The heaviest elements in nature might be formed by one of the most catastrophic events in the universe, according to a new study by American researchers.
The formation of gold, platinum, uranium, and such elements has been a heated debated for about 60 years, but thanks to a very special dwarf galaxy, the scientists were able to confirm the likely culprit. Some thought that these elements were formed in certain types of supernovae, which means the smallest galaxies should have significantly less of these elements as they have fewer stars exploding.
But this is not always the case. The local dwarf galaxy Reticulum II, which is about 100,000 light-years from the Milky Way, is more enriched by heavy elements – especially gold and europium – than similar galaxies. The unusual enrichment indicates that the formation process happened only once in the history of this galaxy, and that it was most likely a neutron star collision.
Stars are powered by nuclear fusion, which is when they fuse lighter atoms, like hydrogen, into heavier ones, from helium all the way to iron. You can’t get any energy out of fusing iron atoms together, so the formation of heavier elements comes from neutron-capture processes. For the heaviest elements, this needs to happen very quickly, thus the name "rapid neutron-capture process," or r-process.
"Understanding how heavy r-process elements are formed is one of the hardest problems in nuclear physics," said Anna Frebel, assistant professor in the Department of Physics at MIT and co-author of the study, in a statement. "The production of these really heavy elements takes so much energy that it's nearly impossible to make them experimentally. The process for making them just doesn't work on Earth. So we have had to use the stars and the objects in the cosmos as our lab."
Neutron stars are incredibly compact objects that are formed in supernovae explosions. Some of these neutron stars have a companion, and for a tiny fraction, the companion is another neutron star. The gravity between the two objects is so great that they move closer and closer together until they merge. At that point, the intense gravity rips some of the material away from the stars and shoots it into the galaxy. In this manner, the galaxy becomes enriched.
The findings, published in Nature, show that Reticulum II has between 100 and 1,000 times more heavy elements than comparable dwarf galaxies. The finding also highlights that by studying in detail the property of a handful of stars, it is possible to extrapolate the history of the entire galaxy.
"I really think these findings have opened a new door for studying galaxy formation with individual stars and to some extent individual elements," added Frebel. "We are seriously connecting the really small scales of stars with the really big scales of galaxies."