Clues to the origin of heavy elements

Just over 300 years ago, in late 1717, the price of gold was first standardised by Sir Isaac Newton. It continues to be the economic standard to this day. In the form of gold coins and later as backing for paper money, it fluctuated with world crises, market forces and economic policies. After World War II, when the European nations were economically shattered, gold was tied to the US dollar, with the delegates at the Bretton Woods Conference, held in 1944, fixing the rate of 35 dollars to an ounce of gold. After 1971, when the dominant US dollar was no longer tied to gold, the metal became freely traded.

Gold, as a chemical element, is not vital to human existence like oxygen but with its imperishable shine and unusual properties like its malleability and density, it has become one of the most coveted commodities. From the ancient Egyptian pharaohs who insisted on being buried in the 'flesh of the gods' to the financiers who made gold the bedrock of the global economy, humankind's attachment to the yellow metal is evident.

As the warden of the Royal Mint, UK, Newton had more to do with gold than standardising it. It appears that he was fascinated by the possibility of transforming base metals to gold, according to his handwritten manuscript that was discovered last year. In other words, he pursued alchemy. Apparently, Newton's laboratory notebooks, even the one containing the first full description of his seminal discovery that white light is actually a mixture of spectral colours, are also filled with recipes that were patently elaborated from several alchemical sources. Along with his explanation of optical and physical phenomena like freezing and boiling, we find the mention of 'Neptune', 'Trident' and 'Mercury's caducean rod', among others. All of these represent the alchemical symbol.

Gold in stars

For many of the prominent 17th century chemists, the attempt to make gold from base metals was a viable research project. As spin-offs, a lot of contributions were made to pharmacology (mineral-based drugs), making of pigments and dyes, distillation of spirits and other areas. Mercury is often used to separate gold from rock, and millions of miners worldwide have inhaled the toxic vapour during the refining process with much damage to health and environment.

Now, what do gravitationally collapsing massive stars have to do with gold? Newton's law of gravity is universal, accounting for the motion and stability of celestial objects. A stable star like the sun is in hydrostatic and radiative equilibrium. The immense gravitational force that results in the collapse of the star is balanced by the outward pressure which is exerted by the superhot gas and radiation in its interior.

The stars maintain their hot core temperatures by nuclear reactions converting hydrogen to helium. In one second, the sun converts 600 million tonnes of hydrogen to helium, the nuclear energy released supplying the immense power it radiates. After the hydrogen fuel in its interior is used up, the core collapses under its gravity, and heats up again to 200 million degrees, when helium undergoes thermonuclear reactions. Stars like the sun are not massive enough to produce elements beyond carbon or oxygen.  

In more massive stars, the core collapses again after the helium is used up, so that carbon, oxygen, and other lighter elements produce heavier elements like calcium and silicon, among others. When one set of heavier elements is used up, the core collapses again becoming hotter, and heavier elements up to iron and nickel are produced. However, the iron nucleus has the maximum nuclear binding energy, so that the thermonuclear reactions cannot go on to form elements beyond iron. So, in that case, how are much heavier elements like gold or uranium produced in the stars?

The iron core of the massive star now collapses till it turns into neutrons (the electrons and protons in the nuclei are all squeezed to super high density when they fuse to form neutrons). The envelope of the star meanwhile explodes to become a supernova. The elements in Group 8 of the periodic table, to which iron belongs to, are bathed in a huge flux of neutrons. The nuclei capture the neutrons in succession, giving elements of higher mass numbers. Together, with successive beta decays where the atomic number keeps increasing, the heaviest elements like gold or uranium are produced.

In this terminology, elements like gold are r-process elements, implying rapid neutron capture. Elements like silver and lead are, on the contrary, produced by the so-called s-process or slow neutron capture which occurs in pulsating red giant stars. So, rapid neutron capture is required to produce elements like gold (atomic number 79) from iron (with atomic number 26). After the supernova explosion of a massive star, the remnant is a dense neutron star with a density hundred trillion times that of water. Neutron stars were first detected as pulsars about 50 years ago. It is a star the size of Bengaluru and it spins more than 100 times every second.

Merging together

Neutron stars are expected to merge together (collide) after millions or billions of years (depending on their initial separation). Neutron star mergers were predicted to produce gravitational wave bursts and also to produce the heaviest elements like gold, platinum and thorium as so many free neutrons could be captured rapidly by the lighter elements like iron.

In August 2017, scientists at the LIGO observatory detected the gravitational wave burst from a merger of neutron stars. Gamma ray bursts were also generated by the merging neutron stars. The aftermath of the neutron star collision resulted in the emission of optical and infrared radiation. Scientists at several telescope observatories were able to spot optical spectral signatures from the glow of gold, platinum and other heavy elements for the first time.

This provided strong evidence that it was indeed the formation of neutron stars by gravitational collapse and their subsequent merger can result in the formation of elements like gold. Thus, all the gold present in the universe was actually produced in a chain of nuclear processes starting from the gravitational collapse of massive stars. This fully follows Newton's law of universal gravitation.

The heavier elements released are incorporated into interstellar matter, the interstellar clouds later collapsing to form stars like the sun, hosting planets whose inhabitants use these elements for their own ends.

(The author is with Indian Institute of Astrophysics, Bengaluru)