thin disk, about 700 light-years high, contains most of the stars that appear to us at night as-one fog stripeas we commonly call the Milky Way.

The thick disk is more than twice as tall (about 3,000 light-years), but its radius is smaller; it contains only a tiny fraction of the stars in our galaxy located near the Sun. Thanks to the sub-giant stars located in these two regions, the researchers were able to establish a chronology of the formation of the Milky Way, revealing two different phases.

Artist impression of the Milky Way.

© (left) NASA / JPL-Caltech / (right) ESA / (layout) ESA / ATG media lab

Artist impression of the Milky Way. The disk is about 100,000 light-years across; it consists of a thin part, and a thicker part about 3000 light-years high and populated by older stars.

The first phase began about 13 billion years ago, or just 800 million years after the Big Bang: it was at this time that the thick disk began to form stars. The final collection of the inner galactic halo was completed about two billion years later, when a dwarf galaxy called Gaia-Enceladus (and called the “Gaia sausage” because of its elongated shape) merged with the Milky Way.

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This fusion not only filled the halo with stars, but also triggered the formation of the majority of stars in the thick disk. ” Over the next 5-6 billion years, the galaxy experienced continuous enrichment of chemical elements, eventually by a factor of 10, as star-forming gas managed to remain well mixed. “, The researchers specify in Nature. In other words, the early regions of the Milky Way disk must have been formed by very turbulent gas, which allowed the metals to spread on a larger scale. The thin disk that contains our sun was formed during the second phase of galaxy formation.

Future observations, especially those made with the James Webb Telescope, which is designed to observe the oldest galaxies in the universe, may bring new elements to the formation of galaxies. In addition, the Gaia mission is scheduled to release its entire third data catalog in June, further enriching the history of our Milky Way.

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