In the dense nebulae where stars are produced, much of the hydrogen is in the molecular (H) form, so these nebulae are called molecular clouds. The largest such formations, giant molecular clouds, have densities of 100 particles per cm, diameters of , and masses up to 6 million solar masses, and an average interior temperature of 10 K. About half the total mass of the galactic ISM is found in molecular clouds. In the Milky Way there are about 6,000 molecular clouds, each with more than 100,000 solar masses. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula, away. However, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex.