Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of roughly and in diameter. Over millions of years, giant molecular clouds are prone to collapse and fragmentation. These fragments then form small, dense cores, which in turn collapse into stars. The cores range in mass from a fraction to several times that of the Sun and are called protostellar (protosolar) nebulae. They possess diameters of and a particle number density of roughly . Compare it with the particle number density of the air at the sea level—.