No matter where you are on Earth, if you have a clear view of the night sky, you can witness the breathtaking beauty of the Milky Way. This stunning galaxy, with its faint clouds of light and shadow, has been a source of inspiration for cultures worldwide. However, despite its proximity, our understanding of the Milky Way has historically been limited by our perspective. In fact, astronomers have often learned more about distant galaxies than our own. This shifted dramatically with the launch of the Gaia spacecraft in 2013.
Mapping the galaxy we inhabit is no simple task. Dense nebulae and star clusters obscure much of the Milky Way from our view, making it akin to trying to understand the layout of New York City while standing in the midst of Times Square. It wasn't until 1918 that astronomer Harlow Shapley established that our Sun is not at the center of the galaxy. Even into the 1920s, a debate raged among astronomers about whether the Milky Way was an isolated universe containing all of creation.
Since those early days, our knowledge has expanded significantly, but the Gaia spacecraft has taken this understanding to unprecedented heights. Designed to create a highly detailed map of our galaxy, Gaia has meticulously cataloged the positions, distances, motions, and spectra of over two billion stars and celestial objects. This extensive data collection revealed that the Milky Way is far from a simple galaxy in a quiet corner of the cosmos. Instead, its stars narrate a history marked by turbulent changes, influenced by galactic collisions and mergers throughout its existence.
Among the fascinating findings from the Gaia data is the discovery of arched trails of stars, which are remnants of smaller galaxies that the Milky Way has absorbed over time. Additionally, some stars have been propelled away at such high speeds that they will eventually escape our galaxy, embarking on a journey through the intergalactic void. These revelations have transformed our perception of the Milky Way.
One of the most surprising insights is that the Milky Way is not a flat spiral disk, as previously thought. Instead, its outer edges exhibit a warped shape that wobbles as the galaxy rotates. This dynamic motion is likely a result of gravitational interactions with nearby galaxies. Furthermore, Gaia's observations indicated that our galaxy is not solely characterized by two prominent spiral arms. Rather, it boasts a delicate web of fainter arms, giving it a unique structure. The Milky Way is also classified as a barred spiral galaxy, featuring a central bulge that is more spheroidal than spherical.
What we have explored so far represents just the initial glimpse into our galactic home. The complete set of observations from the Gaia mission will be released in two additional data updates, promising to provide even more intricate details about our galaxy's structure and history.
As of March 27, 2025, the Gaia mission has officially concluded. The European Space Agency's (ESA) European Space Operations Centre has deactivated the spacecraft's subsystems and placed it into a retirement orbit. What remains now is the invaluable data collected over more than a decade, along with the countless stories that this data can tell us about the Milky Way and its place in the universe.
