Silver Sliver Galaxy (NGC 891 / Caldwell 23)
J2000 coordinates, TRGB distance (31.8 Mly), magnitude 9.93, 89-degree edge-on Milky Way twin, extraplanar dust, and observing guide for NGC 891.
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) | 9.93 mag | ± 0.09 mag | Integrated CCD aperture photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Distance in light-years | 31800000 ly | ± 1000000 ly | HST GHOSTS Tip of the Red Giant Branch (TRGB: 9.77 ± 0.31 Mpc) | definitive | SH0ES Local Distance Ladder Hubble Constant |
| Apparent major axis | 13.5 arcmin | ± 0.4 arcmin | Optical 25 mag/arcsec² isophotal dimensions (13.5′ × 2.5′) | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric recession velocity | 528 km/s | ± 2 km/s | H I 21-cm synthesis rotation curve (v_max = 225 km/s) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Silver Sliver Galaxy (NGC 891 Edge-On) at RA 02h 22m 33.41s, Dec +42° 20′ 56.9″ within Andromeda, 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
Discovered by Caroline Herschel in August 1783 and independently cataloged by William Herschel on October 6, 1784, NGC 891 (Caldwell 23, the Silver Sliver or 'Outer Limits' Galaxy) is a member of the NGC 1023 Group located 31.8 million light-years away in eastern Andromeda. Inclined 89.8 degrees to our line of sight—within two-tenths of a degree of being perfectly edge-on—NGC 891 spans ~120,000 light-years across and rotates at 225 km/s, making its luminosity, size, and morphological type nearly identical to what our own Milky Way would look like viewed from the outside.
Because NGC 891 is viewed almost exactly at 90° inclination, its dark equatorial dust lane splits the luminous stellar disk and boxy central bulge into two nearly symmetrical halves. High-resolution Hubble Space Telescope images of that dust lane reveal hundreds of vertical 'dust chimneys' and dark filaments rising up to 6,500 light-years (2 kiloparsecs) perpendicular to the galactic plane—blown upward by superbubbles from correlated supernova explosions in the disk.
In August 1986 NGC 891 produced the famous radio- and X-ray-luminous Type IIn supernova SN 1986J, whose expanding remnant still interacts fiercely with a dense circumstellar shell and contains a compact central neutron star or black hole resolved by Very Long Baseline Interferometry.
Practical Visual Observing Guide
- Located 3.5° east of the colorful double star Almach (γ Andromedae), a third of the way toward the open cluster M34 in Perseus.
- Because NGC 891's edge-on dust lane blocks much of its own starlight, its visual surface brightness is noticeably lower than NGC 4565's—dark skies (Bortle 1–4) are essential.
- In a 200–300 mm (8–12 inch) telescope at 100×–160×, NGC 891 floats amid a rich Milky Way starfield as a ghostly 13′-long needle (oriented NNE–SSW at PA 22°) split down the middle by its dark central rift.
Catalog Cross-Identifications & Aliases
Common Aliases: NGC 891 · Silver Sliver Galaxy · Outer Limits Galaxy · Caldwell 23 · UGC 1831
| Catalog Authority | Identifier |
|---|---|
| NGC | NGC 891 |
| SIMBAD | NGC 891 |
| WIKIDATA | Q212098 |
Related Objects, Comparisons & Guides
Needle Galaxy (NGC 4565 Edge-On Spiral)
J2000 coordinates, TRGB distance (38.8 Mly), magnitude 9.56, 87-degree edge-on inclination, razor-sharp dust lane, and observing guide for NGC 4565.
Andromeda Galaxy (Messier 31 / NGC 224)
Coordinates, Cepheid/TRGB distance (765 kpc / 2.50 Mly), magnitude 3.44, blueshift (−300 km/s), and observing guide for Andromeda (M31).
Sombrero Galaxy (Messier 104 / NGC 4594)
Coordinates, TRGB distance (9.55 Mpc / 31.1 Mly), edge-on dust lane, 1-billion-M☉ central black hole, and observing guide for M104.
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
- 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
- SH0ES Local Distance Ladder Hubble Constant (2022 Baseline Cepheid + SN Ia): Riess, A. G., et al. 2022, ApJL, 934, L7 (A Comprehensive Measurement of the Local Value of the Hubble Constant) — SH0ES Collaboration (HST / JWST)
- 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