Sulfur (16S) has 23 known isotopes with mass numbers ranging from 27 to 49, four of which are stable: S (94.85%), S (0.76%), S (4.37%), and S (0.016%). The preponderance of sulfur-32 is explained by its production from carbon-12 plus successive fusion capture of five helium-4 nuclei in the alpha process of nucleosynthesis.
The main radioisotope S is formed from cosmic ray spallation of Ar in the atmosphere. Other radioactive isotopes of sulfur are all comparatively short-lived. The next longest-lived radioisotope is sulfur-38, with a half-life of 170 minutes. Isotopes lighter than S mostly decay to isotopes of phosphorus or silicon, while S and heavier radioisotopes decay to isotopes of chlorine.