Gaia reveals the hidden architecture of the Milky Way

Gaia has transformed how we see the night sky. What looks calm and unchanging is actually full of motion – stars racing across space, clusters forming and breaking apart, and entire star families stretching thousands of light-years.

For the past decade, the Gaia space observatory has been quietly watching it all unfold with its twin telescopes. Since 2014, Gaia has collected more precise information about our galaxy than any mission before.

The telescopes tracked nearly two billion stars and watched how they move, change, and where they came from. The mission has reshaped our understanding of the Milky Way – and we’re still just scratching the surface.

The accuracy that sets Gaia apart

Gaia didn’t just capture stunning images. It measured the exact positions, movements, brightness, and colors of stars with mind-blowing accuracy. Most telescopes focus on a few objects, but the Gaia mission surveyed billions. And this huge-scale approach has changed everything.

Gaia has caught stars doing strange things like wobbling, expanding, shrinking, or flying into our galaxy after being kicked out of another.

The space observatory achieved its mission of creating the most precise and comprehensive 3D map of the Milky Way, pinpointing exactly where stars live in space, and how fast they’re headed in any direction.

Gaia’s view of star clusters

For decades, scientists believed star clusters were just small, separate groups. There are two main types: open clusters (with hundreds or thousands of stars, mostly near the galaxy’s disc), and globular clusters (which are older and live near the galaxy’s center or edges).

Most stars are born in clusters, but over time, these families break up and drift apart. But Gaia’s data showed that clusters are more connected than we thought.

Some clusters move together in chains or families, stretching far beyond what we could previously detect. The data also revealed that clusters that don’t behave like typical families – some stars fly off in random directions, which is something we never expected.

The mission made it easier to tell which stars actually belong to a cluster and which are just nearby by chance.

“Thanks to Gaia, we can find and remove rogue stars that don’t actually belong to a cluster, making all of our science far more accurate,” said Antonella Vallenari, deputy chair of the Gaia Data Processing and Analysis Consortium (DPAC).

“Gaia can spot and group stars that are born together and moving similarly, even if they’re spread out through space. We’ve used Gaia to find new open clusters ranging from the very small – just a few pairs of co-moving stars! – all the way up those a few thousands strong.”

The neighborhood around the Sun

Gaia didn’t just zoom in on faraway corners of the galaxy. It revolutionized our understanding of the space around our own Sun. It mapped young stars, dark clouds, and even entire star nurseries – places where stars are still being born.

Scientists used to think that some loose groups of young stars, like Orion OB1, were just old clusters that had drifted apart. Gaia proved they were actually born loose, not broken apart. And many of these young stars don’t live in isolation – they’re often part of larger “families” or chains of clusters with common origins.

Gaia’s ability to trace where young stars come from helped scientists study how stars influence their surroundings. As stars explode or throw off gas, they shape the clouds around them and affect how new stars form. This process, called stellar feedback, is messy and complex, and Gaia’s data brought it into focus.

Connections in the Milky Way

Using Gaia’s giant dataset, scientists have discovered that the Milky Way is much more connected than we thought. Star-forming regions and clusters are linked over massive distances.

For instance, a star ring named the Gould Belt proved to be an illusion. Gaia instead revealed stars aligned in long, thin gas structures such as the Radcliffe Wave and the Split.

The mission also taught scientists more about the spiral arms of the Milky Way. It appears young clusters behave differently based on their location in a spiral arm. The arms aren’t as permanent as we assumed – they’re more like temporary features.

Gaia revealed that young clusters often form in unusual shapes – like strings, beads, rings, or filaments. These patterns stick around for millions of years.

Tidal tails: Star clusters unraveling

As star clusters move through the galaxy, they get pulled and stretched by gravity from clouds, dark matter, and other galactic features. This tug-of-war creates long trails of stars called tidal tails.

“Tidal tails aren’t just remnants of a cluster’s past: they’re powerful dynamical tracers that tell the tale of a cluster’s lifetime and place in the galaxy,” said Tereza Jeřábková of Masaryk University.

Before Gaia, these tails were hard to spot – especially in crowded areas of the galaxy. But Gaia’s precision made them clear. It spotted massive tails around the Hyades cluster, which look small to us but stretch thousands of light-years across the sky.

Scientists also used Gaia to confirm that these stars truly broke off from clusters, not just random stars passing by. By studying how the stars in these tails spin, researchers could prove they came from the same place.

Gaia’s methods are now being used to find more tidal tails, even in clusters like Coma Berenices and Praesepe. “Current efforts are focused on detecting more tidal tails, pushing to larger extents and fainter limits, and refining what we know of the constituent stars,” noted Tereza.

Gaia’s legacy and what comes next

Gaia stopped collecting data in March 2025, but its mission is far from over. At this point, less than one-third of its data has been released.

A major data release is coming in December 2026, with the final one expected around the end of 2030. These releases will add even more depth to our understanding of the galaxy.

“Gaia’s datasets are significantly more detailed and precise than any that have come before. It’s no exaggeration to say that the mission has brought about a revolution in Milky Way astronomy, especially when it comes to star clusters,” said Johannes Sahlmann, ESA project scientist for Gaia.

According to Sahlmann, the mission’s discoveries aren’t ending anytime soon. “Gaia’s spacecraft operations may have ended, but its contributions to science are in full swing.”

Image Credit: ESA / Gaia / DPAC / Stefan Payne-Wardenaar.

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