The wind changes from a slow high-density wind into a fast low-density wind that collides with the older circumstellar cloud
. At the same time the star's surface temperature increases to as high as 30,000 kelvin, and the intense radiation ionizes the surrounding gas clouds, causing them to glow.
(The only exceptions to this orientation are a few objects much closer to the young hot star [Theta.sup.2] A Orionis.) Through photoionization these circumstellar clouds
are losing material, which is being pushed away by the radiation pressure of starlight and strong stellar winds.