In massive stars, nuclear fusion occurs in several stages as they evolve. The process begins with hydrogen fusing into helium, followed by helium fusing into heavier elements like carbon, oxygen, and so on, up to iron on the periodic table.
Iron (Fe) is the heaviest element that can be produced through nuclear fusion in the core of a star while still releasing energy. Fusion of elements heavier than iron requires an input of energy rather than releasing it, which is why the fusion process stops at iron in massive stars.
Elements heavier than iron, such as uranium, are primarily formed through processes like neutron capture, specifically the s-process (slow neutron capture) and r-process (rapid neutron capture), which usually occur during supernova explosions or in neutron star mergers rather than in the core fusion of the stars themselves.
So, to clarify, while iron is the heaviest element produced through nuclear fusion in large stars, elements like uranium are produced through other processes after the star's death.