Multiphase view of the Horsehead nebula (Barnard 33). Left: composite color image of the Horsehead nebula and part of the H II region IC 434 observed with VLT (ESO). The size of the image is about 7′ × 7′, where 1′ is equal to 0.12 pc at the Horsehead distance. Right: zoom of the edge of the molecular cloud imaged with ALMA in the CO J = 3−2 line emission (blue). The hot ionized gas coming from the H II region is traced by the Hα line emission imaged with the 0.9m KPNO telescope (Pound et al. 2003), resolving scales of about 1″ (red). The field of view in this case is approximately 50″ × 50″ or, equivalently, 0.1 pc × 0.1 pc. The synthesized beam size of the ALMA observations is shown in the bottom left corner, as well as the corresponding physical scale. The dark region between the right edge of the CO emission and the bright rim of Hα is the neutral atomic layer.