The oxygen in a galaxy does not sit still. It spreads, thins out, piles up, and leaves behind a record of where stars formed, where gas moved, and when smaller galaxies crashed in. In the nearby spiral galaxy NGC 1365, astronomers have now read that record in unusual detail, using oxygen as a kind of fossil trail. That work, published in Nature Astronomy, marks what the researchers describe as the first use of galactic archaeology beyond the Milky Way at this level of precision. The team, led by the Center for Astrophysics | Harvard and Smithsonian, calls the approach “extragalactic archaeology,” a way to reconstruct how a distant galaxy grew by studying the chemical fingerprints in its gas. “This is the first time that a chemical archaeology method has been used with such fine detail outside our own galaxy,” said Lisa Kewley, lead author, Harvard professor, and director of the Center for Astrophysics. “We want to understand how we got here. How did our own Milky Way form, and how did we end up breathing …