H II regions are luminous clouds of gas containing ionized atomic hydrogen and are known for star formation. Young hot blue stars cause ionization, but star formation is hidden in Bok globules. H II regions have a density between giant molecular clouds and Bok globules and are only found in spiral galaxies.
H II regions, also known as emission nebulae, are luminous clouds of gas and plasma up to several hundred light-years across. H II regions get their name from the large amounts of ionized atomic hydrogen they contain: in astronomical parlance, HI regions refer to neutral atomic hydrogen, H II refers to ionized atomic hydrogen, and H2 refers to molecular hydrogen. the primary form in which hydrogen takes when synthesized in the laboratory. The H II regions are well known as nebulae where star formation takes place.
The light emitted by the H II regions comes from all the ionized gas within. The cause of the ionization is young hot blue stars that emit large amounts of heat and light. The H II regions are among the most fascinating targets in astronomy due to the star formation that takes place in them. Unfortunately, star formation itself is hidden from us in the form of Bok globules, pockets of gas within H II regions so dense that they obscure their contents. Typically, a Bok globule contains gas of about 10-50 solar masses in an area of one light-year. Typically more than one star forms within such a globule, which is why stars are often found in clusters.
An H II region can be thought of as a nebula with a density somewhere between that of a giant molecular cloud (which is too large and diffuse to have ionized hydrogen, so its major component is molecular hydrogen) and Bok globules denser, in which the actual star formation takes place. H II regions are found only in spiral galaxies and always in spiral arms. They are never found in elliptical galaxies because these galaxies are thought to have formed as a result of galactic collisions. During galactic collisions, any pre-existing H II regions are likely to be stirred up and their density increased to the point where nearly all of the gas turns into stars, leaving little else.
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