Rho Ophiuchi Cloud

Molecular cloud, star-forming region

Ophiuchus, Scorpius

RA: 16 28 06
DEC: -24 32 30

Diam: 4.5º x 6.5º

I usually aim for an object that culminates around midnight or 1 AM but not this time. The season for observing this region is coming to an end for northern observers. For southern observers, however, it is still well placed in the evening sky and it culminates about an hour after astronomical twilight ends.

The region contains two dense clouds of gas and dust, so actually the name should be Rho Ophiuchi Clouds, not Cloud. Anyway, deep images of the region show that the bright nebulae form part of one, large cloud. The various parts of it, however, have separate designations.

The bright, mostly reflection, nebulae are IC 4604, IC 4605, IC 4606, Sh2-9 and Ced 130 with RCW 129 probably also being part of the cloud. The dark nebulae are LDN 1688, (which Simbad identifies as the Rho Ophiuchi Cloud) with the two filaments LDN 1709 and LDN 1755, and LDN 1689, with the filament LDN 1712-1729.

The LDN dark nebulae in the filaments have the Barnard designations B 44 and B 45, otherwise known as the Dark River Clouds or Rho Ophiuchi Streamer. The dark cloud in (front of) IC 4604 is B 42.

Here is an annotated map that shows the non-LDN designations. As you can see, there are many star clusters (both open and globular) and other bright and dark nebulae in this area, several of which are worthy of being OOTW themselves so I won't mention those here. Many of the open clusters are not labeled in the image.

Oph-Region-Annotated.jpeg
Source: https://tomjmartinez.blogspot.com/20...tares-and.html

And here one with LDN designations.

IMG_0034.jpeg
Source: https://sternenhimmel-fotografieren....en.de_1600.jpg

Between the two you should be able to figure out which part of the region has which designation(s).

At a distance of about 140 pc or 460 ly, the cloud is one of the closest star forming regions to earth. As with all dark clouds, the temperatures are low, in this case in the 13 to 22 K range. The total mass of the clouds is about 3000 solar masses of which over half is in LDN 1688. This is also the most active star forming region of the two. In this cloud 425 infrared sources have been identified which are presumed to be young stellar objects (protostars and T Tauri stars with estimated ages of 105 to 106 years). Also, the first brown dwarf ever to be discovered in a star forming region was discovered in this cloud.

IMG_0035.jpg
JWST image, source: https://webbtelescope.org/contents/m...4Q6GFBKXAZ3S03

The large bright and dark nebulae are best observed at low magnifications because they all are of low surface brightness. Dark, transparent skies are not required to see them but will bring out more of the subtle glows in the region. A year ago in the Chilean winter I stayed over at Cerro Pachon near Rubin Observatory and Gemini South and observed the region with 10x70 binoculars and a 94 mm refractor at 17x as well. I found the view with the binoculars more pleasing, probably because I could use both eyes. Or maybe because I was using a 31 mm Nagler which yields a 5.6 mm exit pupil while I could have used a 35 mm Panoptic which yields a 6.4 mm exit pupil.

Here are my observations with the 10x70 binoculars with the nebulae near zenith. There were high clouds in the sky and the highest SQM value I measured was 21.5 which isn’t a good as I have measured there on other occasions. The center of the Milky Way at zenith and very bright zodiac light probably didn’t help.

IC 4606: An obvious, large glow that touched Antares.

IC 4605: Connected to IC 4606. A brighter glow lies in it around i Scorpii and two close by stars.

IC 4603: Also connected to IC 4606 with a brighter glow in it that is brighter than the glow in IC 4605.

IC 4604: A very large, oval glow with an extension like an "arm" to the west. Rho Ophiuchi has a whitish yellow glow and two stars close by bluish green.

B 42: A twisting dark area just west of Rho Ophiuchi. This is not the dark cloud that causes the "arm" in IC 4606.

B 44: A very long dark cloud that starts just east of IC 4605 running east north east. I can follow the dark cloud two fields of view of 4.4º each. The east end is split in two.

B 45: Very long but not as long as B 44. The nebula contains two to three parts that are separated by brighter parts of the Milky Way.


As always,

"Give it a go and let us know!
Good luck and great viewing!"