Iris Nebula (NGC 7023 / Caldwell 4)
Coordinates, Gaia DR3 distance (334 pc), Herbig Be star HD 200775, extended red luminescence (ERE), and observing guide for NGC 7023 in Cepheus.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Integrated visual magnitude (V) | 6.8 mag | ± 0.2 mag | Integrated reflection nebula + V=7.39 HD 200775 photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric distance | 1090 ly | ± 20 ly | Gaia DR3 astrometric parallax of HD 200775 & Cepheus Flare YSOs (334 ± 6 pc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent angular dimensions | 18 arcmin | ± 2 arcmin | Optical reflection petals (18′ × 18′) | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Iris Nebula (NGC 7023 Reflection Nebula) at RA 21h 01m 36.90s, Dec +68° 09′ 48.0″ within Cepheus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
The Iris Nebula (NGC 7023, Caldwell 4) is one of the brightest and most celebrated blue reflection nebulae in the northern circumpolar sky, located 1,090 light-years (334 ± 6 parsecs) away in central Cepheus. Discovered by William Herschel on October 18, 1794—who described its central star as 'surrounded veryahananly by a nebulous atmosphere'—the nebula strictly shares the designation NGC 7023 with the small embedded open cluster (Collinder 429) of pre-main-sequence T Tauri stars.
Illuminating the 6-light-year-wide flower petals of the Iris is HD 200775 (V380 Cephei), a 7.39-magnitude Herbig Ae/Be pre-main-sequence spectroscopic binary consisting of two massive B2.5Ve stars (totaling ~19 solar masses) in a 3.68-year eccentric orbit. During its infancy less than 100,000 years ago, a powerful bipolar molecular outflow from HD 200775 carved an hourglass-shaped biconical cavity inside the dark cloud Lynds 1174, whose inner dust walls now scatter sapphire-blue continuum starlight.
In infrared and deep optical spectroscopy, the inner filaments of NGC 7023 are a premier laboratory for 'Extended Red Emission' (ERE) and polycyclic aromatic hydrocarbon (PAH) photoluminescence: ultraviolet photons from HD 200775 excite nanoscale carbon dust rings that fluoresce with a rusty rose-red glow along the edges of the blue cavity.
Practical Visual Observing Guide
- Located 3.5° southwest of Alfirk (β Cephei) and 5.5° northwest of Alderamin (α Cephei).
- Because NGC 7023 is a pure continuum reflection nebula rather than an emission nebula, narrowband O-III or UHC filters dim it rather than help; observe unfiltered under dark, transparent Bortle 1–3 skies with clean optics.
- In a 100–250 mm telescope at 50×–100×, the 7.4-magnitude star HD 200775 is enveloped by a bright 5-arcminute rectangular core surrounded by dark dust bays (LDN 1172/1174) that block out background Milky Way stars.
Catalog Cross-Identifications & Aliases
Common Aliases: NGC 7023 · Iris Nebula · Caldwell 4 · LBN 487
| Catalog Authority | Identifier |
|---|---|
| NGC | NGC 7023 |
| SIMBAD | NGC 7023 |
| WIKIDATA | Q488506 |
Related Objects, Comparisons & Guides
Messier 78 (NGC 2068 Reflection Nebula)
Coordinates, Gaia DR3 distance (410 pc), magnitude 8.3, McNeil's Nebula variable outburst, and observing guide for M78 in Orion.
Witch Head Nebula (IC 2118 near Rigel)
Coordinates, Rigel illumination distance (264 pc), Rayleigh dust scattering physics, and wide-field observing guide for IC 2118 in Eridanus.
Pleiades (Messier 45 Open Cluster)
Coordinates, Gaia DR3 distance (136.2 pc), magnitude 1.6, reflection nebulosity, and observing guide for the Pleiades (M45) in Taurus.
Surface Brightness & Extended Objects Guide
Why surface brightness (mag/arcsec²) governs nebula and galaxy visibility more than integrated magnitude, and how exit pupil controls eyepiece contrast.
Bortle Dark-Sky Scale & Sky Brightness Guide
Reference guide to the 9-class Bortle Dark-Sky Scale, zenith Sky Quality Meter (SQM) readings in mag/arcsec², naked-eye limiting magnitude, and light pollution.
Scientific Sources & Dataset Provenance
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg