Based on data from the Sun and other stars, hydrogen and helium are by far the most abundant elements of the cosmos (e.g. the Sun’s atmosphere may contain 70% hydrogen and 28% helium by mass). In general, elements show an exponential decrease in abundance with increasing atomic number (Z), up to about 45; the abundances of heavier elements thereafter appears fairly constant. Other regularities are superimposed on this general pattern: elements of even atomic number are more abundant than adjacent elements of odd atomic number (Oddo–Harkins rule); there is a pronounced peak at atomic number 26 (the ‘iron’ peak); and isotopes whose mass numbers are multiples of four, i.e. multiples of the alpha-particle (helium nucleus) mass, e.g. carbon, oxygen, neon, magnesium, silicon, sulphur, and iron, have enhanced abundances. Cosmic abundances of elements constrain the way in which the solar system evolves and dictate the composition of its members (which include the Earth and ourselves on it). See also nucleosynthesis.