New research has shed light on the complex evolution of the Milky Way galaxy, home to billions of stars, including our sun. Scientists have uncovered evidence of a significant event in the galaxy’s early history by studying star clusters near its center. Approximately 11.8 billion years ago, when the universe was much younger, the Milky Way merged with a smaller galaxy. This merger marked the earliest-known galactic collision for the Milky Way, demonstrating that its growth involved not only internal star formation but also the absorption of smaller galaxies.
The smaller galaxy involved in the merger was classified as a dwarf galaxy, containing stars equivalent to 500 million times the mass of the sun at that time, about a quarter of the Milky Way’s size then. Researchers utilized the Hubble and Gaia telescopes to analyze stellar clusters spanning the Milky Way’s innermost 20,000 light-years, examining their age, composition, and trajectory.
The merger took place roughly two billion years after the Big Bang, integrating stars from the dwarf galaxy, named Low-energy-Kraken-Heracles (LKH), into the Milky Way. This discovery, led by astrophysicist Davide Massari, provides conclusive evidence of the Milky Way’s first merger event.
The study suggests that the early growth of the Milky Way was influenced by the merger, introducing a significant amount of stars, interstellar gas, and dark matter into the galaxy. Massari noted that while the merger was peaceful for stars, it triggered explosive gas collisions, likely fueling the formation of many new stars.
The Milky Way has been involved in two other major mergers, absorbing the Gaia-Sausage-Enceladus dwarf galaxy and merging with the Sagittarius dwarf galaxy over the past six billion years. Each merger has played a role in shaping the galaxy and, ultimately, our solar system and Earth’s formation. Massari emphasized the importance of understanding the Milky Way’s history in answering fundamental questions about our origins.