Oxygen-16 is a stable isotope of oxygen, having 8 neutrons and the standard 8 protons in its nucleus. This isotope has a mass of 15.9949 amu. From Isotopes of oxygen: 16O is "the most abundant (99.762% natural abundance) isotope [of oxygen]. The relative and absolute abundance of 16O is high because it is a principal product of stellar evolution and because it is a primary isotope, meaning it can be made by stars that were initially made exclusively of hydrogen. Most 16O is synthesized at the end of the helium fusion process in stars; the triple-alpha reaction creates 12C, which captures an additional 4He to make 16O. The neon burning process creates additional 16O."