What are molecular clouds in the interstellar medium?

   Interstellar space contains more than sixty types of polyatomic (containing more than one atom) molecules. The substance formed in the greatest abundance is molecular hydrogen (H2); others include water, carbon monoxide, and ammonia. Since these molecules are broken down by starlight, they are found primarily in dense, dark nebu­lae, where they are protected from the light by cosmic dust. These nebulae—known as molecular clouds—are enormous. They stretch across several light years and are one thousand to one million times as massive as the Sun.