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Astro Bling: Finding gold in our stars

August 15, 2013

Moby had it right: we are all made of stars. Astronomers have long known that stars fuse hydrogen and helium, ultimately creating somewhat heavier elements, which are then expelled through the universe when stars explode as supernovae. All the carbon and calcium in our bodies, all the oxygen we breathe, originated in stars.

But where is the cosmic factory that manufactures the heavier elements—say, gold?

Now a Cal computational astrophysicist may have helped solve that mystery.

As far as the periodic table goes, run-of-the-mill stars only “cook” up elements in any quantity up to the weight of iron. They may produce heavier elements in small amounts—but you simply can’t account for all the gold, lead, platinum and transuranic elements drifting around in the interstellar void through supernovae alone. Some other process has to be in play. And it seems the mechanism has at last been identified: colliding neutron stars.

Without doubt, neutron stars are some of the weirdest critters in the intergalactic menagerie. They are the remnants of the gravitational collapses of massive stars, and are composed almost entirely of neutronium, a popular term for a substance composed mostly of neutrons—particles found in atomic nuclei that are without electrical charge. To say neutron stars are massive is an understatement. And yet, their volume is tiny. A typical neutron star contains from 1.4 to 3.2 masses, but has a radius of only a few kilometers. Neutronium is the densest material imaginable—if it can be imagined at all. On earth, a mere teaspoon of neutronium would weigh about 5 billion tons.

Daniel Kasen, the astrophysicist at UC Berkeley, has been thinking about neutron stars for a long time, and predicted what “heavy element outflow” might look like when two neutron stars smacked into each other. He and other researchers determined that such collisions yield much of their energy in exotic forms, including gravity waves and neutrinos.  But these events could also produce heaps of the heavier elements. In other words, there should be gold in them thar neutron star collisions. Kasen has coined the term “blingnovae” to describe the process.

Neutron star collisions are good places to find gold because you need lots of neutrons to create heavier elements.

 “And certainly, neutron stars are rich in neutrons,” Kasen told us. “In a given collision, you’re likely to see gold produced on the order of several (earth) moon masses. But it’s not like you’re going to find mountains of gold floating around—it’s more like there’ll be little bits of gold intermingled with huge amounts of radioactive waste.  But you add it up, of course, and it’s still a lot of gold.”

Kasen’s models were elegant, but they ultimately lacked proof until June 3, when NASA’s Swift Satellite noticed a gamma-ray burst about 4 billion light years from earth. Subsequent photos snapped by the Hubble Space Telescope indicated quantities of platinum, gold, lead, uranium and other heavy elements totaling about 1 percent of the mass of the sun.

“The event corresponded to our models of a neutron star collision,” said Kasen. “It was pretty gratifying.”

Kasen notes neutron star collisions are rare—maybe one every 100,000 years per galaxy. “But given that the universe is about 14 billion years old and you consider the sheer number of galaxies,” he says, “they’re still probably common enough to account for the heavy elements that exist.”

—Glen Martin


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