Gaia spots odd family of stars desperate to leave home

Stars in the Milky Way tend to form in families, with similar stars springing to life in roughly the same place at roughly the same time. These stars later head out into the wider galaxy when they’re ready to fly the nest. While smaller groups can completely dissipate, siblings from sizable families usually move similarly and largely travel together.

We have seen many star families with Gaia. We’ve spotted strings of stars stretching out across the Milky Way and remaining intact for billions of years, mapped the ancient star streams that wound together to form the earliest structure of our galaxy, and put together a stellar “family portrait” of our cosmic home. By studying star families we can piece together not only the characteristics and behavior of the stars themselves, but also learn about how our galaxy is evolving as a whole.

A family like no other

Using Gaia data, scientists have now spotted a star family unlike any other: a massive family of over 1000 young stars behaving oddly. Despite its size, the family—dubbed Ophion—will soon have completely dispersed in record time, leaving just an empty nest behind.

“Ophion is filled with stars that are set to rush out across the galaxy in a totally haphazard, uncoordinated way, which is far from what we’d expect for a family so big,” says Dylan Huson of Western Washington University (WWU), U.S., and lead author of the discovery paper. “What’s more, this will happen in a fraction of the time it’d usually take for such a large family to scatter. It’s like no other star family we’ve seen before.”

The research is published in The Astrophysical Journal.

A new model

To find Ophion, Dylan and colleagues developed a new model to explore Gaia’s vast, unrivaled trove of spectroscopic data and learn more about young, low-mass stars lying reasonably near to the sun. They applied this model, named Gaia Net, to the hundreds of millions of stellar spectra released as part of Gaia’s data release 3. They then narrowed their search to “young” stars of under 20 million years in age—and out jumped Ophion.

“This is the first time that it’s been possible to use a model like this for young stars, due to the immense volume and high quality of spectroscopic observations needed to make it work,” adds ESA Gaia Project Scientist Johannes Sahlmann. “It’s still pretty new to be able to reliably measure the parameters of lots of young stars at once. This kind of bulk observing is one of Gaia’s truly unprecedented achievements.

“Another is how the Gaia mission is creating opportunities for new collaborative and interdisciplinary science through its open data policy. Several members of the Ophion discovery team are undergraduate and postgraduate students in computer science, who used Gaia data to innovate and develop new methods that are now offering new insights into the stars of the Milky Way.”

Solving the mystery

The question remains: why is Ophion behaving so unusually?

The scientists discuss several options. The star family resides around 650 light-years away near to some other massive gatherings of young stars; energetic events within and interactions between these colossal neighbors may have influenced Ophion through the years.

There are also signs that stars have exploded here in the past. These supernova bursts could have swept material away from Ophion and caused its stars to move far more rapidly and erratically than before.

“We don’t know exactly what happened to this star family to make it behave this way, as we haven’t found anything quite like it before. It’s a mystery,” says co-author Marina Kounkel of the University of North Florida, U.S..

“Excitingly, it changes how we think about star groups, and how to find them. Previous methods identified families by clustering similarly moving stars together, but Ophion would have slipped through this net. Without the huge, high-quality datasets from Gaia, and the new models we can now use to dig into these, we may have been missing a big piece of the stellar puzzle.”

After more than a decade spent mapping our skies, Gaia stopped observing in March. This marks the end of the spacecraft’s operations—but it’s just the beginning of the science. Many more discoveries are anticipated in the coming years, along with Gaia’s biggest data releases yet. (Data Release 4 is planned towards the end of 2026, and the Gaia legacy data release is planned for publication not before the end of 2030).