Interstellar space is filled with a gas of relativistic ions and electrons-the Galactic cosmic rays. These energetic particles tie interstellar gas to ambient magnetic fields by ionizing the component molecules and atoms, and so play a role in stabilizing molecular clouds against collapse(1) and regulating the collapse of protostellar clouds(2). The observed energy spectrum of cosmic rays up to greater than or similar to 10(15) eV is consistent with their acceleration by supernova shock waves(3), but the original source of cosmic-ray nuclei remains unclear. There has been a widely held belief that the source consists of a solar-like ionized medium(4), probably the chromospheres of late-type Sun-like stars(5). This model predicts an overabundance of easily ionized elements, Here we show that lead, which is easily ionized, is underabundant in the Galactic cosmic rays in contradiction with this model. Rather, our measurements are consistent with two other possible models : one in which the nuclei originate in interstellar gas, and in entire grains accelerated to about one per cent of the speed of light by supernova shock : waves(6,7); and another in which the cosmic rays contain an admixture of an exotic, freshly synthesized component(8), probably originating in neutrino-driven winds from newly born neutron stars(9).