Stars are thought to form inside giant clouds of cold molecular hydrogen — giant molecular clouds of roughly 300,000 M ☉ and 65 light-years (20 pc) 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 2,000–20,000 astronomical units (0.01–0.1 pc) and a particle number density of roughly 10,000 to 100,000/cm (160,000 to 1,600,000/cu in). Compare it with the particle number density of the air at the sea level—2.8 × 10 /cm (4.6 × 10 /cu in).