Andromeda Galaxy (Messier 31)
Coordinates, Cepheid/TRGB distance (765 kpc / 2.50 Mly), magnitude 3.44, blueshift (−300 km/s), and observing guide for Andromeda (M31).
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) | 3.44 mag | ± 0.05 mag | Wide-field integrated galaxy surface photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Extragalactic distance | 765000 pc | ± 11000 pc | HST/JWST Cepheids + Tip of the Red Giant Branch (TRGB) + detached eclipsing binaries (2.495 ± 0.036 Mly) | definitive | SH0ES Local Distance Ladder Hubble Constant |
| Apparent major axis (25 mag/arcsec² isophote) | 178 arcmin | ± 5 arcmin | Optical disk isophotal diameter (~3.0° × 1.0°) | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric radial velocity | -300 km/s | ± 4 km/s | Optical & H I 21-cm systemic Doppler shift (~−110 km/s galactocentric approach) | definitive | SIMBAD Astronomical Database (CDS) |
| Total virial halo mass | 1500000000000 M☉ | ± 400000000000 M☉ | Satellite galaxy kinematics & H I rotation curve (~1.5 × 10^12 M☉) | model-dependent | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Andromeda Galaxy (Messier 31 / NGC 224) at RA 00h 42m 44.33s, Dec +41° 16′ 07.5″ 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
The Andromeda Galaxy (Messier 31, NGC 224) is the nearest major spiral galaxy to the Milky Way and the largest stellar disk in the Local Group, situated 765 ± 11 kiloparsecs (2.50 million light-years) away in Andromeda. Recorded as a 'little cloud' by the Persian astronomer Abd al-Rahman al-Sufi around 964 CE, its extragalactic nature was proved in 1923–1924 when Edwin Hubble used the 100-inch Hooker Telescope to identify Classical Cepheid variable stars (starting with 'V1') in its outer spiral arms.
Spanning roughly 152,000 light-years across its main stellar disk and containing ~1 trillion stars (roughly twice the stellar count of the Milky Way), M31 is inclined at 77° to our line of sight. In the 1970s Vera Rubin and Kent Ford measured the Doppler rotation speeds of H II regions out to the edge of M31's disk and discovered that the rotation curve stays flat at ~250 km/s rather than falling off in Keplerian fashion—providing seminal observational evidence for dark matter halos.
Unlike virtually all distant galaxies receding with the cosmic expansion, Andromeda shows a heliocentric blueshift of −300 km/s (corresponding to a net approach speed of ~110 km/s relative to the Milky Way's center after subtracting the Sun's galactic orbit). Combined HST and Gaia DR3 proper motions show M31 and the Milky Way will undergo their first close pericenter passage and eventual merger into a giant elliptical galaxy in roughly 4.5 billion years.
Practical Visual Observing Guide
- At magnitude 3.44, M31 is the most distant object regularly visible to the unaided human eye—photons entering your eye left Andromeda 2.5 million years ago during Earth's early Pleistocene epoch.
- Star-hop from Alpheratz (α Andromedae) two stars northeast along Andromeda to Mirach (β And), then hop two stars north-northwest through Mu (μ) and Nu (ν) Andromedae directly onto M31.
- In 10×50 binoculars or a wide-field telescope at 20×–40×, M31 stretches up to 3° across (six full Moons wide!), flanked by its two bright dwarf elliptical satellite galaxies: compact M32 (mag 8.1) 24′ south of the core and elongated M110 (NGC 205, mag 8.1) 37′ northwest.
Catalog Cross-Identifications & Aliases
Common Aliases: M31 · NGC 224 · UGC 454
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 31 |
| NGC | NGC 224 |
| SIMBAD | M 31 |
| WIKIDATA | Q2469 |
Related Objects, Comparisons & Guides
Triangulum Galaxy (Messier 33 / NGC 598)
Coordinates, Cepheid/maser distance (859 kpc / 2.80 Mly), low surface brightness (23.0 mag/arcsec²), NGC 604 H II region, and observing guide for M33.
Delta Cephei (Cepheid Distance Ladder Anchor)
Coordinates, Gaia/HST distance (290 pc), 5.366-day pulsation period, Leavitt law calibration, and observing guide for Delta Cephei.
Whirlpool Galaxy (Messier 51 / NGC 5194)
Coordinates, SN II-P / TRGB distance (8.58 Mpc / 28.0 Mly), grand-design spiral arms, NGC 5195 tidal companion, and observing guide for M51.
Bode's Galaxy (Messier 81 / NGC 3031)
Coordinates, Cepheid/TRGB distance (3.63 Mpc / 11.8 Mly), magnitude 6.94, M81–M82 tidal pair, and observing guide for Bode's Galaxy in Ursa Major.
Andromeda (M31) vs Triangulum (M33) Comparison
Compare the Local Group spirals Andromeda (M31) and Triangulum (M33): mass, inclination, surface brightness, and why M33 is a classic dark-sky test.
Autumn Sky Tour: Andromeda, Perseus & Cassiopeia
Autumn observing guide from Cassiopeia and Perseus (Double Cluster, Algol, Cas A) across Andromeda and Triangulum to Local Group galaxies M31 and M33.
Binocular Deep-Sky Objects Observing Guide
How to observe open clusters, globular clusters, diffuse nebulae, and Local Group galaxies with 7x50, 10x50, and 15x70 astronomy binoculars.
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.
Proper Motion, Radial Velocity & Redshift
How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.
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.
Messier Catalog (M) Objects & Cross-Reference
Guide and cross-walk to Charles Messier's flagship deep-sky catalog (M1 to M104 in FastAstro), linking Messier numbers to NGC IDs, distances, and magnitudes.
NGC & Caldwell Deep-Sky Cross-Walk Catalog
Cross-reference guide to Dreyer's New General Catalogue (NGC) showpieces, including southern gems Omega Centauri, 47 Tucanae, Carina Nebula, and the Veil.
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)
- 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