Explosive origins
However, when a more massive star (greater than about eight solar masses) reaches the end of its life, it can explode as a core-collapse supernova. Such supernovae can leave behind neutron stars that produce highly neutron-rich winds.
In these winds, additional elements are formed through the rapid neutron-capture process, or r-process. When the nuclei of existing atoms capture extra free neutrons, the resulting product can be radioactive — meaning it will decay into a different version of itself or a new element entirely — or it can remain stable.
The r-process is very similar to the s-process, except it’s much quicker. The s-process can take decades or centuries to capture successive neutrons, with the entire elemental transformation taking tens of thousands of years. However, a supernova can produce roughly a billion billion billion neutrons per cubic inch, so the r-process is nearly instantaneous — at least in astronomical terms. For example, through the r-process, an iron atom can be transformed into uranium in less than a second.
Astronomers also recently confirmed another suspected r-process site: merging neutron stars.
Signatures of elements that are only created by the r-process were observed coming from the location of a confirmed neutron star merger picked up by gravitational waves. Even though such mergers are rarer than supernovae, astronomers now think that neutron star mergers are the primary sites of most heavy r-process elements. After all, this observed gravitational-wave event alone is expected to have produced an estimated three to 13 Earth-masses worth of gold.
Now that astronomers know how the universe forges all (or at least most) of its elements, the next step is working to understand exactly how much of each element is produced through various processes, as well as where they tend to occur. By building on this knowledge, researchers ultimately hope it will allow them to easily probe the complex history of any galaxy by simply looking at the ratios of its elements.