Galaxies · Constellation Leo

Messier 66 (NGC 3627 / Arp 16)

J2000 coordinates, Cepheid/TRGB distance (31.3 Mly), magnitude 8.92, tidally warped arms, five supernovae, and observing guide for Messier 66 in Leo.

Right Ascension (J2000.0)11h 20m 15.03s
Declination (J2000.0)+12° 59′ 29.6″
Primary ClassificationTidally asymmetric barred spiral galaxy (dominant Leo Triplet member)
Best Observing SeasonSpring (February–June)

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.

Astrometric, Photometric, and Physical Parameters for Messier 66 (NGC 3627 / Arp 16)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Integrated visual magnitude (V)8.92 mag± 0.08 magIntegrated CCD aperture photometrydefinitiveRevised New General Catalogue and Index Catalogue (RNGC/IC)
Distance in light-years31300000 ly± 1300000 lyHST Cepheids (calibrated via SN 1989B) & TRGB (9.6 ± 0.4 Mpc)definitiveSH0ES Local Distance Ladder Hubble Constant
Apparent major axis9.1 arcmin± 0.4 arcminOptical isophotal dimensions (9.1′ × 4.2′)definitiveRevised New General Catalogue and Index Catalogue (RNGC/IC)
Heliocentric recession velocity727 km/s± 3 km/sH I 21-cm and CO molecular rotation spectroscopydefinitiveSIMBAD Astronomical Database (CDS)
Measurement Uncertainties & Competing Solutions: Hubble Space Telescope discovery of 68 Classical Cepheid variables ( originally calibrated for the Type Ia supernova SN 1989B) combined with TRGB star counts fixes M66's distance at 9.6 ± 0.4 Mpc (31.3 ± 1.3 Mly).

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Messier 66 (NGC 3627 Asymmetric Spiral) at RA 11h 20m 15.03s, Dec +12° 59′ 29.6″ within Leo, 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.

J2000 Equatorial Sky Position (Leo) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 63° 40°N Lat 47° 30°S Lat RA: 11h 20m 15.03s Dec: +12° 59′ 29.6″ Epoch: J2000.0 · V = 8.92 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Discovered by Charles Messier on March 1, 1780, alongside M65, Messier 66 (NGC 3627) is the largest and brightest member of the iconic Leo Triplet (Arp 317), shining at visual magnitude 8.92 from a distance of 31.3 million light-years. Spanning roughly 95,000 light-years across, M66 was separately cataloged by Halton Arp as Arp 16 ('Spiral Galaxies with Detached Segments') because its two main spiral arms are dramatically warped and displaced above the plane of its central bar.

N-body hydrodynamical models show that about 800 million years ago M66 suffered a close gravitational flyby with NGC 3628 (lying 35 arcminutes to the north). That tidal tug pulled M66's southern spiral arm far out of symmetry, stripped an outer neutral-hydrogen plume from its western side, and drove massive molecular gas inflows along its bar that fuel luminous pink H II star-forming knots and dust lanes observed at sub-arcsecond resolution by Hubble and JWST.

Because of this tidally induced starburst, M66 has hosted five observed supernovae since 1973: SN 1973R, the bright Type Ia calibrator SN 1989B (which peaked at mag 11.9), SN 1997bs (a famous supernova impostor / luminous blue variable eruption), SN 2009hd, and SN 2016cok.

Practical Visual Observing Guide

Minimum Instrument10×50 binoculars or 70 mm telescope (200 mm+ for asymmetric spiral arms)
Recommended Magnification50×–80× with M65; 130×–180× for spiral structure
Best Evening SeasonSpring (February–June)
  • Forms the southeastern corner of the Leo Triplet, just 20′ east-southeast of M65; visible in 10×50 binoculars under dark skies.
  • In 100–150 mm telescopes, M66 is noticeably broader and brighter than M65, with an elongated central bar running northwest-southeast.
  • In a 250–350 mm (10–14 inch) aperture at 130×–180×, the sweeping, hook-like southern spiral arm and dark dust pocket west of the core become clearly visible.

Catalog Cross-Identifications & Aliases

Common Aliases: M66 · NGC 3627 · UGC 6346 · Arp 16

Cross-Catalog Identifiers for Messier 66 (NGC 3627 / Arp 16)
Catalog AuthorityIdentifier
MESSIERM 66
NGCNGC 3627
SIMBADM 66
WIKIDATAQ13991
Related Object · Leo

Messier 65 (NGC 3623 Leo Triplet Spiral)

J2000 coordinates, distance (34.6 Mly), magnitude 9.31, tightly wound SA(rs)a arms, prominent eastern dust lane, and observing guide for Messier 65.

Related Object · Leo

Messier 96 (NGC 3368 Leo I Group Anchor)

J2000 coordinates, HST Cepheid distance (34.9 Mly), magnitude 9.25, asymmetric dusty spiral arms, double bar, and observing guide for Messier 96 in Leo.

Related Object · Canes Venatici

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.

Reference Guide

Apparent vs Absolute Magnitude Explained

Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.

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