 In our How Older Stars segment, we've covered how the Circumsteller disk feeds the central object, leading to the formation of protostars, t-tarry stars, and eventually fully formed helium fusion-burning stars. The process from the beginning of the protostar phase to a fully-fledged star is estimated to take from one to two hundred million years. During this period, the debris in the disk is forming planets. In the Nucleosynthesis segment of the Lambda-Cold-Ark-Matter Big Bang Theory chapter, we covered the content of clouds like these for the first generation of stars. At that time it was limited to hydrogen, some helium, and just traces of beryllium and lithium. Planetary nebula form when normal stars run out of hydrogen. The violent process created pressures and temperatures large enough to fuse hydrogen and helium into heavier elements and eject them into the interstellar medium from which giant molecular clouds like this are formed. These include carbon, nitrogen, and oxygen along with even smaller amounts of heavier elements like silicon, sulfur, and iron. But this kind of star transition does not have energy to fuse enough protons to create atoms larger than iron. Supernova explosions occur when supermassive stars run out of hydrogen. These seed even heavier elements into the interstellar medium like lead, zirconium, silver, tungsten, and gold. But even at these extreme energies it is unlikely that a supernova could produce elements larger than lead. This is because of the repulsive force of like charges is so strong. The Coulomb force creates the Coulomb barrier. You may recall from our last chapter on how older stars that a proton in our sun can collide a trillion times a second with other protons and still not fuse for a billion years. But neutrons have no charge and their fusing has no such barrier. It has long been theorized that the heaviest elements like thorium, protactinium, and uranium were created by neutron star mergers. In 2017, using large laser interferometers that we covered in the gravitational waves segment of the How Fast Is It? video book, just such a merger was detected. Now known as a kilonova, it's understood that neutron star mergers are the origin of the majority of all the heaviest elements found throughout the universe. For our collapsing cloud we know that it contained all 94 natural elements because we find them here on earth.