One of the pursuits of the ancient alchemists was the transmutation of “base metals” (such as lead) into “noble metals” (especially gold). In the twentieth century, astronomers and physicists made great strides toward understanding the origin and abundance of the various chemical elements as they are found in nature.

Below is a color-coded periodic table of the elements based on a graphic created by Jennifer Johnson. The colors indicate current scientific estimates as to where the elements were (and are) produced. Notice that only the two lightest elements (hydrogen and helium) were produced in significant quantity in the “Big Bang” from which our expanding universe emerged 14 billion years ago. Researchers believe that all the remaining elements have been produced in various ways from that initial hydrogen and helium. That’s cosmic alchemy!

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The blue colors in the table show that most of the elements up to rubidium (symbol Rb, atomic number 37) are thought to be produced in supernova explosions (exploding massive stars and exploding white dwarfs). There is ample and longstanding evidence for this. Most of the chemical elements essential to our lives – notably carbon, nitrogen, oxygen, phosphorus, sulfur, calcium, and iron – were produced in the cataclysmic deaths of stars.

But the heavier elements (atomic number 38 and higher) are thought to be produced mainly by pairs of neutron stars that collide after millions of years spent slowly spiraling inward toward one another. One of those heavier elements is gold (symbol Au, atomic number 79). Researchers have had theoretical reasons (based on nuclear physics) for believing that gold and other heavy elements are produced by colliding neutron stars. But there has been no good observational evidence – at least not until August 17, 2017.

On that day, a tell-tale chirp of gravitational waves was detected by the two Advanced Laser Interferometric Gravitational-Wave Observatories (Advanced LIGO) detectors: one in Hanford, Washington and the other in Livingston, Louisiana. A third detector (Advanced VIRGO, near Pisa, Italy) did not detect the signal due to a combination of that detector’s limits and the source’s location on the sky. Based on the information from all three detectors, the source (formally called GW 20170817) was located to within an area on the sky of about 28 square degrees. That’s not exactly a pinpoint: it’s about 140 times the area of the full moon! However, it was good enough for astronomers worldwide to find an optical counterpart: a “kilonova” (a visible-light outburst about 1000 times as luminous as an ordinary nova, but much less intense than a supernova). The photographs below were taken by NASA’s Hubble Space Telescope. The main photograph shows the location of the kilonova within the galaxy NGC 4993, about 130 million light years from Earth. The insets show that the optical event faded over several days.

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A few kilonovae had been observed previously, but GW 2017817 is the first one which was also detected by the gravitational waves it emitted. Based on the details of the gravitational-wave chirp, LIGO and VIRGO scientists are confident that the waves came from the in-spiral and merger of two neutron stars.

The Fermi Gamma-Ray Space Telescope also detected a short-duration gamma-ray burst, confirming suspicions that such gamma-ray bursts are produced by neutron-star collisions.

Furthermore, astronomers found that the decay of the visible light from the kilonova was consistent with the theory in which gold and other heavy elements are produced by the collision of two neutron stars.

Beginning on August 17, 2017, Mother Nature really smiled on researchers. Through good fortune and hard work (funded by the citizens of several countries), humanity confirmed several hunches about how the universe works. A commentator for the British journal Nature referred to this pair of neutron stars as “a golden binary.” The image below is a conception of the explosive merger of those two stars, created by Robin Dienel of the Carnegie Institution for Science.

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--Bill Ingham