Dying stars called collapsars may forge much of the universe’s gold

The gold in your favorite jewelry could be the messy leftovers from a newborn black hole’s first meal.

Heavy elements such as gold, platinum and uranium might be formed in collapsars — rapidly spinning, massive stars that collapse into black holes as their outer layers explode in a rare type of supernova. A disk of material, swirling around the new black hole as it feeds, can create the conditions necessary for the astronomical alchemy, scientists report online May 8 in Nature.

“Black holes in these extreme environments are fussy eaters,” says astrophysicist Brian Metzger of Columbia University, a coauthor of the study. They can gulp down only so much matter at a time, and what they don’t swallow blows off in a wind that is rich in neutrons — just the right conditions for the creation of heavy elements, computer simulations reveal.

Astronomers have long puzzled over the origins of the heaviest elements in the universe. Lighter elements like carbon, oxygen and iron form inside stars, before being spewed out in stellar explosions called supernovas. But to create elements further down the periodic table, an extreme environment densely packed with neutrons is required. That’s where a chain of reactions known as the r-process can occur, in which atomic nuclei rapidly absorb neutrons and undergo radioactive decay to create new elements.

Scientists had suspected that when two dead stars known as neutron stars collide, the r-process could occur in material churned up by the merger. Astronomers recently clinched the case for that idea when they spotted a collision between two neutron stars that produced spacetime ripples known as gravitational waves and light. The fireworks show revealed signs of the formation of a medley of heavy elements including gold, silver and platinum (SN: 11/11/17, p. 6).

The neutron star explanation has shortcomings, though. These dense dead stars can take a long time to coalesce. But heavy elements have been found in ancient stars that formed early in the universe’s history. It’s not clear whether a neutron star merger could happen fast enough to explain the elements’ presence in those early stars.

Collapsars, however, can occur shortly after stars begin to form. And the phenomenon could be a prolific producer of heavy elements. A single collapsar might generate 30 times as much r-process material as a neutron star merger, and could generate a few hundred times the Earth’s mass in gold, Metzger says. The researchers report that collapsars might be responsible for 80 percent of the r-process elements in the universe, with neutron star mergers making up the rest.

The study sheds new light on the 2016 discovery that a dwarf galaxy called Reticulum II experienced a cataclysm early in the history of the universe that left r-process elements in its stars (SN: 5/14/16, p. 9). Scientists had proposed that an ancient neutron star merger seeded the galaxy with those elements. Now, a collapsar is another candidate.

“It’s very exciting,” says astrophysicist Anna Frebel of MIT, a coauthor of the 2016 study.  Neutron star mergers are rare, so “it felt a little bit like we were proposing to win the lottery.” But collapsars are about 10 times as rare, so if they are the explanation, “it feels like we’ve won the lottery twice.” 

But it’s still not clear if collapsars happen frequently enough, or if they produce the right amount of material, to explain the abundances of heavy elements seen in the universe. “I think the jury’s still out,” says astrophysicist Alexander Ji of Carnegie Observatories in Pasadena, Calif., who coauthored the 2016 paper on Reticulum II.

“Now we’re really excitedly thinking about how you might be able to tell the difference” — whether collapsars or neutron stars better explain galaxies like Reticulum II, Ji says. Future observations of the aftermath of the supernovas produced by collapsars could also help nail down their role.