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 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, the things it obscures are visible only using radio waves in radio astronomy or infrared in infrared astronomy .
6-576: The Dark Horse Nebula or Great Dark Horse (sometimes called the Prancing Horse ) is a large dark nebula that, from Earth 's perspective, obscures part of the upper central bulge of the Milky Way . The Dark Horse lies in the equatorial constellation Ophiuchus (the Serpent Bearer), near its borders with the more famous constellations Scorpius and Sagittarius . It is a large, visible feature of
12-603: Is also known as "Great" because it is one of the largest (in apparent size ) groups of dark nebulae in the sky. The rear of the Great Dark Horse (its rump and hind legs), is also known as the Pipe Nebula , which itself carries the designation B77 , B78 , and B59 . (The 'B' numbers reference entries in the Barnard Catalogue of dark nebulae.) The Snake Nebula (B72) is by comparison a small S-shaped nebula emerging from
18-541: The O isotope ), CS, NH 3 ( ammonia ), H 2 CO ( formaldehyde ), c-C 3 H 2 ( cyclopropenylidene ) and a molecular ion N 2 H ( 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 closest and largest dark nebulae are visible to
24-461: The Milky Way's Great Rift , uniting several individually catalogued dark nebulae, including the Pipe Nebula . It is visible from Earth only on clear moonless nights without light pollution and with low humidity . This region of dark nebulae is called Dark Horse because it resembles the side silhouette of a horse and appears dark as compared with the background glow of stars and star clouds . It
30-569: The naked eye, since they are the least obscured by stars in between Earth and the nebula, and because they have the largest angular size , 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 molecular regions of dark nebulae, important events take place, such as
36-568: The west side of the northern part of the bowl of the Pipe (B77). Barnard 68 is another named dark patch of molecular gas and dust appearing in the Dark Horse Nebula. Dark nebula 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, C O (CO with oxygen as
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