With only hydrogen, helium and a bit of lithium to work with initially, the earliest generations of stars kept things simple. So how to explain abnormally high concentrations of gold, platinum, uranium and other heavy elements in a handful of the Milky Way's senior residents?
One theory is that they could have been enriched when a partner star exploded nearby, giving the survivor a rich dusting in heavy elements.
A new study, however, unhinges that theory.
By tracking the orbital motions of the metal-rich ancient stars, scientists realized the vast majority of them are traveling solo in a halo above and below the galaxy's flat central disc.
"A companion star — if there was one — would have left behind an optically undetectable black hole, neutron star or faint white dwarf," astrophysicist Terese Hansen, with the Niels Bohr Institute at the University of Copenhagen, told Discovery News.
But the remnant companion would have a gravitational influence on the visible star's orbit, which would be detectable, she added.
The other explanation for why 1 to 2 percent of the galaxy's most ancient stars are flush with heavy metals is that they formed from a cloud of molecular gas enriched with the exploded remains of earlier stars.
"The great thing about these stars is that there has only been one generation of stars before then. So by looking at these stars we see clear footprints from the first generation of stars and can hence look even further back in time," Hansen wrote in an email.
More work remains. The team plans to expand its analysis of heavy element-enriched ancient stars and turn its attention to another chemical mystery, a group of carbon-enriched stars.
The carbon production associated with some of the stars appears to date back to the earliest time of the universe's history.
"It may even be due to the explosions of the very first stars that formed," astronomer Timothy Beers of Michigan State University told Discovery News.
The research appears online at Astrophysical Journal Letters.