Formation of Elements

How then are elements heavier than iron made? There are two processes, both triggered by the addition of neutrons to atomic nuclei: the s (slow) process and the r (rapid) process. In both processes, a nucleus captures a neutron, which emits an electron and decays into a proton, a reaction called a beta decay. One proton at a time, these processes build up elements heavier than iron. Some elements can be made by either process, but the s process can only make elements up to bismuth (83 protons) on the periodic table. Elements heavier than bismuth require the r process.

The s process occurs while the star is still in the red giant stage. This is possible because the reactions create excess energy, which keeps the star stable. Once iron has formed in the star's core, however, further reactions suck heat energy from the core, leading to catastrophic collapse, followed by rebound and explosion. The r process occurs rapidly when the star explodes.

During a supernova, the star releases as much energy as the Sun does in 10 billion years and also releases the large number of neutrons needed for the r process, creating new elements during the outburst. The elements that were made during the red giant stage, and those that are made during the supernova explosion, are spewed out into space. The atoms are then available as raw materials for the next generation of stars, which can contain elements that were not made during the big bang. These elements are the basic materials for life as we know it. During their death throes, massive stars sow the seeds for life in the universe.