Mirror Galaxy (Messier 100 / NGC 4321)
J2000 coordinates, HST Cepheid distance (53.8 Mly), magnitude 9.35, grand-design spiral arms, five supernovae, and observing guide for Messier 100.
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) | 9.35 mag | ± 0.08 mag | Integrated CCD aperture photometry | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Distance in light-years | 53800000 ly | ± 1900000 ly | HST Key Project Classical Cepheids + TRGB (16.5 ± 0.6 Mpc) | definitive | SH0ES Local Distance Ladder Hubble Constant |
| Apparent major axis | 7.4 arcmin | ± 0.3 arcmin | Optical isophotal dimensions (7.4′ × 6.3′) | definitive | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Heliocentric recession velocity | 1571 km/s | ± 1 km/s | H I 21-cm and CO molecular rotational spectroscopy | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Mirror Galaxy (Messier 100 / NGC 4321) at RA 12h 22m 54.90s, Dec +15° 49′ 20.3″ within Coma Berenices, 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 Pierre Méchain on March 15, 1781, along with neighboring M98 and M99, Messier 100 (NGC 4321) is the grandest face-on spiral galaxy in the entire Virgo Cluster, shining at visual magnitude 9.35 from a distance of 53.8 million light-years in southern Coma Berenices. Spanning roughly 115,000 light-years across its two sweeping major spiral arms and several subsidiary branches, M100 is physically larger than the Milky Way and contains ~100 billion stars.
M100 occupies a landmark chapter in the history of space astronomy: in December 1993, following the first Hubble Space Telescope servicing mission that installed the corrective WFPC2 camera, NASA released before-and-after images of M100's core to demonstrate Hubble's restored optical vision, and months later Wendy Freedman and the HST Key Project team used 20 Cepheids in M100 to measure the first direct Cepheid distance to the Virgo Cluster.
At the center of M100, a short bar funnels gas into a compact 3,000-light-year circumnuclear starburst ring, while the galaxy's outer northern spiral arm is connected by a faint luminous bridge of stars to the dwarf elliptical satellite NGC 4323 (located 5′ north-northeast). Six supernovae have been recorded in M100: SN 1901B, 1914A, 1959E, 1979C (whose X-ray afterglow from a wind-fed remnant or black hole remains bright decades later), 2006X, and 2019ehk.
Practical Visual Observing Guide
- Located 1.8° east-northeast of 6 Comae Berenices and 1.6° northeast of M99.
- In a 100–150 mm telescope at 60×–100×, M100 appears as a large 7′ circular glow with a bright, concentrated nuclear starburst region.
- Under dark skies (Bortle 1–3), a 250–350 mm (10–14 inch) reflector at 130×–180× traces the two broad S-shaped spiral arms and picks up the faint 13.5-mag dwarf companion galaxies NGC 4323 and NGC 4328 right alongside M100's halo.
Catalog Cross-Identifications & Aliases
Common Aliases: M100 · NGC 4321 · UGC 7450 · Mirror Galaxy · Blowdryer Galaxy
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 100 |
| NGC | NGC 4321 |
| SIMBAD | M 100 |
| WIKIDATA | Q14074 |
Related Objects, Comparisons & Guides
Coma Pinwheel Galaxy (Messier 99 / NGC 4254)
J2000 coordinates, Tully-Fisher distance (49.6 Mly), magnitude 9.86, asymmetric one-armed spiral, VIRGOHI21 gas tail, and observing guide for M99.
Messier 61 (NGC 4303 Supernova-Rich Spiral)
J2000 coordinates, distance (52.5 Mly), magnitude 9.65, nuclear starburst ring, eight recorded supernovae, and observing guide for Messier 61.
Messier 85 (NGC 4382 Merger-Remnant S0)
J2000 coordinates, SBF distance (58.7 Mly), magnitude 9.10, concentric stellar shells, NGC 4394 companion pair, and observing guide for Messier 85.
Measurement Uncertainty & Optics in Astronomy
How random vs systematic errors, asymmetric error bars, model dependencies, and optical aperture/exit-pupil limits shape astronomical measurements.
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.
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