First proof shows role of neutron stars as cosmic element factories

Insight into a cosmic factory: The first proof of a neutron-star collision by gravitational waves and radiation is not only a milestone in astronomy - it also provides us with new insights into how gold, platinum and other heavy elements are created. Only the cataclysmic fusion of neutron stars create the appropriate conditions for this. But the now observed neutron-star fusion could have released 200 earth masses of gold and 500 earth masses of platinum.

Astronomers have long been speculating that only the collision of neutron stars creates elements heavier than iron. Now they have proof of this. © National Science Foundation / LIGO / Sonoma State University / A. Simonnet

After the Big Bang, there were only the lightest elements in the cosmos: hydrogen, helium and traces of lithium. It was only when the first stars were formed that even heavier elements, such as oxygen, carbon or iron, were formed by the core fusion. When these stars exploded in supernovae, these elements became free.

Puzzling heavy elements

But one question remained open: Where do elements that are heavier than iron originate from? In order for them to arise, more than just the stellar core fusion is necessary. The atoms must be bombarded directly with neutrons, in order to gradually grow into increasingly heavy nuclei. However, where and when this growth took place in the cosmos, was one of the greatest riddles of astrophysics for a long time.

One of the candidates for the role as an element factory has long been the collision of neutron stars - the extremely dense remains of dead stars. In this cataclysmic event, matter is extremely heated and thrown out into space. The accelerated neutrons collide with atomic nuclei and initially form unstable heavy particles, which then decay to more stable heavy atoms, including gold, silver or platinum, by radioactive decay.

A radiant "Kilonova"

According to theory, this element-forming process should be revealed by a characteristic sequence and composition of electromagnetic radiation.

As this debris cloud spreads in space, the decay of radioactive elements makes them hot and makes them glow, says Brian Metzger of Columbia University.

Metzger and his colleagues have calculated that the radiation emitted in the optical and infrared range should be about a thousand times brighter than in normal nova explosions, but less bright than in a supernova. Such a fusion of neutron stars should therefore be optically traced as so-called "Kilonova" - theoretically. But the evidence was still pending, because no one had ever seen a neutron-star collision.

First silver, then platinum, uranium and gold

This has changed. By means of gravitational waves and dozens of telescopes, GW170817 was registered on August 17, 2017, and for the first time astronomers detected one of these cosmic factories of the heaviest elements.

Perhaps the biggest surprise was how well the optical signals of this neutron-collision coincided with our theoretical expectations, says Metzger.

As predicted, the radiation emitted by the collision corresponded to that of a kilonova. At first the light was rather short-wave and bluish. According to the models, this light comes from the outer regions of the debris cloud, the region where lighter precious metals such as silver are formed. A few days later the light spectrum of the kilonova moved into the red area. As the astronomers explain, this radiation is generated by radioactive decay inside the debris cloud, the disintegrations that make up the heaviest elements such as platinum, uranium, or gold.

200 earth masses of Gold

Because the fusion of the neutron stars has been observed by so many different telescopes and in all wavelengths, the astronomers have even been able to determine how many heavy elements have arisen during this cataclysmic event. They estimate that an equivalent of about six percent of the mass of our sun were produced on heavy elements.

In gold alone, the neutron-star collision generated probably enough to produce 200 completely gold-bearing earths, as the researchers report. At the same time, about 500 earth masses of platinum and other heavy elements were created.

"Holy Grail of Astronomy"

The gravitational waves tell us the movements and masses of the neutron stars involved, and the radiation reveals the astrophysical side of the event - what happened during the collision and how many heavy elements were produced, explains Marcelle Soares-Santos of the Fermi National Accelerator Laboratory. That is why the detection of light and gravitational waves from one source is a holy grail of the present astronomy.

Source: University of California Berkeley, National Optical Astronomy Observatory