O and B Stars and Their Relation to H II Regions

• The most massive protostars to form out of a dark nebula rapidly become main sequence O and B stars

• They emit strong ultraviolet radiation that ionizes hydrogen in the surrounding cloud, thus creating the reddish emission nebulae called H II regions

• Ultraviolet radiation and stellar winds from the O and B stars at the core of an H II region create shock waves that move outward through the gas cloud, compressing the gas and triggering the formation of more protostars