Object of the Week September 18, 2016 - NGC 7023 the Iris Nebula

[Uwe Glahn]

NGC 7023 “Iris Nebula”,

Alias: vdB 139 (LBN 487), Collinder 429, Caldwell 4
Near: GN 21.02.4.02

Reflection Nebula

Constellation: Cepheus
RA: 21 01 37
DEC: +68 09 48
(SIMBAD ~ position of star HD 200775 = V380 Cep)

This week we visit a nebula which is visible during a long time of the year but culminates in summer.
NGC 7023 – the so called “Iris Nebula” lies in the western part of Cepheus and marked one of the brightest nebulae in this region.

Not unusual, that a famous object has a funny or better confusing discovery history. Found by Wilhelm Herschel in 1794 with his 18.7-inch speculum mirror telescope he describes correctly a large nebula around a 7th magnitude star and some 9th and 10th magnitude stars around. The entry refers to an open cluster. In the 19th century Collinder decided to name the open cluster as Cr 429 and van den Bergh named the nebula as vdB 139. Last but not least the whole object was named as Caldwell 4. Today there are still the parallel object designations and we understand under “NGC 7023” the nebula and the cluster – exactly as Herschel described.

Looking at colored deep sky images we see the bright blue shining around the 7th magnitude star, a lot of dust in a large area of nearly 1° area, but also some reddish structures and the smaller offside reflection nebula GN 21.02.4.02 (8’ east of HD 200775). HD 200775 itself is a very luminous star which illuminates the field around. The red color is created as a combination of reflection energy and the emission of special molecules. The Spitzer Space Telescope discovers so called PAH’s (Polycyclic Aromatic Hydrocarbons), molecules which are the cause of the red color.

Already in the smallest telescopes or even mid size binoculars the nebula is visible as a showy brightening around the bright “central star”. Because of the reflection character no nebula filters are required. First detail could be a brighter “bar” to the south. Especially small telescopes with larger fields show the ghostly area around with dark structures and fainter nebulae. Larger telescopes show tremendous details which are difficult to describe. Beside the details of NGC 7023 itself, the small GN 21.02.4.02 could be a challenge for larger apertures.

Stefan Westphal
2250


sketch: 20x125 binocular, 3° field, NELM 6m0+
2251


sketch: 27", 172x-244x, Seeing III, NELM 6m5+
2252

And as always - "Give it a go and let us now!"

[Ivan Maly]

I looked at this one again a week or two ago. The notch from the WSW was clearly seen (20", 200x). The sky wasn't very good (SQM 21.0). Next time, the outer segments.

[Steve Gottlieb]

Great target – perhaps the most detailed reflection nebula in the sky! Here's another observation with my 18-inch.

18" (7/24/06): viewed at 160x, this detailed reflection nebula displayed a great deal of interesting structure! Surrounding the mag 7.4 illuminating star is a bright halo of nebulosity extending mostly north of the star and ending just south of the star in a well-defined slightly curving border. A wide dark lane intrudes into the nebulosity from the southwest towards the bright star. To the south of the star is a triangular region of haze (brightest just south of the central star) roughly filling in the region defined by a mag 13.5 star 5.5' SSE and a fainter star a similar distance southwest. A larger section of faint haze extending N-S is seemingly detached at the periphery on the east side of the nebula (this is an outer "wing" on photographs) and a more vaguely defined region of low surface brightness haze is detached on the western side (also oriented N-S). These two detached wings give a diameter of at least 7'.

[Ivan Maly]

Here is a new observation. Brightest central glow fanning N of the central star, S of it a "bar" pointing at the stem of a "Y". The brighter E branch of the "Y" passes by the other bright star and points at a separated arc that suggests a loop on the E side. The other branch does connect back to the central glow via a deformed faint loop. A small faint loop (or simply an area with a dark hole in it) is adjoined to it on the NW side of the central glow. 12", 230x, SQM 21.7.