Have you ever seen an image of a colored cloud in night sky with the caption HII (read "H-two") region under it? Or heard that the Orion Nebula, pictured below,is an HII region? HII regions make for nice telescope images, but do you know what they are?
Clouds of hydrogen gas in the form of molecular hydrogen (H2)or atomic hydrogen (HI) are where star formation often takes place. These gas clouds collapse in various places, triggering the formation of stars. Groups of young, hot stars emit lots of ultraviolet light which is very energetic. This light is absorbed by some of the atomic hydrogen, causing the hydrogen to lose a electron or become ionized (now called HII). The more stars that form, the more hydrogen they ionize, and thus the larger the HII region. These regions exist most often within the spiral arms of galaxies, where we know star formation is occurring. HII regions exist until some of the young stars within them age and die in a supernova explosion. This explosion causes any HII in the area to be blown away, leaving behind an open cluster of stars.
Image Credit: NASA/ESA
