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July 18, 2013

ONLINE ONLY: Origin of gold found in rare neutron-star collisions

Gold — atomic number 79, element symbol Au, the most widely beloved of the precious metals — might have its origin in extremely rare and violent explosions in the far reaches of outer space. The bling apparently begins with a blam.

For many years, scientists had theorized that the heavy elements of the periodic table, such as gold, platinum, lead and uranium, had their origin in supernova explosions. But the source, scientists announced Wednesday, might be even more exotic: the collision of two ultra-dense objects called neutron stars.

"We now have kind of a smoking gun," said Edo Berger, an astronomer who led the research at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The elements on Earth are all of cosmic origin. Carbon and oxygen atoms in our bodies, for example, come from the interior of stars, where they were formed under high pressure and heat. They were later spewed into the universe in supernova explosions. It is literally true, as Carl Sagan was fond of saying, that we are all star stuff.

But what hasn't been known is whether these supernova events could account for the heaviest of the elements. A heavy element such as gold has 79 protons, 79 electrons and 118 neutrons. That's a lot of mass for one atom. Most of the elements are simpler; gold and the other heavy elements are cosmically extravagant.

The neutron stars might provide the explanation.

Neutron stars are the collapsed cores of stars that have exploded in a supernova. A neutron star might be roughly the diameter of Washington but contain as much mass as our sun, all of it crammed together by the force of gravity, until even the atoms have collapsed, leaving the object with the density of an atomic nucleus.

A teaspoon full of neutron-star material would weigh, on Earth, about 5 billion tons, Berger said.

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