Clouds and Cloud Collapse

About 50% of interstellar gas is concentrated into denser interstellar gas clouds that occupy about 2% of the volume of the Galaxy. The temperature of the gas clouds is about 100 K, the densities range from 1 to 100 atoms per cubic centimeter. 

In the interstellar space molecules do not survive easily if the gas is too hot they would collide with other atoms or molecules and break into their constiment atoms. In neutral hydrogen clouds, the temperature may be low enough but another mechanism may destroy them photons of high energy from neighboring stars. As a consequence, molecules survive only in cold dark interstellar clouds where dust blocks photons. These are the molecular clouds. 

Inside such molecular clouds temperatures are only up to 10 K and the densities are from 100 to 1000 molecules per cubic centimeter. The most important molecule is hydrogen, H2. Other molecules were mentioned already, such as CO and even complex organic compounds are found there. The masses of these molecular clouds are up to 10 solar masses and large clouds may be ovo 1000 light years in size. There exist about 4000 molecular clouds in our galaxy. 

How are these clouds affected by their environment One mechanism is the heating by UV radiation from massive hot stars. Another mechanism are blast waves from supernova explosions. In most molecular clouds self gravity is unimportant. Each part of the cloud is attracted by every other part. But the clouds are not in hydrostatic equilibrium like stars. In most clouds, pressure is much stronger than gravity. Such a pressure causes them to expand, however the expansion is stopped by their surroundings. Some clouds are massive enough so that they start to collapse 

For a cloud to collapse, the so called Jeans criterion has to be fulfilled Let us assume a cloud with mass M, radius R, total number of particles N, average particle mass m, and temperature T. The gravitational potential energy of this cloud is 

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