Dark clouds appear so because of sub-micrometre-sized dust particles, coated with frozencarbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C18O (CO withoxygen as the18O isotope), CS, NH3 (ammonia), H2CO (formaldehyde), c-C3H2 (cyclopropenylidene) and a molecular ion N2H+ (diazenylium), all of which are relatively transparent.[1][2]
The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The closest and largest dark nebulae are visible to the naked eye, since they are the least obscured by stars in between Earth and the nebula, and because they have the largestangular size, appearing as dark patches against the brighter background of theMilky Way like theCoalsack Nebula and theGreat Rift. These naked-eye objects are sometimes known asdark cloud constellations and take on a variety of names.
In the inner molecular regions of dark nebulae, events such as theformation of stars andmasers take place.
The Milky Way as seen byGaia, with prominent dark features labeled in white, as well as prominentstar clouds labeled in blackMain dark nebulae of thesolar apex half of thegalactic planeMain dark nebulae of thesolar antapex half of the galactic planeDust extinction within 1250 pc
