A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy.
Dark clouds appear so because of sub-micrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C18O (CO with oxygen as the 18O isotope), CS, NH3 (ammonia), H2CO (formaldehyde), c-C3H2 (cyclopropenylidene) and a molecular ion N2H+ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation.
The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift. These naked-eye objects are sometimes known as dark cloud constellations and take on a variety of names.
In the inner outer molecular regions of dark nebulae, important events take place, such as the formation of stars and masers.
The Coalsack Nebula (Southern Coalsack, or simply the Coalsack) is the most prominent dark nebula in the skies, with a designation TGU H1867, first referred to in Cataloging 1850, being easily visible to the naked eye as a dark patch obscuring a brief section of Milky Way stars as they cross their southernmost region of the sky, east of Acrux (Alpha Crucis) which is the bright, southern pointer star of the southern cross. It dominates and overspills the southeast corner of what is considered the extent of the constellation Crux at a little less than twice the distance of Acrux, 180 parsecs (590 ly) away from Earth.
The Coalsack Nebula covers nearly 7° by 5° and overlaps somewhat with the neighbor constellations Centaurus and Musca.
The first observation was reported by Vicente Yáñez Pinzón in 1499. It was named “il Canopo fosco” (the dark Canopus) by Amerigo Vespucci and was also called “Macula Magellani” (Magellan’s Spot) or “Black Magellanic Cloud” in opposition to the Magellanic Clouds.
The Coalsack is omitted in most of today’s standard catalogs on the Milky Way such as the New General Catalogue and its only mainstream identification number is in the somewhat specialist Caldwell catalogue, in which it is C99.
A depiction of the emu in the sky, which is an Australian Aboriginal constellation consisting of dark clouds rather than of stars. The European constellation on the right is Crux, or the Southern Cross, and on the left is Scorpius. The head of the emu is the Coalsack.
The Coalsack in Australian Aboriginal astronomy forms the head of the emu in the sky in several Aboriginal cultures. Amongst the Wardaman people, it is said to be the head and shoulders of a law-man watching the people to ensure they do not break traditional law. According to a legend reported by W. E. Harney, this being is called Utdjungon and only adherence to the tribal law by surviving tribe members could prevent him from destroying the world with a fiery star. There is also a reference by Gaiarbau (1880) regarding the coalsacks replicating bora rings on Earth. These astronomical sites allowed the spirits to continue ceremony similar to their human counterparts on Earth. As bora grounds are generally located on the compass points north–south, the southern coal sack indicates the initiation/ceremonial ring.
In Inca astronomy this nebula was called Yutu, meaning a partridge-like southern bir or Tinamou.