Published online by Cambridge University Press: 25 April 2016
It is generally agreed that early-type stars, and hence their HII regions, appear to develop preferentially in dense concentrations within molecular clouds. These parent molecular clouds owe their existence in two ways to the dust grains in the interstellar gas, a component which comprises about 1% by mass. First, dust grains provide stable surfaces upon which hydrogen atoms can come together to form molecules. Then, once formed, the molecules are protected in cloud interiors by dust absorption of the dissociating ultraviolet radiation which pervades the low-density interstellar medium.