Molecular clouds.

             
                                                  Sometimes called a stellar nursery.

 What is molecular clouds? 
        Molecular clouds isa large and relatively dense of cold gas and dust in interstellar space from which new stars are born. It also contain a high fraction of molecules.
     
              

       Molecular clouds are the principal sites of ongoing star formation. Therefore, they tend to be associated with young stars and star-forming regions. The nearest star-forming clouds are found in the constellations Ophiuchus, Taurus, and Perseus, at distances of 125, 140, and 300 parsecs (1 parsec = 3 × 10¹³ km or 2 × 10¹³ mi), where the nearest regions of active low- and intermediate-mass star formation are found. These cloud complexes have masses ranging from several thousand to perhaps over 10,000 times the mass of the Sun. However, most of the molecular gas in the Milky Way Galaxy is concentrated into giant clouds with masses more than 100,000 times the mass of the Sun. The nearest giant molecular clouds are located at a distance of 460 parsecs toward the constellation Orion where, over the last 10⁷ years, they gave birth to tens of thousands of stars, including several dozen relatively rare high-mass stars.Most molecular clouds have temperatures of only 10 K. Molecular clouds are orders of magnitude more dense than the general interstellar medium, with gas densities ranging from about 10 molecules per cubic centimeter on large scales to over 10⁶ molecules per cubic centimeter in cloud cores. The sizes of individual clouds range from less than 0.1 parsec for small clouds and dense cores to over 100 parsecs for giant molecular clouds. In addition to star-forming molecular clouds concentrated toward the plane of the Milky Way, there are many smaller and lower-density molecular clouds visible most clearly far away from the galactic plane, with the closest ones only about 50 parsecs from the Sun.

Molecular clouds have a very complex internal structure consisting of clumps and filaments of dense gas surrounded by interclump gas of much lower density. Individual clumps usually have supersonic internal motions with a velocity of severalkilometers per second. The powerful outflows produced by young stars during the first 100,000 years of their existence may be a major source of these chaotic motions. Magnetic fields which thread molecular clouds may play a role in thelongevity of turbulent motions and may support clouds against gravitational collapse.

About 100 different chemical species have been so far identified within molecular clouds, indicating that there is a rich chemistry taking place. See also Interstellar extinction.