TheZone of Avoidance (ZOA,ZoA), orZone of Galactic Obscuration (ZGO),[1][2] is the area of the sky that is obscured by theMilky Way.[3]
The Zone of Avoidance was originally called theZone of Few Nebulae in an 1878 paper by English astronomerRichard Proctor that referred to the distribution of "nebulae" inJohn Herschel'sGeneral Catalogue of Nebulae.[4]
When viewing space from Earth, theattenuation,interstellar dust andstars in the plane of the Milky Way (thegalactic plane) obstruct the view of around 20% of the extragalactic sky atvisible wavelengths. As a result, optical galaxy catalogues are usually incomplete close to the galactic plane.
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Many projects have attempted to bridge the gap in knowledge caused by the Zone of Avoidance. The dust and gas in the Milky Way causeextinction at optical wavelengths, and foreground stars can be confused with background galaxies. However, the effect of extinction drops at longer wavelengths, such as theinfrared, and the Milky Way is effectively transparent at radio wavelengths. Surveys in the infrared, such asIRAS and2MASS, have given a more complete picture of the extragalactic sky. Two very large nearby galaxies,Maffei 1 andMaffei 2, were discovered in the Zone of Avoidance byPaolo Maffei by their infrared emission in 1968. Even so, approximately 10% of the sky remains difficult to survey as extragalactic objects can be confused with stars in the Milky Way.
Projects to survey the Zone of Avoidance at radio wavelengths, particularly using the21 cm spin-flip emission line of neutral atomichydrogen (known in astronomical parlance asH I line), have detected many galaxies that could not be detected in the infrared. Examples of galaxies detected from their HI emission includeDwingeloo 1 andDwingeloo 2, discovered in 1994 and 1996, respectively.
Recent astronomical studies revealed a supercluster of galaxies, termed theVela Supercluster, in theGreat Attractor's theorized location.[5]
