What event is believed to create elements heavier than iron?

The explosion of a supernova. Some astrophysicists don't believe that even THAT would suffice to make some of the very heavy elements such as gold or uranium; they believe that only the collision of two neutron stars would release enough energy to do that.

The problem is the "packing fraction" curve. Two atomic nuclei can smash into each other at high energy and release a little bit of energy as the nuclei come together, or "fuse". When two or more hydrogen atoms smash into each other in the cores of stars, they fuse into helium, and we call this "nuclear fusion".

As we smash heavier and heavier elements together, they release smaller and smaller amounts of energy in fusing - until we get to iron. Once you start fusing elements together to get stuff heavier than iron, you have to PROVIDE energy to complete the reaction. Think of the packing fraction curve as a valley, with iron at the bottom of the valley. As you roll your bike down the hill from one side, you can coast because gravity is providing energy. Once you pass iron (at the bottom of the hill) you need to start putting in your OWN energy, by pedaling.