Multiple populations, defined by light-element correlations, are an almost universal property of globular clusters, but heavy-element spread is seen only in rare cases. We present measurements of Mg, Y, Ba, La, and Eu abundances for 89 stars in M15 from medium-resolution spectra obtained with Keck/DEIMOS (errors <0.4 dex). We find that the dispersion of Ba, La, and Eu in first-generation stars is higher than in second-generation stars, at a significance level ≥2σ. This indicates inhomogeneous gas mixing during the formation epoch of the first generation, which then became well-mixed by the time the second generation was born. If the r-process event that caused the spread occurred with the first population, it must have a small delay time.
Globular cluster M15 in our galaxy is an ancient stellar soup. Why do its stars differ in composition? By spreading starlight into a rainbow (spectroscopy), the Keck telescope revealed: in the first generation, heavy element abundances vary, while in the second they’re uniform. Like a poorly stirred soup: at first, you scoop up chunks or broth, but later it evens out. This means the gas for early stars was clumpy, and only later mixed.
This difference suggests the source of heavy elements ignited early and then quickly fizzled out — otherwise the unevenness would have persisted into the second generation. This fits the signature of neutron star mergers. And here’s a twist: the gold in your ring might have been forged in just such a burst, which “salted” M15’s gas over 10 billion years ago.
🎯 M15 boasts one of the densest stellar regions: in its core, stars can be closer together than the distance from Earth to Alpha Centauri.
🎬 Science fiction writers have often placed ancient civilizations in globular clusters — these stellar metropolises seem like the perfect setting for a long history.