11 Billion-Year-Old Collision

The reconstruction of the Milky Way was not a sudden event but a process of chaotic reorganization. For much of the early universe, the Milky Way existed as a much smaller, less organized collection of stars and gas. The collision with the Gaia-Enceladus galaxy—a smaller, satellite system—interrupted this early growth, injecting massive amounts of kinetic energy into the existing stellar populations.

This energy transfer caused a phenomenon known as disk heating, where the orderly, circular orbits of stars in the original galactic disk were disrupted. Instead of moving in a flat, predictable plane, the stars were kicked into highly elliptical, chaotic orbits. This scattering effectively dissolved the coherent structure of the early Milky Way, leaving behind a scattered population of stars that now forms part of the galactic halo.

The Mechanics of the Gaia-Enceladus Merger

The merger event, which occurred roughly 10 to 11 billion years ago, is characterized by the absorption of the Gaia-Enceladus galaxy into the Milky Way’s gravitational well. This was not a simple addition of mass; it was a violent interaction that reshaped the host galaxy’s geometry. As the two systems collided, the gravitational tidal forces stripped stars from the incoming galaxy and redistributed them throughout the Milky Way.

The resulting debris created a distinct population of stars that move differently from the stars in the modern disk. By studying the kinematics—the specific patterns of motion—of these stars, researchers can trace the trajectory of the collision. The stars from the merger exhibit high eccentricity, meaning they follow long, stretched-out paths rather than the tight, circular orbits seen in the majority of the Milky Way today.

This collision also provided a significant influx of gas. While the collision disrupted the existing stellar order, the gas brought in by Gaia-Enceladus, combined with the gas already present in the Milky Way, began to settle into a new, more stable plane. This settlement facilitated the birth of a new generation of stars, eventually leading to the formation of the thin disk that characterizes our galaxy in the current epoch.

Chemical Signatures and Stellar Archeology

To confirm the timeline and nature of this destruction, scientists employ a method known as stellar archeology. This involves analyzing the chemical composition of individual stars to determine when and where they were formed. The primary indicator used is metallicity, which refers to the proportion of elements in a star that are heavier than hydrogen and helium.

Stars born in the early universe or in smaller, less evolved galaxies typically have lower metallicity because they formed before multiple generations of supernovae could enrich the interstellar medium with heavy elements. The stars identified as remnants of the Gaia-Enceladus merger show a specific chemical fingerprint: they are enriched in alpha-elements—such as magnesium, silicon, and calcium—relative to their iron content.

This chemical ratio acts as a stopwatch. Alpha-elements are produced in the rapid explosions of massive stars, while iron is produced more slowly by Type Ia supernovae. The specific ratio found in these “halo stars” indicates they formed in an environment with a high rate of star formation that was abruptly interrupted, consistent with the timeline of a major galactic collision 11 billion years ago.

Spanish Contributions to Galactic Mapping

A significant portion of the data processing and theoretical modeling used to identify this merger has been driven by researchers within the European astronomical community, including major contributions from Spain. Scientists at the Instituto de Astrofísica de Canarias (IAC) and the Centro de Astrobiología (CAB) have been integral to the analysis of the Gaia mission’s data releases.

The complexity of the Gaia dataset requires massive computational power and sophisticated algorithms to separate the movement of billions of stars. Spanish research teams have focused on the high-precision astrometry required to detect the subtle gravitational wobbles and velocity shifts that signal a past merger. By refining these models, they have allowed for the distinction between stars that are native to the Milky Way and those that were “stolen” from Gaia-Enceladus.

The work of these institutions has helped move the theory of a major early merger from a mathematical possibility to a documented historical event. This research clarifies how the Milky Way transitioned from a chaotic, merging system into the stable spiral galaxy observed by modern telescopes.

The Transition to the Modern Milky Way

Following the period of intense disruption, the Milky Way entered a phase of relative stability. The gas that survived the merger and the gas that accumulated through subsequent accretion began to settle into a rotating disk. This process, known as secular evolution, allowed for the gradual buildup of the thin disk where most of the Sun and other star systems reside.

The current structure of the Milky Way is therefore a hybrid. It consists of an old, chaotic halo composed of the remnants of the original disk and the Gaia-Enceladus galaxy, and a newer, more organized disk formed from the subsequent settling of gas. This dual nature is essential for understanding the lifecycle of galaxies.

Understanding this history is not merely an exercise in looking backward. The way the Milky Way responded to the Gaia-Enceladus merger provides a template for how larger galaxies in the universe evolve through mergers. As astronomers continue to refine the data from the Gaia mission, the timeline of our galaxy’s destruction and subsequent reconstruction will likely become even more precise, offering a clearer view of the violent origins of our cosmic neighborhood.