Silhouettes of Cold Hydrogen Clouds.
INTERSTELLAR SILHOUETTES: Canadian Galactic Plane Survey Traces the Shapes of Cold Hydrogen Clouds
Despite the violent motions in the interstellar medium and the impact of energetic radiation, pockets of quiescence exist where gas clouds can cool and condense, leading to the formation of the next generation of stars. This image (Map 3) magnifies a section of the Canadian Galactic Plane Survey panorama Map 1, exposing clumps and filaments of cold hydrogen gas silhouetted against the hotter diffuse hydrogen. This is the first time the cold hydrogen clouds have been seen in detail, emphasizing the superb quality of the data collected by National Research Council of Canada's Dominion Radio Astrophysical Observatory (DRAO). These interstellar silhouettes appear purple-ish due to the colours assigned to the DRAO hydrogen gas dataset. The origin of these clouds is still a mystery, but they may be first step in the evolution of the diffuse interstellar gas to a cold, dense state in which molecules can coalesce, clouds collapse, and new stars form.
This detail from Map 1 (CGPS pr_14012000) displays the stuff between stars in the Perseus Spiral Arm (6500 light-years distant) of our Milky Way Galaxy by assigning different colours to radiation not detected by the human eye. Invisible to optical telescopes, hydrogen gas can only be revealed by observations of faint radio waves emitted at a wavelength of 21 centimetres. Besides radio data showing this gas, from the National Research Council of Canada's Dominion Radio Astrophysical Observatory (and assigned colour combinations of blue, orange, yellow, and green), this composite image includes Infrared Astronomical Satellite (IRAS) data (coloured pale pink), and data (grey-blue) from the Westerbork Synthesis Radio Telescope (WRST) in the Netherlands.