How is gold technically created by colliding stars?

Question

Accepted Answer

Gold is created through a process known as rapid neutron capture, or the r-process, which occurs during cataclysmic cosmic events, most notably neutron star mergers.

The Mechanism of the r-process
For elements heavier than iron, standard stellar fusion (which powers stars like the Sun) is energetically unfavorable. Instead, gold is synthesized through the following steps:

Extreme Neutron Flux: When two neutron stars collide, they release an incredibly dense cloud of free neutrons. The environment is so extreme that atomic nuclei are bombarded with neutrons faster than they can undergo radioactive decay.
   Rapid Capture: An existing seed nucleus (often iron) rapidly captures these neutrons, building up mass almost instantaneously. Because the capture happens faster than the nucleus can decay, it can reach very high atomic masses.
   Beta Decay: Once the neutron bombardment ceases, the resulting unstable, neutron-rich atoms undergo a series of beta decays. During this process, neutrons within the nucleus transform into protons, changing the identity of the atom and stabilizing it into heavy elements like gold (Au), platinum, and uranium.

Why Colliding Stars?
Ordinary supernova explosions were long thought to be the primary source of heavy elements. However, modern astrophysical simulations and observations—specifically the GW170817 gravitational wave event detected in 2017—confirmed that neutron star mergers are the dominant "factories" for the universe's gold supply.

Density Requirements: A neutron star is essentially a giant atomic nucleus the size of a city. When two collide, they provide the necessary density of neutrons (exceeding $10^{20}$ neutrons per cubic centimeter) required to drive the r-process.
   Kilonova Explosions: The energy released during these mergers is so immense that it creates a kilonova, a transient astronomical event that ejects the freshly synthesized heavy metals into the surrounding interstellar medium.

Incorporation into Planets
Once these heavy elements are ejected into space, they become part of the nebular cloud from which new solar systems form.

1.  Accretion: As a star system forms, these heavy elements are pulled into the protoplanetary disk.
2.  Differentiation: During the formation of a planet like Earth, heavier elements sink toward the core. 
3.  Late Veneer: Much of the gold found in the Earth’s crust today is believed to have been delivered by a "late veneer" of asteroid impacts after the planet’s core had already formed, explaining why gold is relatively rare and concentrated in the crust rather than being entirely buried in the Earth's center.