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What happens to the trail of ionized gas behind a moving ultracompact H II region?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

As an ultracompact H II region moves through a surrounding molecular cloud, it leaves behind a trail of ionized material. This trail is less bright than the confined, freshly ionized gas located ahead of the moving region. As the ionizing star continues to move forward, the gas in the trail gradually fades as it undergoes recombination.1