Wedged between the star Acrux and the Jewel Box cluster in the Southern Cross, and extending east into Centaurus and south into Musca, lies the Coalsack, the most prominent and easily observed of the so-called dark nebulae that permeate the the star clouds of the Milky Way like black smoke.
Like all dark nebulae, the Coalsack is a region of cold gas and dust that obscures background starlight. It is more remarkable than most because it stands out against the bright background of the Milky Way, and covers a patch of sky some 7×5 degrees… so large it barely fits into the field of view of most binoculars.
The dark nebula called the “Coalsack” in the constellation Crux ((c) 2002 – Vic & Jen Winter)
The dark expanse of the Coalsack impressed preshistoric and renaissance stargazers. Aboriginal stargazers have known about the Coalsack for at least 40,000 years. Bark drawings found on Australia’s Groote Eylandt depicts the Coalsack as a fish speared by two brothers who are represented by the two brightest stars of Crux. Polynesians called the Coalsack “Te Paniwi a Taewa”, the Black Fish. Vespucci called it the “Black Canopus”.
While it looks like a patch of unoccupied space, the Coalsack is the quite the opposite. It’s made of thousands of solar masses of gas molecules and cold dust grains which scatter starlight out of our field of view, much like dust in our atmosphere dims the setting Sun (see inset). This nebula spans about 50 light years at its widest. But it’s not a single, simple blob. By studying light extinction of dim backgrounds stars, astronomers conclude the Coalsack is two overlapping dust clouds 610 and 790 light years away. And it’s not completely dark. Ground and space-based observation show that dust grains of the nebula reflect a modest amount of blue and ultraviolet light from nearby stars.
In good sky, the Coalsack presents a grand sight for stargazers without optics. But it’s worth exploring with binoculars, and even with a telescope at low power. Look for structure among the blackness. Keen-eyed observers see a rib-like structure in the Coalsack, especially in the area nearest Acrux.
Look also for the small, dim star cluster NGC 4609 about 2 degrees east-northeast of Acrux. It lies eight times farther away than the Coalsack, yet its young stars pack enough punch to remain visible through this interstellar cloud.