How does gold emit gamma rays

Gold was created by the collision of dying stars

It is heavy, shiny and is the epitome of wealth and power: gold is a precious metal that fascinated our ancestors. Now another peculiarity of this element can be seen: It did not originate on earth, but in space. A gamma-ray flash has now revealed to US researchers where there. According to this, the collision of two neutron stars could cause these flashes - and in the process produce so much gold that our moon could be outweighed by it, as the researchers report in the specialist magazine "The Astrophysical Journal Letters".

Gold is not only the symbol of wealth par excellence - it is also quite rare on earth. In a thousand tons of crustal rock there are only around four grams of it - on average. It was only washed out over the course of millions of years and then concentrated in some places in the ores of the gold veins or in nuggets. Where the gold originally came from, however, was unclear until now.

Born in the stars - only not the gold

Almost all chemical elements have their origin in stars and their explosions. It was only after the Big Bang that they produced almost all heavier substances from hydrogen. Because the nuclear fusion in their interior fused hydrogen atoms to form larger structures, including helium, carbon, iron and oxygen. This element production takes place in the stars to this day - also in our sun. The saying of the astronomer Carl Sagan: "We are all made of stardust" sounds poetic, but describes hard scientific facts.

However, gold cannot have been created in this way, as Edo Berger from the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues explain. Because it is too heavy and its formation requires more energy than is released when a star merges or explodes. But how did the noble metal come about?

A gamma-ray burst with an unusual afterglow

A rare and dramatic event in the cosmos provided the astronomers with an indication of this: a gamma-ray burst (GRB). This brief flash in the gamma-ray spectrum of light glowed 3.9 billion light-years from Earth and was captured by NASA's Swift Gamma-Ray Observatory on June 3, 2013. With a duration of just two tenths of a second, the GRB 130603B lightning bolt is one of the so-called short gamma ray bursts. Typically, they only emit high-energy radiation for a fraction of a second and then glow in the infrared range for a few days.

The collision of two neutron stars creates a short gamma-ray flash - and gold © Dana Berry / SkyWorks Digital

The astronomers also registered this afterglow with GRB 130603B - however, its spectrum deviated from that of a normal afterglow, as they report. The radiation's characteristics suggested that it could have been emitted by heavy radioactive elements, the researchers say. Such elements can arise, among other things, in one of the greatest catastrophes in the cosmos: the collision of two neutron stars.

Because the collision of two objects with such extremely densely concentrated matter releases energy that atoms can fuse to form these extremely heavy elements. Because these elements are not stable, they decay after a short time, releasing radiation - the radiation that the astronomers have now also captured in GRB 130603B.

A lot of gold from the mass of the moon

But that means two things, as Berger and his colleagues explain: On the one hand, this observation could provide the long-sought proof that short gamma-ray flashes are actually caused by a collision of two neutron stars. “The radioactive glow from GRB 130603B could be exactly the smoking gun that proves it,” says co-author Wen-Fai Wong of the Harvard-Smithsonian Center for Astrophysics.

On the other hand, however, this collision evidently not only created radioactive elements, but also gold - and in large quantities: "We estimate that the merger of the two neutron stars produced and ejected an amount of gold that corresponds to the mass of the entire moon - so a lot of 'bling' ”, says Berger. If you add up the number of short gamma-ray bursts that have occurred in the universe since the Big Bang, all of the gold in the cosmos could actually have been created by these collisions.

"To paraphrase Carl Sagan: We are all stardust and our jewelry is the dust of colliding stars," says Berger. (The Astrophysical Journal Letters, 2013; in press

(Harvard-Smithsonian Center for Astrophysics, 07/19/2013 - NPO)

19th July 2013