Lisa Lock and her colleagues at the Harvard-Smithsonian Center for Astrophysics did something previously unthinkable in March 2026: they rewound the cosmic clock on a distant galaxy and watched it grow, piece by piece, across 12 billion years of history. The tool they used wasn't a time machine but something far more elegant—the chemical fingerprints embedded in the spiral galaxy NGC 1365, read like the rings in a tree or the layers in an archaeological dig.
Spiral galaxies like our own Milky Way hold secrets about how the universe built itself up, how heavy elements like oxygen were forged in stars and scattered across space, and how the galaxies we see today came together from much smaller pieces. Yet tracing that history has always been elusive. Lock's team developed a technique they call "chemical archaeology," analyzing oxygen distribution across thousands of star-forming gas clouds in NGC 1365 using the du Pont telescope at Las Campanas Observatory in Chile. That observational data became the key to unlocking the galaxy's past.
The researchers searched through simulations of approximately 20,000 model galaxies, hunting for one that matched the chemical signature of NGC 1365. When they found it, they could rewind the simulation and watch the galaxy's life unfold in reverse—a kind of cosmic replay showing how the galaxy grew, how it collided with smaller dwarf galaxies, and how fresh gas and stars arrived to build up its elegant spiral arms.
What they discovered tells a story of patient assembly. NGC 1365's central region formed early in its history and quickly became rich in oxygen—the product of massive stars burning bright and then exploding as supernovae. But the outer disk grew much more slowly. Over billions of years, the galaxy probably collided multiple times with smaller companion galaxies, pulling in their gas and stars and using them to construct the spiral arms we see today. Much of the material now decorating those outer edges, it turns out, arrived relatively late in the galaxy's life.
This approach represents a genuine leap forward. By tying ultra-high-resolution observations directly to state-of-the-art computer simulations, Lock's team has opened a new window onto how distant galaxies assembled themselves across cosmic time. The work is among the first to apply such detailed chemical archaeology beyond the Milky Way, suggesting that scientists can now read the past in ways previously impossible.
Yet mysteries remain. Different combinations of mergers and gas flows can sometimes produce similar chemical patterns, leaving room for uncertainty. The researchers don't yet know whether NGC 1365's particular life story—building the center first, the outer disk slowly—is typical for large spiral galaxies or unusual in ways still hidden from view. And perhaps most tantalizingly, they don't yet know how this distant galaxy's history compares to that of our own cosmic home.
Lock and her colleagues have given us a new tool for understanding how the universe assembled itself. The questions it raises are just beginning.