Since an H+ region is at a much higher temperature than the neutral gas which surrounds it, it exerts an outward pressure at the ionization front. The main characteristic of the evolution of an H+ region is therefore one of expansion. The birth of an H+ region is generally a by-product of the birth of an 0 star. The gas which becomes ionized is part of the cloud out of which the star formed. At first, the H+ region is very dense and compact, and is invisible optically because of the dust in the outer, neutral layers of the cloud. A group of five compact H+ regions each of these contains at least one newly-formed 0 star. The differences in size and brightness between these five objects is a result of differences in their ages and in the 0 stars at their centers. These very young H+ regions are embedded in a warm cloud of molecular hydrogen and are expanding into it at a rate of the order of one parsec every 100000 years. As they expand they will ionize more and more of the surrounding neutral hydrogen and disperse the obscuring material that is currently concealing them from the view of optical astronomers.

Optical nebulae such as the Rosette Nebula and the North American nebula are much older than the com¬pact H+ regions, but are probably evolved from objects not unlike them. As these H1″ regions continue to expand they will become less and less dense and will eventually blend into the background of the interstellar medium.

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