The second is a smaller list of about 500 stars, all ionizing stars that are known to be associated with HII regions: large regions of ionized hydrogen gas. (galaxymap.org)
Blue, green, and red colors are attributed to the B - band, R - band, and H - alpha (emission line from ionized hydrogen gas) images, respectively. (sciencedaily.com)
One strong possibility is that the ultraviolet (UV) radiation from the first - generation galaxies in the one - billion - year - old Universe ionized the hydrogen gas. (sciencedaily.com)