There is an enormous star cluster lurking within the inner reaches of the Milky Way and its existence could help astronomers unravel a mystery about the distribution of these vast, glittering objects within our galaxy.
Star clusters like the one studied are immense gatherings of stars swarming together within the swirling disc of the Milky Way. So far, researchers haven’t detected many large clusters in the inner parts of the Milky Way compared to the numbers they see when they look outwards through our galaxy. Now, a new study suggests this rarity is mainly because those in the inner Milky Way are trickier to discern.
Hints of the shadowy cluster – in the constellation Scutum – were first spotted around a decade ago. A study published last year, which involved using machine learning to examine data from the Milky Way-mapping Gaia satellite, also found evidence for the cluster’s existence.
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Now, a team of researchers led by Ignacio Negueruela at the University of Alicante, Spain, have confirmed the cluster is real, using Gaia measurements alongside observations of the light from the cluster. They have dubbed the cluster Valparaiso 1.
Like identifying a group of friends running together in a busy marathon, the Gaia data – which records, in unprecedented detail, the positions and motions of millions of stars in our galaxy – enabled the scientists to see the relationships between stars in a broader, bustling mass.
“By looking at this information for all the stars in the region, we can identify the cluster as a group of stars that are at the same distance and moving in exactly the same way,” says Negueruela.
The researchers believe Valparaiso 1 could be as large as 80 light-years wide and may be home to between 15,000 and 20,000 stars. “This number firmly places Valparaiso 1 among the 1 per cent most massive open clusters in the galaxy,”says Negueruela. Additionally, examination of the starlight from Valparaiso 1 hints that it is roughly 75 million years old.
One idea put forward to explain the seeming absence of large star clusters closer in to the Milky Way’s centre is that powerful gravitational forces there might disrupt and scatter them, like a breeze dispersing a pall of smoke.
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That may be happening. However, the new research indicates another effect is primarily responsible: the clusters are there, but just hidden from view. Significantly, Valparaiso 1 sits in a part of the night sky that contains particularly dense fields of stars. The region is also home to dark dust clouds that weave through the galaxy and partially block our view of the cluster.
“Because it is large and some parts are hidden by dust, you won’t recognise a typical cluster shape,” says Negueruela.
, a member of the Gaia science team and star cluster expert at the University of Barcelona, who wasn’t involved with the new study, describes the confirmation that the cluster is real as “very important” and says more of these objects should be uncovered in the latest data from the Gaia mission.
“There are several research groups already looking for such unnoticed clusters,” she adds.
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