H+ regions
As its name implies an H+ REGION (or mi REGION as it is often written) is a part of space where the hydrogen in the interstellar medium is in an ionized rather than in a neutral state. The most common situation where this occurs is in the immediate vicinity of O-type stars, since these stars have a large ultraviolet flux; an05 main-sequence star emits 6 x 1O19 photons per second which have an energy great enough to ionize hydrogen atoms. If there is enough gas close to an O star, then a prominent nebula may be formed . Within this region, almost all the gas is ionized, and there is a close balance between the rate at which electrons and ions recombine and the rate at which the resultant neutral atoms are re-ionized. There is often a comparatively sharp boundary, known as an IONIZATION FRONT between the H+ region and the surrounding neutral gas.

H+ regions sometimes look approximately circular but more often appear quite irregular, either because of the odd shape; of the gas cloud itself, or because of patchy obscuring matter in front of it . The ionization may be from a single star or from a cluster of them. The temperature of the ionized gas in an H+ region is always close to 10000K, but the density can vary from about 107 to 1012 ions per cubic meter. Sizes can range up to several parsecs across, depending on the density of the gas and the ultraviolet luminosity of the ionizing stars.

Of all interstellar clouds, the H+ regions are the easiest and most rewarding to study; they emit many different kinds of radiation and can bo detected in large numbers all over the Galaxy. In H+ regions, the interaction between the stars and the interstellar medium is noon at its strongest; an H+ region usually comes into being as the by-product of star formation, and is made visible only an a result of the radiation from the stars it contains.

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