Most chemical elements heavier than helium were born in the death throes of stars; the explosive energy of a supernova is responsible for generating most of the contents of the periodic table. Now, a new observation hints that another type of explosion—caused by the collision of two neutron stars—could be responsible for the production of many heavy nuclei, including gold.

Astronomers at the Harvard-Smithsonian Center for Astrophysics identified a red point of light at the same location as a powerful explosion known as a short duration gamma-ray burst. This is the first identification of an optical counterpart to this type of gamma-ray burst, and it could be the signature of new neutron-rich elements being produced in the aftermath of the explosion. If that conclusion is correct, then this observation is powerful support for the idea that colliding neutron stars are responsible for many gamma-ray bursts and the origin of some heavy elements.

As their name suggests, gamma-ray bursts (GRBs) are intense sources of gamma rays that peak, then fade on a timescale that depends on the event that produced them. Long-duration GRBs are currently thought to be particularly energetic supernovae, or hypernovae, since they occur in regions of vigorous star formation. Short-duration GRBs, on the other hand, happen in galaxies containing old stars or other regions lacking the high-mass stars that go supernova. Additionally, no supernova explosion has yet been observed at the location of a short-duration GRB, indicating that something else is at work.

In the past 10 years, astronomers have gathered increasing amounts of data supporting the idea that short-duration GRBs are the result of collisions between neutron stars. Neutron stars are the remains of the cores of stars at least eight times more massive than the Sun. They have masses comparable to a star, but that mass is compressed into an object roughly 10 kilometers in diameter, comparable to a city on Earth. The incredible density of neutron stars means they are crucibles of exotic physics, with strong gravitational and nuclear forces shaping them.

Two neutron stars in mutual orbit can collide when gravitational waves carry enough energy away from the system to destabilize the orbit. Astronomers have measured this effect in other systems, including the famous Hulse-Taylor binary pulsar. All observed binary neutron stars are within the Milky Way, but we have yet to watch any collide. Most GRBs are located in faraway galaxies. That makes it more difficult to unambiguously connect the two phenomena; however, the circumstantial evidence is strong, based on the correlation between the location of GRBs and where neutron stars would live.

The new observations help make the correlation stronger. E. Berger, W. Fong, and R. Chornock combined optical and infrared (IR) observations of the region of the sky where a short-duration GRB had gone off. The explosion, known as GRB 130603B, appeared on June 3, 2013 (hence the designation); the astronomers performed follow-up observations nine days later using the Hubble Space Telescope and the Magellan/Baade Telescope in Chile.

They found a red-colored point of light at the same position in the sky as GRB 130603B and determined both the GRB and the red point were at the same distance from Earth: about 3.9 billion light-years. Most GRBs are farther away, so the ability to spot an optical/IR counterpart is partly due to the relative closeness of the system. (Note cosmologists are insane enough to say 3.9 billion light-years is relatively close.)

Assuming the red point was actually part of the same event as the GRB, the astronomers considered two possibilities: either it was the afterglow of an explosion or it was light emitted by the production of new nuclei. The latter option is being called a "kilonova," since it produces new elements like a supernova does but with a lot less light and matter output. (The idea is that "kilo" refers to a smaller quantity than "super." The name doesn't do much for me, either.) If a short-duration GRB leads to a kilonova, then it could be a source of heavy, neutron-rich atoms. That list could include gold, though the research paper doesn't mention any element by name.

If GRB 130603B is a kilonova, then it's strong evidence that short-duration GRBs are caused by colliding neutron stars. The researchers compared the light output from the optical/IR counterpart to models and found it most strongly agreed with the collision between two objects of roughly equal mass, along with the nuclear processes forming heavy elements. That favors the kilonova interpretation.

The true alchemy of making gold doesn't require the philosopher's stone, but something far harder to obtain: neutron stars.

The arXiv. Abstract number: 1306.3960 (About the arXiv). To be published in Astrophysical Journal Letters.