A population of incredibly old stars has been identified at the centre of our Galaxy - a cluster that appeared more than 12 billion years ago, becoming the 'germ' of the future Milky Way.
The centre of our Galaxy is in the constellation Sagittarius. Extremely old stars can also be found there, once part of the protogalaxy around which the wider Milky Way gathered. Observations from the Gaia telescope have helped to identify these stars and estimate the size of the 'embryo' of our Galaxy.
An international team of astronomers worked under the direction of Hans-Walter Rix of the German Max Planck Society Institute for Astronomy. They used data from the Gaia space telescope's DR3 survey. This astrometric instrument is designed to collect the most accurate information possible on the positions, movements and spectra of stars and to produce a detailed map of the Milky Way. The telescope has collected information on more than a billion stars, but this time scientists have examined only those close to the galactic centre, within 30 angular degrees. Within these boundaries, about two million stars fell.
Of these, the astronomers selected stars with very low metallicity - less than three percent of the metallicity of the Sun. The index shows that the star contains elements heavier than hydrogen and helium, elements which once formed in the bowels of other light sources and were incorporated into new generations of stars. Low metallicity is typical of older stars. In addition, scientists sifted out those whose movements don't allow them to be associated with the centre of the galaxy. As a result, they've identified 18,000 stars that can be attributed to the remnants of the "embryo"-protogalaxy.
These are exceptionally old stars, perhaps more than 12.5 billion years old. They still form a fairly compact cluster, and their motions reflect the initial set of rotations of the growing galaxy. The oldest stars in this group move little or no around the centre, but the younger the star, the faster it orbits. Gaia was not able to see all of them, many remained hidden behind clouds. But if you approximate the total number of such stars, they have a mass between 50 and 200 million solar masses. That's about 0.2 per cent of the total mass of the Milky Way, which has been 'growing' around the protogalaxy for billions of years.