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Within the plane of the Milky Way Galaxy, a stretch of dark clouds extends through much of the interior of the galactic disc. From Earth, these clouds are seen as a dark band running through the lighter Milky Way in the night sky, stretching along about a third of its length. The entire dark band is known as the Great Rift, but in places darker and denser regions form. One of these denser regions runs from Aquila through Serpens and into Ophiuchus, and is known as the Aquila Rift or, more fully, as the Serpens-Aquila Rift.

Imagery provided by Aladin sky atlas

The nature of the dark clouds of the rift means that it is difficult to establish their distances or relationships with precision. Indeed the 'rift' may not be a single entity, instead being made up of several different overlapping complexes of dark nebulae at different distances. Its inner edge appears to be some 750 light years from the Sun, and it extends inward from there towards the central regions of the Galaxy. The depth of the rift is uncertain, but objects within it appear to be more than twice as far away as its inner edge, and so the entire structure is probably at least a thousand light years from its outer to its inner edge.

The Aquila Rift contains several regions of active star formation, in which filaments of material within the rift have become dense enough to create stellar nurseries. Within these regions, nebulous material is condensing to form open star clusters, as well as swarms of protostars evolving towards true stars. The most active such region known is designated Westerhout 40, a cluster of hundreds of young stellar objects in northern Serpens Cauda. Several other similar star-forming regions have also been identified among the dark clouds of the Aquila Rift.


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