 As Diogenes Laertius reports Leucippus' cosmology, worlds or kosmoi are formed when groups of atoms combine to form a cosmic whirl, which causes the atoms to separate out and sort by like kind. A sort of membrane of atoms forms out of the circling atoms, enclosing others within it, and creating pressure by whirling. The outer membrane continually acquires other atoms from outside when it contacts them, which take fire as they revolve and form the stars, with the sun in the outermost circle. Worlds are formed, grow and perish, according to a kind of necessity (DK 67A1).