That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.
Que o gas molecular atópese predominantemente nos brazos espirales suxire que as nubes moleculares deben formarse e disolverse nunha escala de tempo non superior a 10 millóns de anos, o tempo que tarda a materia en pasar a través da rexión do brazo.