Universe Today
Stars don't exist in isolation. They have siblings and exist in clusters, associations, and groups. The ESA's Gaia mission found an unusual group of stars rapidly leaving its birthplace behind and dispersing into the wider galaxy. While that's not necessarily unusual behaviour, it is for such a large group. Could supernovae explosions be responsible?
Gaia's mission is complete, and it has bequeathed to scientists a vast database of the Milky Way's stellar population. It measured the distances, positions, and motions of about two billion stars. It also measured their brightness in different wavelengths and classified stars by age, mass, and temperature. The result is a three-dimensional map of the Milky Way that amounts to a searchable database of stars.
Researchers have used this data to identify different groupings of co-moving stars in the galaxy. These stellar families help astronomers understand how the Milky Way has formed and evolved over time. In new research, astronomers have found an odd group of stars rapidly dispersing into the wider Milky Way. While it's normal for groups of stars to disperse, this group contains more than 1,000 young stars that are dispersing much more rapidly than they should be.
The research is "Gaia Net: Toward Robust Spectroscopic Parameters of Stars of all Evolutionary Stages." It's published in The Astrophysical Journal, and the lead author is Dylan Huson from Western Washington University.
The work is based on 220 million stars in the Gaia Data Release 3. The stars span multitudes of different masses and ages. "This includes for the first time ever robust processing of spectroscopic parameters for pre-main-sequence stars," the authors explain.
This all-sky map from the research shows the 220 million stars in the study. The darker the colour, the greater the density. The rapidly dissipating group is named Ophion and is near the star-forming region p Oph. Image Credit: Huson et al. 2025, Astrophysical Journal.
The researchers developed a new model to probe Gaia's vast collection of spectroscopic data for young, low-mass stars that they call Gaia Net. Using it to filter through hundreds of millions of stellar spectra, the researchers examined how young low-mass stars up to 20 million years old behave in our solar neighbourhood.
"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," said Johannes Sahlmann, an ESA Gaia Project Scientist. "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.:
Huson and his co-researchers identified a new group of more than 1,000 stars, which they named Ophion.
"This population appears to be fully disrupted, with negligible kinematic coherence," the authors write in their paper. "Nonetheless, due to its young age, it appears to still persist as a spatial overdensity." A spatial overdensity means the region has more stars than its surroundings. A spatial overdensity with minimal kinematic coherence indicates a group of stars born in the same molecular cloud transitioning from an identifiable group to a dissipated one.
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," said lead author Huson in a press release. "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.
This figure from the research helps explain some of the findings. Ophion is in the black outline in the upper left, and the purple circle outlines a bubble around the star-forming Rho Ophiuchi Cloud. The upper right is an HR diagram of the stars in Ophion. The bottom row of panels shows the distances and velocities of the stars in Ophion. Image Credit: Huson et al. 2025, Astrophysical Journal.
What caused Ophion to disperse so rapidly? The bubble near Rho Ophiuchi in the image above and other bubbles nearby may offer an explanation.
It is possible that these are remnants of supernovae that went off in these regions," the authors write. "Indeed, there have been several voids already identified in the vicinity of ρ Oph that have been attributable to supernovae.
When supernovae explode, they sweep away star-forming gas and create a bubble shape. The gas becomes more dense on the edge of this bubble, triggering the birth of young stars. "Additionally, gravitational feedback from the rapid mass loss following gas dispersal can also accelerate already formed stars away from the eruption site," the authors explain. "As such, supernovae have significantly impacted star formation in the region."
The rapid mass loss in the bubble means that the young stars in Ophion had less gravity restraining their dispersal. "This resulted in the acceleration of the bulk of the stars to very high velocities," the authors write. The only reason that Ophion is coherent is that it hasn't had enough time to disperse completely.
However plausible that is, it's speculative at this point.
"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," said co-author Marina Kounkel of the University of North Florida, USA.
Excitingly, it changes how we think about star groups, and how to find them," said Kounkel. "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.
Astronomers will have to find more groups like Ophion to solve this mystery. Even though the Gaia mission ended, another massive data release, the mission's fourth one, is coming in the future. The researchers think that with better age data for stars in the solar neighbourhood, they can find more groups like Ophion and study them.
Combined with improved radial velocity determination for the pre-main-sequence stars, which should be possible with Gaia DR4," the authors write in their conclusion, "it may be possible to shed light on their evolving kinematics.
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