In the Infrared and X-ray images, the black stripes indicate missing data. If data were lost in transmission or if the instrument were turned off for a span of time, the resulting gap in the coverage appears as a stripe. At this frequency, most of the emission is from the scattering of free electrons in interstellar plasmas, i.e., hot, ionized interstellar gas. Column density of neutral atomic hydrogen, derived on the assumption of optically thin emission, from radio surveys of the 21-cm transition of hydrogen. The data shown here are a composite of several surveys with ground-based telescopes in the northern and southern hemispheres.
Most molecular clouds are sites of star formation.
The black areas away from the plane were not observed systematically, but little CO emission is likely to have been missed. The images are encoded in the blue, green, and red color ranges, respectively. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds. The broad, sideways S-shaped curve traces the ecliptic plane and is the residual from the subtraction. The images are encoded in the blue, green, and red color ranges, respectively.