In the entire universe, heavier elements than hydrogen and helium are extremely rare. The universe consists of about three-fourths of hydrogen and a fourth of helium; all other elements together make up less than 1% of the universe.
That is because the universe at the big bang came about with the current supply of hydrogen and helium. All other elements were then formed with great difficulty in the stars. How difficult that process must have been is demonstrated by the fact that, in the more than 13 billion years that the universe has existed, less than one percent of the original amount of hydrogen has been converted into heavier elements.
These elements are built up in the stars through a series of fusion processes. However, only iron can be produced from this, which has atomic number 26. This is because iron fusion processes use much more energy than they release. The formation of heavier elements than iron, such as uranium with atomic number 92, is such a difficult process that it is virtually impossible. Elements that are heavier than iron make up only a millionth of all elements on a cosmic scale. And that is why elements such as gold, platinum and uranium are so rare and expensive.
Originated in supernova explosions
All elements heavier than iron arise in supernova explosions when trapping neutrons. A huge amount of neutrons are produced in a supernova burst. Because neutrons are electrically neutral, they can effortlessly penetrate the existing atomic nuclei of iron or lighter elements. Isotopes are formed with a substantial neutron surplus, resulting in unstable radioactive nuclei.
For example, an iron atom can rapidly capture five neutrons, making it so unstable that it decays instantly to cobalt with atomic number 27. In this way, elements that are heavier than iron can be formed. However, there is not much time for that in a supernova explosion, because the neutron radiation soon disappears. And of all the stars, only a very few - only the heaviest - end in the form of a supernova.
A new theory therefore assumes that the lion's share of the heaviest elements such as gold and uranium originated in the even more intense processes that take place when two neutron stars collide.