The Coalsack Nebula is one of the best known dark nebulae in the sky, along with the nearby Dark Doodad Nebula and the Horsehead Nebula.
The Coalsack is located in the constellation Crux and overlaps into the neighbouring constellations Musca and Centaurus. The dark cloud of cold gas and dust lies at a distance of 600 light years from Earth and is about 30 to 35 light years across.
The Coalsack Nebula occupies an area of 7 by 5 degrees and is the most conspicuous object of its kind in the sky, barely fitting into a single binocular field of view. The nebula’s darkness is the result of dust particles blocking the visible light of stars in the background. The little background starlight that can be seen through the thick dust clouds appears reddish because dust absorbs and scatters blue light more easily than red.
The Coalsack is easily visible to southern observers, appearing as a dark patch near the Southern Cross asterism and silhouetted against the star fields of the Milky Way. The nebula lies roughly between the bright star Acrux and the Jewel Box Cluster.
The Coalsack Nebula was known in the southern hemisphere since prehistoric times. It was first seen by a European in 1499, when Spanish navigator and conquistador Vicente Yáñez Pinzón reported observing it during his voyage to South America.
Italian explorer Amerigo Vespucci called the nebula “il Canopo fosco,” or “the dark Canopus.“ The Coalsack was also known as the Black Magellanic Cloud or “Macula Magellani” (Magellan’s Spot), in reference to the Magellanic Clouds, bright dwarf galaxies visible in the southern sky.
Italian-born historian Peter Martyr d’Anghiera was the first to provide a formal description of the nebula between 1511 and 1521. Another description came from the French astronomer Nicolas Louis de Lacaille, who mapped the southern skies from the Cape of Good Hope and included a note about the Coalsack Nebula in an appendix to his 1755 catalogue.
Consisting of thick clouds of gas and dust – material used to form new stars – the Coalsack Nebula will not stay dark forever. Millions of years from now, parts of it will be illuminated by the light of young stars.
In 1970, Finnish astronomer Kalevi Mattila proved that the nebula is not completely black, but has 10% of the brightness of the surrounding Milky Way. The dim light comes from the reflection of stars the nebula obscures.
Observations of light extinction of the background stars have revealed that the Coalsack consists of two large overlapping clouds of dust located 610 and 790 light years from Earth.
In Australian Aboriginal astronomy, the Coalsack Nebula forms the head of the Emu in the Sky, an indigenous Australian constellation. The Emu’s neck can be seen between the Southern Pointers (Alpha and Beta Centauri) and its body stretches across Scorpius. Some indigenous groups also see the nebula as the top of a large pine tree that can be climbed into the sky, or as a deep hole in the celestial river represented by the Milky Way.
The Incas associated the Coalsack with the story of the god Ataguchu, who kicked the Milky Way in a fit of anger, causing a fragment to fly off and form the Small Magellanic Cloud. In the story, the Coalsack is the black mark left behind where the broken fragment was.
The Coalsack is identified as C99 in the Caldwell Catalogue. It is not listed in the New General Catalogue and does not have a NGC number.
|Object: Dark nebula|
|Right ascension: 12h 50m|
|Apparent magnitude: –|
|Distance: 600 light years|
|Apparent size: 7 x 5°|
|Radius: 30 – 35 light years|
|Designations: Coalsack Nebula, C99, TGU H1867|
Zooming in on the dark and dusty Coalsack Nebula – European Southern Observatory (ESO)
A close look at part of the Coalsack Nebula – European Southern Observatory (ESO)
Part of the Coalsack Nebula (fulldome) – European Southern Observatory (ESO)