Astronomers Tally All the Gold in Our Galaxy

By Eric Betz | October 16, 2017 2:15 pm
When two neutron stars collide, the aftermath creates heavy elements like gold. (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonne)

When two neutron stars collide, the aftermath creates heavy elements like gold. (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonne)

Before “he went to Jared,” two neutron stars collided.

That’s what scientists learned from studying the debris fallout after a cosmic explosion called a kilonova — 1,000 times brighter than a standard nova — which appeared, and was witnessed by astronomers, in earthly skies Aug. 17.

For decades, astronomers debated the origins of the heaviest elements, which includes precious metals, rare Earth elements and basically everything on the bottom rungs of the periodic table, from platinum to plutonium.

“It’s a very violent process,” says Columbia University astronomer Brian Metzger, whose team predicted neutron star mergers would create a kilonova. “The two neutron stars are moving almost at the speed of light around each other and then they slam into each other.”

Which Star Stuff?

You and I are made from pretty typical stardust — stuff that forms when large stars explode as supernovas. But supernovas make few heavy elements.

So that gold in your wedding ring, science didn’t know for sure how it came to be.

Indirect evidence pointed to colliding binary neutron stars — the dense cores of dead suns. But they don’t happen very often in any particular galaxy, and no one had ever seen such an event before.

“The question was always which one of these wins?” says University of Wisconsin-Milwaukee astronomer David Kaplan, whose team studied the August kilonova. “Is it the really common thing that makes a little? Or was it the rare thing that makes a lot?”

The chance to find out arrived this summer.

Carnegie Observatories' Swope telescope was the first to image the neutron star merger in optical light. It's a small, decades-old telescope at Chile's Las Campanas Observatory. (Courtesy Ryan Foley)

Carnegie Observatories’ Swope telescope was the first to image the neutron star merger in optical light. It’s a small, decades-old telescope at Chile’s Las Campanas Observatory. (Courtesy Ryan Foley)

Chemical Fingerprints

A ripple in space-time — a gravitational wave — stretched and squeezed detectors at the Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as Italy’s Virgo instrument. Electromagnetic light arrived seconds later.

Astronomers used the twin Magellen telescopes at Las Campanas Observatory in Chile to capture the chemical fingerprints, or spectra, of this cosmic collision, along with the Hubble Space Telescope. The results, published Monday in the journal Science, found signs of precious metals and radioactive waste.

“As the matter expands,” Metzger says, “there are nuclear reactions, which turn the neutrons and protons into heavier nuclei, so things like gold and silver and platinum.

Hydrogen and helium emerged after the Big Bang, forming stars. Heavier elements -- up to iron -- formed in supernovas. But astronomers now know the stuff that's heavier than iron is created in neutron star mergers. (Credit: Jennifer Johnson/ SDSS/ CC BY 2.0 (modified)/ Courtesy: CalTech)

Hydrogen and helium emerged after the Big Bang, forming stars. Heavier elements — up to iron — formed in supernovas. But astronomers now know the stuff that’s heavier than iron is created in neutron star mergers. (Credit: Jennifer Johnson/ SDSS/ CC BY 2.0 (modified)/ Courtesy: CalTech)

All That Glitters

The merger produced somewhere between 10 and 100 Earths worth of gold — among many other heavy elements.

And based on this one observation in the relatively short period of time gravitational wave detectors were capable of seeing it, scientists can extrapolate to guess how often binary neutron stars merge—it’s about once every 10,000 years, Metzger says.

If you multiply those mergers over the Milky Way’s entire history, it indicates there should be roughly 100 million Earths worth of gold in our galaxy.

“It’s a number that comes with a factor of five uncertainty in either way,” Metzger says. “But that’s the ballpark number.”

Of course, before you pivot careers to become a space pirate and plunder the galaxy, consider that impressive gold yield is all mixed in among hundreds of billions of stars. But it could be good to keep it mind next time you’re at Tiffany & Co.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: cosmology, stars
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  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Sewage downstream of fab plants is golden, but not commercially so. Blowing hot chlorine through discarded catalytic converter cores recovers Pt, Pd, Rh as volatile chlorides, then selective electrowinning. Again, marginal.

    Recovering dysprosium from discarded Fe-Nd-B supermagnets by solution then ionic liquid partition should be a national priority, but it isn’t.

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    The merger’s distal ends centripetally fragmented. A cloud of neutron-rich nuclear matter furiously beta-decayed down the Periodic Table. Look for Island of Stability superheavy elements. Gold is small stuff.

    • TLongmire

      You just fumble thru the cascade. True geniuses focus or does move too fast to grasp?

    • yetanotherbob

      The “Island of Stability” where super heavy elements don’t decay rapidly has been conjectured, based on theory, but never yet been detected. It may not exist. No current methods of synthesis for such elements is available.

      There are other “islands of stability”. Iron and lead sit at the center of two such. So does Uranium. There are other predicted peaks in the far beyond numbers of unobtainable elements.

      Stability here is relative. Iron and lead generally don’t decay, while Uranium does. Uranium though is much more stable than the elements around it. Finding an element in the ‘island of stability’ would not mean it doesn’t decay or isn’t radioactive, only that it decays more slowly than the elements around it. decay half life of seconds rather than miliseconds for instance.

  • Luke

    So how did gold end up on earth?

    Wasn’t the earth created through the big bang and not stars colliding?

    So the big bang is the equivalent of stars colliding?

    • http://www.beetrieve.no/ Lars A. Gundersen

      No, our solar system is much younger than the big bang, it was created by debris from earlier generations star explosions afaik. Universe is about 14 billion years old, Earth is about 5 billion years old

      • Luke

        Thanks Lars this makes sense!

  • Suresh Dorbala

    Boy! If a miniscule portion of this reached the earth just imagine what would happen to the price of gold. It would become dirt cheap.

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