According to the present evidence from cosmological research, our universe began about 13.7 × 109 (± 0.2 × 109) years ago with the big bang. Calculation also suggests that only the three lightest elements were formed under the conditions at that time. Elements from about number 4 upwards could only be formed later, under the conditions in stars. Ordinary stars burn hydrogen and then successively heavier elements, and apparently can synthesize all the elements up to iron (number 26) by processes of thermonuclear fusion. All elements above iron must be made by several slow and rapid processes of neutron and proton addition as stars age and sometimes explode. The contents of stars are redistributed into space by gas streaming and under the remarkable conditions when they explode as novae or supernovae. The present mixture of elements on Earth (and in the Solar System generally) seems to require multiple processes for synthesis and thus seems to be the product of more than one generation or episode of star formation and explosion. Information from meteorites also suggests heterogeneity in source material.