Two Ancient Star Clusters Have Been Spotted In Their Final Dance

The merging of star clusters is common in the Magellanic clouds, such as in this image of part of the Tarantula Nebula in the LMC, but few have been found within our own Galaxy. Now one has been identified, that's unlike anything we have seen. Image Credit: NASA, ESA/E.Sabbi, R. O’Connell (University of Virginia), and the Wide Field Camera 3 Science Oversight Committee. 

Open star clusters are the birthplaces of most stars, making their evolution an important part of galactic development. However, only those on the galaxy's outskirts last long. Binary clusters have even shorter lives, with just 17 percent lasting more than 25 million years. Now, however, a pair of open clusters have been found to form an exception, with the older of the two around 2 billion years old. The two clusters are locked in a gravitational dance that is slowly tearing them apart, leaving smaller clusters and trails of stars in their wake.

Open clusters are collections of stars that formed from the same cloud of gas within a galaxy, in contrast to globular clusters that sit outside a galaxy. Where globular clusters are often immensely old, open clusters are more likely to be filled with hot young stars, making them favorite attractions for amateur astronomers.

Since its discovery in 1786, NGC 1605 was considered a fairly ordinary star cluster – until Professor Denilso Camargo of Brazil's Colegio Militar de Porto Alegre discovered it is not one cluster, but two. In the Astrophysical Journal (preprint on ArXiv.org) Camargo refers to the pair as NGC 1605a and NGC 1605b and reveals they are respectively 2 billion and 600 million years old.

We know of few binary open clusters in the Milky Way, but there are more in the Magellanic Clouds. Combined, these have allowed us to learn about their typical characteristics. It's clear NGC 1605a and b are unlike anything else we have seen. Most binary clusters are young, which probably indicates they formed together from a single cloud that broke apart. That can't be the case for a pair of such different ages.

When galaxies merge they can leave a trail of gas behind that indicates their dance. In the middle of a galaxy surrounded by powerful forces the path for open clusters should be more complex and harder to discern, but have some similarities. Image Credit: Subaru, NAOJNASA/ESA/Hubble - Assembly and Processing; Roberto Colombari

Instead, these two must have been drifting past each other, and come close enough their gravitational fields caused them to interact. The two have now come so close their star populations overlap.

However, this is no simple case of a larger cluster capturing a smaller one. For one thing, the forces applied are causing additional stars to break free from each cluster, a process astronomers know as “evaporation”. Moreover; “There are some prominent tidal debris that appear to trace the clusters’ orbits during the close encounter and, unexpectedly, some of them appear to be bound structures,” Camargo writes in the paper. This, “May suggest that additionally to the evaporation the merging clusters are being broken apart into smaller structures by the combination of Galactic disk, Perseus arm, and mutual tidal interactions.”

As a result, Camargo told IFLScience, the clusters are in their last dance, engaged in a process that will tear them apart into a mixture of unaccompanied stars and smaller clusters. “During the dance they lose stars, especially when they twist and turn, leaving stellar streams.” Camargo, who has previously discovered and named many overlooked star clusters has named two of the clusters left behind Camargo 1110 and 1113. “It is the cosmic equivalent of life's perpetuation,” he told IFLScience, “Two clusters merge together to give rise to new clusters,” something never seen before.

Camargo told IFLScience he hopes to simulate the motions of the clusters based on the trails they have left behind to work out how long they have been “dancing” together.

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