Recent observations suggest that type Ia explosions begin 100 million years after stars form. The time lag occurs because white dwarfs evolve from stars that live longer than high-mass stars do. Tsujimoto and Bekki calculate that 17 to 20 exploding white dwarfs cast their iron-rich debris into the gas, which then spawned NGC 1718, whose many new stars inherited the high iron-to-magnesium ratio.
"I agree with the authors," says Rosemary Wyse, an astronomer at Johns Hopkins University. "In all likelihood, the cluster formed from material that was extremely rich in the ejecta from type Ia supernovae." Two decades ago, she and Gerard Gilmore of the University of Cambridge invoked exploding white dwarfs to explain milder abundance anomalies in the Magellanic Clouds. She says the cluster demonstrates how a galaxy's chemical evolution doesn't always proceed smoothly. Instead, irregularities can arise, with chemical abundances that deviate from the norm, allowing underdogs like type Ia supernovae to be occasional champions.