Westerlund 1 cluster: a nursery of giant stars emerging from the darkness

Stars tend to form in clusters, or groups of between ten and several thousand that, although evolutionarily different, share the same age and composition. Among the clusters in the Milky Way, the very young Westerlund 1 (Wd 1) cluster stands out in its inner regions. With an age of less than ten million years -by comparison, the Sun is five billion years old- it is considered the most massive cluster in our Galaxy. Its population is an ideal laboratory for the study of massive stars, but it is hidden behind a dusty region that makes it difficult to study. Now, a group of scientists has managed to break through this "mist", estimate the distance of the cluster with high precision and analyse the surrounding stellar population.

The population of stars associated with Westerlund 1, which looks like a glossary of giant objects, encompasses all kinds of massive stars, from O-type giants and supergiants to red supergiants, several extremely luminous B-type hypergiants or several yellow hypergiants, among others. Some show rare evolutionary phases and different interaction pathways in binary systems, making this group of stars the ideal sample to unravel the evolutionary processes of giant stars. However, the precise determination of the masses and ages of the stars depends on the parameters derived for the cluster, and until now one of the main unknowns has been their distance, as well as the effect of light extinction due to dust in these regions.

“Wd 1 is, without a doubt, one of the most interesting objects in our Galaxy –says Ignacio Negueruela, a professor at the University of Alicante who heads the study–. Due to the enormous amount of dust along our line of sight, even a telescope as advanced as Gaia has difficulty in providing high-quality data. It has been necessary to apply complex statistical processing to the observations in order to give such an accurate value for the distance. But Gaia has provided us with much more information, as it has revealed the true size of the cluster and allowed us to identify stars in the cluster that were not known before”.

Emilio J. Alfaro, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) and co-author of the paper, emphasises the importance of the Gaia data for selecting the stars that belong to the cluster and determining their precise distance. "The cluster is located about thirteen thousand light-years from the Sun, which implies that its mass is closer to a hundred thousand solar masses than to a few tens of thousands, making it the most massive young star cluster in the Local Group, except for R136 in the Large Magellanic Cloud”.

The Gaia-EDR3 data, together with new spectroscopic observations obtained with AAOmega (Anglo-Australian Telescope Omega Spectrograph), have allowed the team to find a large concentration of blue stars that may be located about six and a half thousand light-years from the Sun and that would represent a hitherto unknown star-forming complex or a spiral arm segment.

"The detection of a concentration of blue stars, with an angular motion very close to that of the cluster, but at a smaller distance, requires a more detailed study to show its nature and origin. This direction of the Galactic plane is very rich in young stars, and the determination of the distance of Wd 1 also indicates the probable position of one of the inner spiral arms, a fundamental data to understand the complicated spiral structure of the Milky Way", adds Emilio J. Alfaro (IAA-CSIC).

Only globular clusters, old concentrations of stars in the Galactic halo, have a mass range comparable to or greater than that of Westerlund 1 (between ten thousand and one million solar masses). But they are the oldest objects in the Galaxy, with ages exceeding twelve billion years. Studying how this young stellar swarm formed may give us the keys to understanding how the most massive clusters form today and why they are so rare.

"All the stars we can see in this cluster are much more massive and luminous than the Sun. Some are so huge that, if we were to place them at the centre of the Solar System, they would almost reach as far as the orbit of Saturn. In fact, one of them is a candidate to be the largest star we know of. The importance of the cluster lies in the fact that all these extreme objects can be associated with the population they come from", concludes Ricardo Dorda, a researcher at the Instituto de Astrofísica de Canarias who is participating in the study.