Great Rift (astronomy)

This article is about the band of dark areas in the night-sky running along the Milky Way. Not to be confused with the particular dark nebula called Dark River of theRho Ophiuchi cloud complex.

Thisoverexposedphoto shows just over half of the inward bulk of theMilky Way(including its centre). As in all real images fromour Armof the galaxy much is obscured by the Great Rift, darkdust cloudsthat span fromCygnustoCentaurus.

Inastronomy,theGreat Rift(sometimes called theDark Riftor less commonly theDark River) is adark bandcaused byinterstellar cloudsofcosmic dustthat significantly obscure (extinguish) thecenterand most radial sectors of theMilky WaygalaxyfromEarth's perspective.

In dark, clearnight skies,the rift appears as clear as the brightbulgeof stars around the Galactic Center does to thenaked eyeorbinoculars.The rift is largely between theSolar System(which is close to the inner edge of theOrion Arm) and the next arm, inward, theSagittarius Arm.The clouds are an obstruction to millions of the galaxy's stars detected atvisible wavelengths,which compose a bright hazy band appearing30° wideand arching through the night sky. The clouds within our radial sector of the galaxy span about 800–1,000parsecs(2,600–3,300ly) from Earth.The clouds are estimated to contain about 1 millionsolar massesofplasmaand dust.

Properties

View of the Milky Way and Great Rift fromESO'sVery Large Telescope

To thenaked eye,the Great Rift appears as a dark lane that divides the bright band of theMilky Wayvertically. The Great Rift covers one third of the Milky Way, and is flanked by strips of numerous stars, such as theCygnus Star Cloud.West of theCepheus Clouds,theFunnel cloud/Le Gentil 3and the borderingNorth America Nebula,the Great Rift starts with theNorthern Coalsackat the constellation ofCygnus,where it is known as theCygnus Rift.The Great Rift stretches from there over theSerpens-Aquila Rift;toOphiuchus,where it broadens out; toSagittarius,where it obscures theGalactic Center;essentially ending atCentaurus.

Overview of the Milky Way as seen by Gaia, with prominent dark features andstar cloudslabelled. The Great Rift extends from the far left across the galactic field of stars.

One of the regions obscured by the Great Rift is theCygnus OB2 association,aclusterof young stars and one of the largest regions of star formation within 2 kiloparsecs. Similar dark rifts can be seen in many edge-on galaxies, such asNGC 891inAndromedaandNGC 4565(the Needle Galaxy) inComa Berenices.

Human observation

Layout of some Great Rift "constellations" as represented by the Inca

Dark zones obscuring the night-sky lighting mass of the bulk of the Milky Way in a dry atmosphere (or at long exposures) were recognized by many ancient civilizations in which a seasonally or regularly dry climate is a frequent feature. In South America, theIncagave some patterns of darkness and stars names much as normal stellar constellations were, including a series of animals likellamas,afox,toad,and others, thought to be drinking from the "great river" (the Milky Way) and seen in silhouette.

Theclassical Greekssometimes described the Great Rift as being the path of devastation left byPhaeton,who tried to guide the chariot ofHelios(the Sun god) across the sky before losing control, wreaking havoc, and finally being struck down by a lightning bolt of Zeus.

Modern astronomy first began to notice the rift in the 18th century, but struggled to explain it untilE. E. BarnardandMax Wolfin the early 20th century, who produced the currently accepted explanation after careful photographic study.

Main dark nebulae of theSolar apexhalf of thegalactic plane,with theCygnusandAquila Riftsof the Great Rift at the center right. On the left are the dark clouds of theRadcliffe Wave.

Of this, Barnard said:

I did not at first believe in these dark obscuring masses. The proof was not conclusive. The increase of evidence, however, from my own photographs convinced me later, especially after investigating some of them visually, that many of these markings were not simply due to an actual want of stars but were really obscuring bodies nearer to us than the distant stars. —Astrophysical Journal(1919)

See also

• Coalsack Nebula
• Dark-cloud constellations
• Emu in the sky(a continuation from Scorpius to Crux)
• Large Sagittarius Star Cloud
• Serpens-Aquila Rift,a sub-feature of the Great Rift
• Small Sagittarius Star Cloud

Further reading

• "The Great Rift".Encyclopædia Britannica Online.2009.Retrieved2009-04-25.
• "Great Rift".Sci-Tech Dictionary.Retrieved2009-04-25.
• Kaler, Jim."The Milky Way – From STARS".University of Illinois at Urbana-Champaign.Department of Astronomy. pp. Maps 2 and 5.Retrieved2009-04-25.
• Dark River, Wide Fieldby Rogelio Bernal Andreo, atAPOD