Some galaxies, such as our own (shown), spawn new stars, but many other galaxies ceased star formation long ago. Why did they stop? In most cases, say astronomers online today in Nature, you can blame something called "strangulation": Gas quits falling into the galaxies, depriving them of the fresh material they need to create new stars. The researchers reached this conclusion after comparing 3905 star-forming galaxies with 22,618 others that have retired from the starmaking business. For galaxies less than twice as massive as the Milky Way—the vast majority—star-forming galaxies possess less iron and other heavy elements than quiescent galaxies of the same mass. This is just the pattern expected if infalling gas sustains their star-forming careers, because this gas has little iron and therefore dilutes a galaxy's iron abundance; once the gas stops falling in, the iron abundance rises as exploding stars forge the element. The study finds that about 4 billion years elapse between when the gas stops falling in and when stars stop forming. In the Milky Way's case, lots of gas is raining onto it, so our galactic home won't go from sizzle to fizzle any time soon.