A Space Telescope Solved a Star Mystery

BALTIMORE — Scientists using the Hubble Space Telescope solved an old mystery about the Milky Way, the galaxy our planet lives in. Billions of years ago, a smaller galaxy crashed into the young Milky Way and became part of it. The new study shows our galaxy started growing this way earlier than anyone thought. Galaxies are giant groups of stars, dust, and planets held together in space. By studying the paths of very old stars, scientists can look back in time to see what happened. The Milky Way holds more than 100 billion stars today.

Why it matters: The Sun and Earth are part of the Milky Way, so this is the story of how our own corner of space was built.

BALTIMORE — Astronomers using the Hubble Space Telescope, paired with data from Europe's Gaia mission, have solved a long-standing puzzle about how our galaxy took shape. Their finding: the Milky Way began assembling itself earlier than previously believed, growing in part by absorbing a smaller dwarf galaxy in a collision billions of years ago.

The team traced the motions and chemistry of ancient stars — the ones left over from that early era — to reconstruct events that happened long before Earth existed. Because light and motion carry a record of the past, these stellar "fossils" let researchers read the galaxy's history the way geologists read layers of rock.

The study refines the timeline of galactic growth. It suggests the merger that helped build the Milky Way's inner regions happened sooner than models had assumed. The work builds on years of Gaia's precise measurements of nearly two billion stars, and it has been published in a peer-reviewed journal.

Scientists caution that reconstructing events from billions of years ago always carries uncertainty. The conclusions rest on models of how stars move and age, and future data could shift the picture. Even so, the result adds a firmer chapter to the story of where our galaxy — and the Sun and Earth within it — came from.

Background

Galaxies grow largely by cannibalism — bigger ones swallow smaller ones over billions of years. The Milky Way's history has been reconstructed piece by piece from these leftover stellar streams, but the timing of the earliest mergers has been hard to pin down. This result narrows that gap.

Why it matters: Understanding when and how the Milky Way assembled helps scientists model how galaxies form in general, and places our own Sun and solar system within that much larger story.

What to watch: Follow-up studies using Gaia's next data release, which could test whether the revised timeline holds.