A conspicuous globule in the Southern Coalsack

Abstract

THE Southern Coalsack is the finest dark nebula in the Southern Milky Way. At the Royal Observatory, Edinburgh, in October 1976, we studied two photographs taken with the SRC 1.2-m Schmidt telescope (a Unit of the Royal Observatory, Edinburgh) at Siding Spring Observatory, Australia. Centred at α (1975) = 12 h 45 min, δ (1975) = −63°01′, they show the whole of the Coalsack in all its beauty. The blue plate (Fig. 1) was a 60-min exposure on a hydrogen-sensitised IIIaJ plate, taken through a 2-mm Schott GG395 filter; the near-infrared plate was a 60-min exposure on a water-hypersensitised IN plate, taken through a 4-mm RG715 filter. These plates were taken as part of a more comprehensive study of the Southern Coalsack; this note represents a pilot study of how much can be learned by simple star-counting techniques. Visual inspection of these plates confirms the earlier conclusions^1–3 that the Southern Coalsack is basically a thin obscuring sheet with an average total visual absorption A_v = 1.2 mags. Rodgers¹ derived the distance from the Sun to the Coalsack to be 175 pc.