A small cloud of molecular gas (here in green) is associated with GSH-138-01-24. Molecular clouds are denser and usually colder than clouds of hydrogen atoms. Radiation of carbon monoxide (CO) is used here look for molecules, because hydrogen molecules are difficult to detect.

The black and white image on the left is Figure 5 from the paper by Stil & Irwin (2001). The white contours show the outline of the molecular cloud (greenish in the image avove). The black contours outline radio continuum emission at a wavelength of 21 cm from ionized hydrogen. The ionized hydrogen (HII region) was discovered by De Geus et al. (1993), and has the same velocity as the molecular cloud. The grayscales display the atomic hydrogen with the same velocity as the CO cloud and the HII region. All of the atomic hydrogen is part of he shell GSH 138-01-94. The white crosses indicate the brightest areas of the optical emission of the HII region. The black + indicates the star that is probably responsible for the ionization of the HII region (see De Geus et al. 1993)
The colour image shows a slightly larger area than the black and white image. Only the upper right quadrant of the shell is displayed in these images.