N91 Complex

One of the prettiest objects in the Cloud

Image credit Robert Gendler

RA: 04h 58m 29s Dec: -68° 27′ 02″

Diameter: 8′ x 7′

Local OB Associations: LH 12

NGC Objects: NGC 1770

An arrowhead-shaped association of stars and nebulosity, NGC 1770 is simply superb at all magnifications, both filtered and unfiltered

LH 12 + N91 = NGC 1770 (OB Association + Emission Nebula)

RA 04 57 18 Dec -68 25 22 Mag – Size 6′ x 4′

16″ at 228x: NGC 1770 is a gorgeous mix of 20 or so resolved stars, mag 11 and fainter, scattered over a 5′ area that is rich in unresolved stars and the uneven glow of N91’s nebulosity. The cluster BSDL 305 , lying north-west of mag 11.5 HD 268798, appears as a couple of very small stars and the glow of unresolved stars, ~25″ in diameter.

Unfiltered, the nebulosity appears as a fairly bright, irregular mistiness with two beautifully bright patches. The more northern of the patches, N91B, has three faint stars in an E-W line across its centre, and the southern patch, N91A = IC 2117 (discovered by Williamina Fleming on a Harvard objective prism plate taken in 1901 at the Arequipa station), is a strikingly bright and silky little knot of nebulosity. The complex’s nebulosity has a tremendous response to the UHC filter, it is beautifully irregular and uneven with edges that fade away in all directions. N91A dominates it as a bright intense knot with sharp edges. And N91B encases the trio of stars in a bright and irregular glow that has distinct edges and is brighter on its eastern edge.

SL 134 (Open Cluster)

RA 04 57 29.6 Dec -68 21 49 Mag 12.4 Size 0.9′ x 0.8′

16″ at 228x: The size of OB association LH 12 is surprising. It incorporates SL 134 which lies ~ 3.8′ NNE of the centre of NGC 1770. A jagged 3′ row of faint stars lead from mag 11.2 HD 268804 to the small cluster. SL 134 appears as a pretty bright, round cluster, 30″ in diameter, with a small star lying to its SSE, and a second, very small, star lying to its NNW. Adding the UHC filter and using averted vision, the cluster appears to be lying to the west of an exceedingly faint and very small strip of nebulosity; so faint that it is almost not there.

Brey 10 + Wolf-Rayet Bubble

RA 05 29 33.2 Dec -70 59 35 Mag – Size 3.1′ x 1.7′

16″ at 228x + UHC filter: The bubble appears as the most delicate semi-circular nebula, open to the NE, faint and subtle, and resembling a smoke ring that has been caught in a slight waft of air and fractured as it dissipates. With averted vision and a lot of time, it appears subtly uneven and has disorderly edges; its western side is thicker and quite hazy – but very faintly so – while its southern side appears as a thin, delicate, slightly brighter streak. Without the filter, Wolf-Rayet star Brey 10 is a lovely mag 12.3 spark lying almost dead centre in the bubble’s open area. What a wonderful thing to see!