English: This image shows a ROSAT PSPC false-color image of the Chamaeleon I dark cloud, a well-known site of star formation. The contours show the 100 micron emission from dust in this cloud as measured by the infrared IRAS satellite. This image was made from X-rays with energies between 500 eV and 1100 eV. Since these soft (low energy) X-rays are absorbed by interstellar dust and gas, the observation of a "shadow" provides an indication of the intensity of X-rays coming from behind the absorbing cloud. X-ray "shadows" like this are the X-ray analog of well-known optical dark nebulae such as the Horsehead Nebula, produced by absorption in a dark cloud of optical photons emitted in a bright nebula behind the cloud. In the X-ray case, the entire sky is bright with a diffuse glow known as the diffuse X-ray background (somewhat like the night sky as seen from a modern city, where scattered light from street lamps produces a bright glow in the sky and washes out all but the brightest stars). Observations like this will help us understand the source of the diffuse X-ray background at these energies, which is known to be different from the 1/4 keV diffuse background shown in our image of the Draco nebula in this directory.