In a recent article on the European Space Agency’s (ESA) website titled (ESA – Gaia spots odd family of stars desperate to leave home) a group of scientists reported the discovery of an unusual group of over 1,000 stars, collectively known as “Ophion.” These stars exhibit behaviour that is unexpected for their kind. At less than 20 million years old, they are quite young compared to the average age of stars in the Milky Way, which is estimated to be around 10 billion years, depending on the type of stars.
Stars in the Milky Way — and likely in other galaxies—tend to form “families” that share similar birth times and locations. Over billions of years, these families spread out within the galaxy while retaining certain common characteristics. By studying these star families and their behaviour, as the Gaia mission has enabled, we can learn more about the formation and evolution of our galaxy.
ESA’s Gaia Uncovers Young, Rogue Star Group
What makes the Ophion group particularly intriguing is how its stars move in an unusual, seemingly random manner. The family appears to be dispersing rapidly, and the reason behind this puzzling behaviour is what is currently challenging the astronomy community.
This discovery was made possible by a team of young scientists who delved into the dataset already delivered by Gaia and developed new methods to navigate the vast amount of information available. Gaia is one of the most important scientific missions of the European Space Agency. The spacecraft was launched in December 2013 and was recently deactivated after exhausting its propellant and completing its mission. However, the enormous amount of data stored in the archive of the European Space Astronomy Centre (ESAC) in Villanueva de la Cañada, Spain, only covers the first three and a half years of the mission. The next update will be released in 2026 and will include five and a half years of observations. The final Gaia catalogue will be released in 2031, containing over ten years of data.
The construction and maintenance of such an archive is a massive undertaking performed by the Gaia Data Processing and Analysis Consortium, which includes ESA personnel and scientists from all over Europe. The expected volume of data available to the scientific community next year will exceed 500 TB (terabytes). The dataset is open to anyone who wants to explore the mysteries of our galaxy but finding what you’re looking for requires skill and methods, much like searching for new plant species in a virgin forest.
The main goal of the mission was to study our Milky Way and its stellar content, providing highly accurate astrometric and photometric parameters. The astrometric component involves determining the precise three-dimensional positions and velocities of around 2 billion stars. While this represents only a small fraction (1-2%) of the stars in our galaxy, it is large enough to provide statistically significant data. The key enabler of the mission is the exceptional precision of the measurements, both in terms of position and velocity.
High positional accuracy is achieved for stars depending on their brightness, with measurements ranging from around 6 microarcseconds for stars with magnitudes between 3 and 12, to 25 microarcseconds for 15th magnitude stars, and several hundred microarcseconds for 20th magnitude stars. To put it in perspective, Earth’s pole star, Polaris (Alpha Ursae Minoris), is a very bright star by Gaia’s standards, with a magnitude of 2. Gaia can pinpoint its position in the sky with precision better than the size of a 1€ coin on the Moon as seen from Earth. This is orders of magnitude better than any previous space mission or ground-based observations have achieved. Distances to stars, even those near the galactic center, are measured with an accuracy of 10%, while velocity accuracies are on the order of 1 km/s. It is thanks to these exceptional position and velocity accuracies, along with the spectroscopic data associated with each star (also measured by Gaia using its photometers and spectrometer), that the Ophion star family was discovered.
The Ophion group is just one example of the wealth of discoveries made by Gaia. To mention just a few particularly interesting ones: Gaia data were used to identify multiple star systems, one of which is likely to be a quadruple star system; the orbit of a small galaxy, called Sculptor, around our Milky Way was characterized; the acceleration of our solar system toward the center of the Milky Way was measured at 0.23 nanometers/s²; new exoplanets (planets orbiting other stars) were discovered; and new neutron stars and three black holes were identified. And all of this was achieved with only one-third of the data collected so far. Even more discoveries are yet to come.
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