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This Rare Dwarf Galaxy is a Galactic Gold Mine

The study of a recently-discovered rare dwarf galaxy some 98,000 light years from Earth has uncovered two stars that have an extraordinarily dense presence of gold, platinum and silver. And while no one’s going to be tapping stars for precious elements, the revelation does point to an answer to one of the hardest questions in nuclear physics: how exactly are elements such as gold formed?

Rapid neutron-capture process elements (r-process)–which include a number of our fancy earthly favorites–have been a source of befuddlement to nuclear physicists for over 60 years. As their name suggests, these elements, all of which are heavier than iron, are believed to have been formed billions of years ago during rare, violent collisions between neutron stars and in the supernova explosions of massive stars. A single collision between neutron stars is thought to produce a Jupiter’s mass-worth of gold.

An artist's conception of a supernova forging heavy elements. (Credit: Supernova illustration: Akihiro Ikeshita/Particle CG: Naotsugu Mikami (NAOJ))

An artist’s conception of a supernova forging heavy elements. (Credit: Supernova illustration: Akihiro Ikeshita/Particle CG: Naotsugu Mikami (NAOJ))

But there’s been no way for scientists to test this theory. As Anna Frebel, an Assistant Professor at MIT and coauthor of a Nature paper on the discovery explained in a statement:

The production of these really heavy elements takes so much energy that it’s nearly impossible to make them experimentally, even with current particle accelerators and apparatuses. The process for making them just doesn’t work on Earth.

So instead we have to study the stars where these events take place, which is what exactly what Alexander Ji, a graduate student in physics at MIT was doing. Specifically he was observing the relatively nearby Reticulum II dwarf galaxy, which was discovered last year. Based on the low presence of r-process elements in stars in other similar galaxies, Ji’s expectations were low.

When we read off the r-process content of that first star in our telescope, it just looked wrong, like it could not have come out of this galaxy! I spent a long time making sure the telescope was pointed at the right star.

A Dark Energy Survey image of Reticulum II. The nine stars, described in a recent study, are circled in red, seven of which have high r-process element abundances. (Credit: Alex Ji (background image: Fermilab/Dark Energy Survey))

A Dark Energy Survey image of Reticulum II. The nine stars, described in a recent study, are circled in red, seven of which have high r-process element abundances. (Credit: Alex Ji (background image: Fermilab/Dark Energy Survey))

Shortly after the team observed a second star in the galaxy that the same high content of r-process elements. As for how these elements made their way down here to Earth, into our bodies as atoms and into display cases at Zales, it’s believed that in the course of their formation the gold, silver and platinum became embedded in asteroids. Some of these asteroids hit Earth and, due to the planet’s molten state at the time, embedded themselves and their valuable cargo deep underground.

And one single neutron star collision thousands of light years away and billions of years ago may bind us all. As UC-Santa Barbara Astronomy Professor Enrico Ramirez-Ruiz recently said at a roundtable:

Because just one of these neutron star mergers produced so much gold, probably all of the gold atoms that are in the four of us in this roundtable discussion came from the same event.

Lede image: An artist’s impression of two neutron stars colliding. (Credit: Dana Berry / Skyworks Digital, Inc.)