Two Ancient Star Clusters Have Been Spotted In Their Final Dance

Most stars are born in open star clusters, making their evolution a crucial element of galactic evolution. Only those on the margins of the galaxy, however, survive. Binary clusters have considerably lower lifespans, with only 17% enduring over 25 million years. However, a pair of open clusters have recently been discovered to form an exception, with the older of the two clusters dating back roughly 2 billion years. The two clusters are entangled in a gravitational dance that is pulling them apart, leaving behind smaller clusters and star trails. In contrast to globular clusters, which are found outside of a galaxy, open clusters are collections of stars that arose from the same cloud of gas. Open clusters, in contrast to globular clusters, are more likely to be loaded with blazing new stars, making them popular among amateur astronomers.

NGC 1605 has been thought to be an ordinary star cluster since its discovery in 1786 — until Professor Denilso Camargo of Brazil’s Colegio Militar de Porto Alegre revealed it is actually two clusters. Camargo identifies the pair as NGC 1605a and NGC 1605b in the Astrophysical Journal (preprint on ArXiv.org) and indicates that they are 2 billion and 600 million years old, respectively.

There are fewer binary open clusters in the Milky Way than in the Magellanic Clouds, but there are more in the Magellanic Clouds. We were able to learn about their usual traits because of the combination of these factors. NGC 1605a and b are clearly unlike anything else we have seen before. The majority of binary clusters are young, implying that they started from a single cloud that split apart. That cannot be true for two people of such disparate ages.

Instead, these two must have been drifting past each other, and their gravitational fields collided, as they got close enough. They have gotten so close that their star populations have merged. This is not a straightforward situation of a larger cluster capturing a smaller one. For one thing, the forces are causing more stars to break out from each cluster, a phenomenon known as “evaporation” by astronomers. Furthermore, Camargo says in the report, “There is some notable tidal debris that appears to trace the clusters’ orbits during the near encounter, and, unexpectedly, some of them appear to be bound structures.” This “may imply that, in addition to evaporation, the merging clusters are being torn apart into smaller structures by a mix of Galactic disk, Perseus arm, and mutual tidal interactions,” according to the researchers.

As a result, according to Camargo, the clusters are in their final dance, tearing themselves apart into a jumble of lone star has and smaller clusters. “They lose stars during the dance, especially as they twist and twirl, creating starry streams,” says the narrator. Camargo 1110 and 1113 are two of the clusters left behind by Camargo, who had already located and named several neglected star clusters. “It’s the cosmic equivalent of life’s perpetuation,” he told IFLScience. “Two clusters join to give rise to other clusters,” he explained.