The Milky Way's violent youth: the galaxy we swallowed
What happened
About 10 billion years ago, roughly 3 billion years after the Milky Way's thick disc had already begun forming, our galaxy collided with another galaxy roughly a quarter its size, comparable to one of the Magellanic Clouds still visible in the night sky today. The European Space Agency's Gaia satellite uncovered the wreckage by mapping the positions and motions of roughly 30,000 stars that move on elongated orbits running backward against the rest of the galaxy's hundred billion stars, a signature no ordinary formation process produces. Astronomers named the vanished galaxy Gaia-Enceladus. Its stripped-apart stars now make up most of the Milky Way's inner halo, and its debris carried with it hundreds of variable stars and thirteen globular clusters that still trace the same telltale backward orbits.
Why it matters
The Milky Way's calm, orderly spiral disk hides a genuinely violent adolescence: this one collision reshaped its structure and delivered a large share of the stars in its halo. Reading a galaxy's structure this precisely, recovering a collision from ten billion years ago in the motions of individual stars, is a scale of forensic detail cosmology could not do before Gaia.
How we know
The case rests on independent, converging lines from the same Gaia data: the unusual backward, elongated orbits of tens of thousands of stars; those stars' distinct chemical composition, which marks them as born in a separate galaxy rather than the Milky Way itself; and an associated population of globular clusters and variable stars following the identical orbital signature. Agreement across all three is what turned a statistical anomaly into a confirmed ancient merger.
Sources
- European Space Agency. Galactic ghosts: Gaia uncovers major event in the formation of the Milky Way · Reputable sourceesa.int · The domain "esa.int" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- Big Bang to Now → · Zoomed out: this continues the spine's 'The Milky Way begins to form'