Understanding why galaxies stop forming stars is one of the biggest questions in galaxy evolution because it helps scientists explain how galaxies change over billions of years. The researchers found galaxy mergers are neither necessary nor sufficient to stop galaxies from forming new stars — a process astronomers call quenching. “I’ve always been fascinated by galaxy evolution and the question of why some galaxies continue forming stars while others stop completely,” Casimiro said. For decades, astronomers have viewed galaxy mergers as one of the leading explanations for why otherwise active galaxies eventually stop forming new stars. Perhaps even more surprising, most galaxies that stopped forming stars showed no evidence of a recent merger at all.