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Rho Ophiuchi Cloud Complex

Rho Ophiuchi DN Complex,
Ophiuchus Molecular Cloud, IREC 500

Proper NamesRho Ophiuchi Cloud Complex, Rho Ophiuchi DN Complex, Ophiuchus Molecular Cloud
Messier NumberNone
NGC/IC NumberNone
Other DesignationsIREC 500
ConstellationsOphiuchus, Scorpius
Right Ascension16h 28m 6s (centre)
Declination-24° 32' 30" (centre)
Distance453 light years
139 parsecs
DimensionsApparent: 6.5° x 4.5°
Actual: approximately 50 light years
Optimum VisibilityJune

The colour variationss within the Rho Ophiuchi Cloud Complex are evident in this image. The blue-grey colours in the northern part derive the blue star Rho Ophiuchi itself, while the orange hues to the south are produced by the red light of Antares. In the southwest (bottom right) of the image, a globular cluster is visible: this cluster is Messier 4, which lies thousands of light years beyond the nebulae that make up the cloud complex. Imagery provided by Aladin sky atlas

A wide complex of nebulae that stretches across the sky between between southern Ophiuchus and northern Scorpius. The clouds of the complex are too faint to be seen by the naked eye, but they cover an area of some 6.5° by 4.5° (that is, the disc of the Moon would fit thirteen times along the long axis of this region, which corresponds to an actual diameter of some fifty light years). This is one of the closest star-forming regions to the Solar System, estimated to lie some 450 light years away. The complex forms part of an even larger region of nebulosity running along the inner edge of the Milky Way's Orion Arm (the same minor spiral arm that holds the Solar System).

The cloud complex is subdivided into various nebulous structures, and many of these have their own distinct colour due to the individual stars that illuninate them. The northern lobe, IC 4604, surrounds the blue star Rho Ophiuchi (from which the entire complex takes its name), and so the northern part of the cloud structure has a deep blue colour. The southern lobe extends towards red Antares in Scorpius, and thus the southern section of the complex (Cederblad 132, commonly called the Antares Nebula or the Cloud Nebula) has a rich orange colour. To the west of these main lobes, a lesser patch of nebulosity (Sh2-9) is illuminated by the star Alniyat or Sigma Scorpii, and the hydrogen in this region glows a deep crimson.

Running between the two main nebulae that make up the complex are filaments of material, and the entire area is crossed and patterned by dense dark nebulae (hence the alternative name of the 'Rho Ophiuchi DN Complex', where 'DN" stands for 'Dark Nebula'). Two of these absorption nebulae are particularly striking against the coloured backdrop of the complex, Barnard 44 and Barnard 45, which run across the area where the blue and orange nebulae meet. Dark strands are found throughout this region, obscuring the stars of the Milky Way behind them, and these strands are given the name of dark streamers.

Within the material that makes up the cloud complex, new stars are being formed (the oldest of these new stars appears to be approximately one million years old). These faint young stars stand out strongly in infrared light, forming clusters of glowing points within the gas and dust of the surrounding nebulae. Many of these objects represent protostars in the earliest periods of formation, while others are young T Tauri stars surrounded by the dense circumstellar discs typical of stars early in their evolution.


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