The sky above us is not just a random collection of glittering dots, but a complex and dynamic tapestry of interwoven stellar families, chains, and threads that stretch across unimaginable distances. We owe this revolutionary insight to the European Space Agency's (ESA) Gaia mission, a pioneering space telescope that has reshaped our understanding of the Milky Way over the past decade and written a completely new chapter in the history of astronomy. Gaia, like the most precise cosmic cartographer, has revealed that star clusters, once thought to be isolated islands, are in fact deeply connected in vast structures, unveiling the hidden architecture of our galaxy.

After more than a decade of tirelessly observing the sky, Gaia has been in well-deserved retirement since March 2025, quietly orbiting the Sun. However, the mission's scientific contribution is just getting into full swing, as the vast majority of the collected data is still being processed and awaits publication. Nevertheless, the very first sets of released data, covering only the initial years of the mission, have brought a true revolution. With unprecedented precision, Gaia has determined the positions, movements, and brightness of billions of stars, allowing us to get to know the inhabitants of the Milky Way in a way that was previously unimaginably detailed.

The cosmic census-taker that changed everything

From the moment it "opened its eyes" in 2014, Gaia has become a key tool for understanding the dynamics of our galaxy. It precisely mapped how different stars move through space, recorded their trajectories, and determined their locations with an accuracy that surpassed all previous measurements. The telescope tracked subtle changes in stars, such as their expansion and contraction, and discovered surprising "starquakes" – vibrations on the surface of stars that reveal their internal structure. It followed how stars grow, evolve, and eventually die, crystallizing into cosmic diamonds. It also discovered stellar travelers, exiles from their own galaxies who have found refuge in the Milky Way. The final result is the largest and most accurate multi-dimensional map of our galaxy ever created, a true treasure for generations of astronomers to come.

One of the key tasks of the Gaia mission was to meticulously scan the contents of our galaxy to understand the nature of star clusters. There are two main types of these cosmic families. The first are open clusters: smaller gatherings of a few hundred to a few thousand stars, usually found within the main disk of the galaxy. The second are globular clusters, massive spherical formations that hide on the outer edges or in the central regions of the galaxy and can contain millions of stellar inhabitants.

Most stars are born and grow up together in clusters, but these families do not stay together forever. Over time, gravitational forces and interactions within the galaxy cause them to disperse, and the stars merge with the wider population of the Milky Way. It is for this reason that star clusters define the nature and composition of the galactic disk and hold key information about its past. Understanding the history and evolution of the Milky Way is inextricably linked to understanding star clusters.

Redefining the concept of a star cluster

Gaia has collected trillions of individual observations of billions of stars, creating data catalogs of unprecedented precision that contain information about the motion, age, location, chemical composition, and many other properties of stars. This colossal amount of data, of which less than a third has been released so far, represents the unique strength of the mission. When dealing with such numerous cosmic objects as stars, studying them in large batches offers insights that are simply impossible to obtain by observing a smaller number of individual objects.

The spacecraft conducted a kind of "census" of clusters, mapping their locations, defining their main characteristics (age, size, distance, composition, internal and external motion), and, most importantly, distinguishing true star clusters from random alignments of stars in the sky (asterisms). To achieve this, Gaia carefully tracks stars presumed to be members of a particular cluster, checking if they all move in the same way. Using precise photometry, which is the measurement of light coming from a star, it determines whether they are of the same age and at the same distance from us. Scientists have even applied artificial intelligence (AI) to Gaia's data, using advanced algorithms and machine learning approaches to identify new cluster members and hidden subgroups of stars.

Thanks to Gaia, astronomers can now find and remove "impostors," stars that do not actually belong to a cluster, making scientific analyses far more accurate. An incredible number of new clusters have also been discovered. Gaia can spot and group stars that were born together and move similarly, even if they are scattered over a large area of space. The data has been used to find new open clusters, from the smallest, consisting of only a few pairs of co-moving stars, to those with several thousand members.

Our neighborhood in a new light

The Gaia mission has fundamentally changed our view of the Sun's neighborhood, allowing scientists to comprehensively map all stars and interstellar matter in our vicinity in a way that was not possible before. Its maps of the sky, created in 3D (three spatial coordinates) and 6D (three spatial coordinates plus three velocity components), have revealed the precise movement and position of millions of nearby stars.

These maps have also unveiled the structure of dark molecular clouds, numerous young clusters, stellar associations, and stellar streams in the space near us. With Gaia, astronomers have been able to map "stellar nurseries" where Young Stellar Objects are formed in three dimensions, revealing the true structure and extent of the two nearest stellar associations to us: Orion OB1 and Scorpius-Centaurus. Scientists have also used Gaia to study the diffuse "coronas" of stars around clusters, as well as the family ties between clusters, tracking how stars form near the Sun. It has been shown that many young clusters are not isolated, but are part of larger "chains" or "families" of clusters, sharing a common origin and star formation history.

The interwoven tapestry of the Milky Way

After the Gaia mission, it has become clear that star-forming regions, clusters, and associations are interconnected on truly vast scales. This fundamentally changes our understanding of how gas and stars fill the sky we see and how star formation occurs throughout the Milky Way. The telescope has redefined the nearby ring of stars known as the Gould Belt, revealing it to be an optical illusion. The stars that seemed to be part of the ring are actually aligned along two prominent linear structures. One is a gaseous filament extending from the Scorpius-Centaurus association, and the other is the Radcliffe Wave, a long, wavy string of gas that connects regions like Orion, Perseus, and Taurus and contains a mass equivalent to 3 million Suns. Gaia has also mapped the 3D structure of superbubbles, shells, and filaments, which are shaped by the winds of massive stars and supernova explosions.

Building on this new view of an interconnected sky, astronomers have used Gaia to better understand the spiral structure of our galaxy. Observations show that young clusters move at different speeds and in slightly different ways, depending on their location within the spiral arms of the Milky Way. Star clusters are unevenly distributed throughout the arms, which implies that these vast swirling structures are likely transient, rather than long-lived, in nature.

The long goodbye: Unraveling tidal tails

As star clusters move through the Milky Way, they are affected by the gravitational forces of various structures, from molecular clouds and dark matter clumps to the massive "bar" of stars that cuts through the center of our galaxy. These interactions pull and stretch the clusters, creating long "tidal" tails of stars and gas that extend behind and in front of them on their path.

Tidal tails are not just remnants of a cluster's past; they are powerful dynamic traces that tell the story of the cluster's lifespan and its place in the galaxy. Historically, these tails could only be seen around clusters in less populated areas of the Milky Way, as they stand out better against a darker sky. They were far more difficult to spot in the densest regions of our galaxy, but Gaia changed that. Thanks to its high-precision astrometry, scientists have been able to spot extended tails around the Hyades cluster and track the kinematics of the tails around the Coma Berenices cluster.

It has been shown that these tidal tails can be enormous, stretching for thousands of light-years. The Hyades cluster, for example, although it appears modest in size in the sky, has tidal tails that span vast portions of the sky – a silent testament to the origin, evolution, and ongoing dissolution of the cluster into the galaxy. Gaia was also key in confirming that the stars within the tidal tails are indeed "escapees" from the cluster itself, and not random alignments or stars that happened to be nearby. Open clusters are no longer seen as isolated entities, but as dynamically evolving structures that are slowly dissolving into our galaxy, leaving behind traces of their past life.

A stellar revolution that continues

Although the Gaia spacecraft's operations have concluded, its contribution to science is in full swing. Two more vast data releases are ahead of us by the end of the decade. The fourth data release (Data Release 4), based on 5.5 years of observations, is expected in late 2026, while the fifth and final release (Data Release 5), which will cover all 10.5 years of the mission, is not expected before the end of 2030.

Scientists are already using the treasure trove of published data to explore billions of stars and objects. The discoveries that have emerged from the Gaia mission show that, instead of clusters being solitary in nature, our skies are filled with chains of these stellar gatherings. The universe is permeated with vast threads and filaments that connect cluster to cluster and bind stars in ways we did not expect. As more and more data is released in the coming years, we will witness a new wave of discoveries that will further reshape our knowledge of the sky in a truly transformative way.