As our technology advances and more spacecraft and ground-based telescopes survey our skies, we’re going to keep discovering things that defy our expectations. One of the biggest surprise identifying missions of recent times is Gaia. This ESA-run space telescope is surveying the motions of stars in the sky in a level of detail previously never achieved. In these motions, researchers are finding families of stars moving together through our galaxy: remnants of disrupted dwarf galaxies and other structures that had previously hidden in plain sight. It is also showing that previously known clusters aren’t as they appear. 

In a new analysis of the Hyades Cluster and its surrounding region, researchers have identified populations of stars related to the cluster that were previously unknown. This alone isn’t that surprising; the Hyades cluster is an older group of stars, and with every passage around the Milky Way, variations in the stars’ distances from the Sun cause them to either race ahead or lag behind the cluster center, forming what are called tidal tails. Over time, the cluster will be so stretched out, we can no longer tell there was ever a cluster at all. The thing is, in looking at this region with Gaia, not everything can be explained with plan old orbital dynamics. 

In looking at the trailing tidal tail – those stars that lagged behind – researchers found there just weren’t as many stars as there should be. To explain this discrepancy, the team ran simulations to figure out what could cause stars to disappear. In the simulations, a cloud of matter containing ten million solar masses could do the trick. As put by discoverer Tereza Jerabkova: There must have been a close interaction with this really massive clump, and the Hyades just got smashed. 

The catch is that no such cloud of mass is visible in the sky. Our universe is primarily made of invisible stuff, and there are predictions of dark matter sub-halos — clumps of dark matter left from the formation of our galaxy. Jerabkova goes on to say: With Gaia, the way we see the Milky Way has completely changed. And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before. 

This work is being published in Astronomy & Astrophysics. In follow-up research, this team will look at more clusters and see what hints of additional invisible structures are out there waiting to be discovered in the Gaia data.

More Information

ESA press release

ESA video

“The 800 pc long tidal tails of the Hyades star cluster,” Tereza Jerabkova et al., 2021 March 24, Astronomy & Astrophysics