NGC 7023, Iris Nebula

History

NGC 7023 is a small, poor open star cluster enveloped by a faint dusty nebula (LBN 487). It was discovered on October 18, 1794 by the German-British astronomer Wilhelm Herschel. Dreyer described NGC 7023 as a 7th magnitude star embedded in a very faint but large nebula. The nickname «Iris Nebula» refers to the blue flowers of the iris (Iris versicolor) and not the iris of the eye, because of their color and shape.

NGC 7023
NGC 7023: Iris Nebula in Cepheus; Celestron RASA 11" f/2.22; ZWO ASI6200 Pro; Tentlingen; © 2020 Peter Kocher
NGC 7023
NGC 7023: Iris Nebula in Cepheus; Meade Schmidt-Newton 250 mm f/4, Canon EOS 20Da; ISO 1600, RGB 76.5 min; Observatory Oberes Schlierental, Obwalden; © 2018 Eduard von Bergen
NGC 7023
NGC 7023: Iris Nebula in Cepheus; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 120+45+45+45 min LRGB; Bernese Highlands; © 2011 Radek Chromik

Physical Properties

The star HD 200775 is a hot, blue Herbig Be star with an apparent magnitude of about 7.4. It emits most of its radiation in the UV range. The mass is estimated at around 10 to 15 times that of the Sun and the age at just 100'000 years.

Outside of the reflection nebula, which is illuminated blue by the star, there are other nebulae, some faintly glowing, some made of dark interstellar dust. They are particularly noticeable because the apparent star density is lower there, as can be seen in Fig. 1. The nebula is an active star-forming region. Measurements by the Gaia probe gave a distance of 335 ± 11 pc (1093 ± 36 light years). The open star cluster is about 10 arc minutes in diameter, which is about three light-years at this distance. The reflection nebula is about twice as large. [109, 196, 260]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 7023
TypeEN
Right Ascension21h 01m 35.5s
Declination+68° 10' 10"
Diameter10.00 × 8.0 arcmin
Photographic (blue) magnitude7.1 mag
Dreyer Description* 7 in eF, eL neby
IdentificationLBN 487, OCL 235

Finder Chart

West of Cepheus' large alignment, which looks more like a child's drawing of a house than a king, lies the dust nebula NGC 7023. To find it, it is best to use a wide-field eyepiece and align the telescope with the help of the Telrad finder according to the adjacent map. Although the nebula is circumpolar, it is highest in the night sky from May to December.

Chart NGC 7023
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

Visual Observation

NGC 7023
NGC 7023: Drawing; 150mm-Refraktor, 70x-140x; © 8. 9. 1996 Beat Kohler

150 mm aperture: Its hazy appearance around a relatively bright star immediately gives the impression of a tarnished eyepiece. In fact, one has to be careful to find this rather difficult object to observe! The best magnifications proved to be 65x to 130x, without a filter, but with the eyepiece lens cleaned of dust (and dew). The large extent of the object is best seen with averted vision. Although there are differences in brightness, no structures can be made out. — 1996, Beat Kohler

762 mm aperture: The nebula is clearly visible around the bright star as a large, diffuse glow. With well-rested eyes, the two light-colored trapezoids and each within a darker area were rudimentarily recognizable. — 14. 8. 2021, Bernd Nies, Eduard von Bergen, 30" Slipstream Dobsonian, Hasliberg

References

109A Cepheus Star-Hop by Alan M. MacRobert; S&T 9/94, p.40
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
196Celestial Atlas by Curtney Seligman; cseligman.com/text/atlas.htm (2020-12-28)
260«A census of young stellar population associated with the Herbig Be star HD 200775» Piyali Saha, Maheswar G., Kamath U. S., Lee C. W, Manoj P., Blesson Mathew, Ekta Sharma; arXiv:2005.00519
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)