The Age of Westerlund 1 Revisited

The status of a nearby cluster of stars, known to be the most massive in the galaxy, is being challenged by new observations. Westerlund 1, thought to contain a total mass of more than 10,000 times that of our Sun, is likely to need its total mass revised downward after astronomers show it could be more than twice as old as previously thought. This hints at a complicated evolutionary history where the ages of the stars provide clues to how massive they can really be.

Hubble image of Westerlund 1
Image of young star cluster Westerlund 1 taken by the Hubble Space Telescope toward the southern constellation of the Altar. Westerlund 1 is home to a variety of the largest and most massive stars known, including red, yellow, and blue supergiants as well as an exotic object known as a magnetar. Westerlund 1 is relatively close-by for a star cluster, at a distance of 15,000 light years, giving astronomers a good laboratory to study the development of massive stars. Credit: ESA/Hubble & NASA

Star clusters are groups of stars smaller than a galaxy, bound together by a shared gravitational pull. They are used by astronomers as distant laboratories to understand how stars evolve. They are formed by giant collapsing clouds of gas, which eventually collapse into smaller clouds that collapse even further. This compression generates extreme pressure, eventually causing the material to ignite and form individual stars.

This means that all stars in a cluster are born at the same time from the same material. Since each prenatal cloud will contain a different amount of material, each star will be born with a different mass. Those that formed from larger clouds, the more massive stars, are those which live the shortest lives – quickly burning through their fuel and dying as energetic explosions known as supernovae. Those with lower masses, similar to our Sun, will live far longer, slowly using their fuel for hundreds of millions of years.

“By looking at clusters we can see the evolution of stars in action,” says Emma Beasor, a postdoctoral researcher at NOIRLab in Tucson, Arizona, and lead author of a recent paper in The Astrophysical Journal. “We can follow stars of different masses and learn how the mass they are born with affects how they will end their lives.”

One particularly interesting cluster, Westerlund 1, has long been considered the most massive cluster in the local universe, and is thought to contain a total mass of more than 10,000 times that of our Sun. In addition, Westerlund 1 is used as a benchmark object for studies of distant starburst galaxies and for calibrating stellar evolutionary models. Westerlund 1 also contains a high number of rare massive stars, including red, yellow, and blue supergiants, as well as an exotic object known as a magnetar. This unprecedented diversity makes Westerlund 1 truly unique.

The presence of a large number of massive stars means the cluster is young enough that these objects have not yet gone supernova, suggesting that Westerlund 1 formed around four million years ago – a cosmic newborn by astronomical standards. However, new data from NASA’s Stratospheric Observatory for Infrared Astronomy, or SOFIA, has revealed a complicated evolutionary past for Westerlund 1.

“One of the most important parameters we need to measure is the brightness of a star, since this can tell us how old or young the star is,” adds Emma Beasor. “Different stars emit light in different wavebands that we can’t see with our naked eye.”

Red supergiant stars, such as Betelgeuse, emit most of their radiation as infrared light. Using new data from SOFIA, Beasor and collaborators directly measured the brightness of the red supergiants in Westerlund 1, for the first time. In doing so, they found that the stars were all too faint to be a mere four million years old. The observations instead suggest that the red supergiants are more than double that age, around 10 million years old.

This new age estimation hints at a complicated past for Westerlund 1. The presence of the young, hot stars implies an age of around four million years. If the cluster were truly 10 million years old, these stars would all have ended their lives as supernova explosions many years before the red supergiants had time to form. Instead, researchers conclude that Westerlund 1 was not born in a simple, single starburst, but rather via a sustained period of star formation, likely over a period of millions of years. If this is the case, the stars in Westerlund 1 cannot all be as young or as massive as once thought.

Although Westerlund 1 can no longer serve as a benchmark for studies of distant starburst galaxies or stellar evolutionary models, it highlights the complexity of the astrophysical systems we struggle to understand and reminds us how wondrous and surprising the universe can be.