The Milky Way is a typical spiral galaxy, a flattened disc where most stars and interstellar clouds reside close to the plane of the sky. Most studies of star forming clouds have concentrated on nearby clouds and regions near the central plane of our Galaxy. This is particularly true for the studies carried out with ground-based telescopes. Previous infrared satellites like IRAS and more recently AKARI have made all-sky maps of the emission of dust, a tracer of the interstellar clouds. They were mostly sensitive to the normal "warm" dust that is found in the interstellar space. In this connection "warm" means temperatures about twenty degrees above the absolute zero, some –260 degrees centigrade. The densest cores, especially those cores that can collapse and form stars, the temperatures are still colder. This means that the prestellar cores radiate only weakly at infrared wavelengths (wavelengths 100µm and below) and one needs data at longer wavelengths.
Planck has carried out observations at several wavelengths, especially in the sub-millimetre range at wavelengths beyond 350µm. Combined with excellent sensitivity and a sufficient resolution for the detection of compact objects, Planck is providing the first all-sky census of cold clouds in the Milky Way. This is especially the first time we have the opportunity to survey all the potential star formation sites that are outside the better studies Galactic plane.
Below is an all-sky image that is combined from Planck observations between wavelengths of 350µm and 10mm (credit: ESA). The image is dominated by the emission from the dust particles inside Galactic interstellar clouds. The centre of Milky Way is at the centre of the image, the plane of the galaxy running horizontally across the figure.