Open Clusters · Constellation Sagittarius

Messier 25 (IC 4725)

Coordinates, Gaia DR3 distance (661 pc), magnitude 4.6, U Sagittarii Cepheid distance-ladder calibrator, and observing guide for M25.

Right Ascension (J2000.0)18h 31m 47.00s
Declination (J2000.0)-19° 06′ 51.0″
Primary ClassificationOpen star cluster (hosts classical Cepheid U Sagittarii)
Best Observing SeasonSummer (June–August)

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 25 (IC 4725)
ParameterValueUncertaintyMethod & EpochQuality StatusSource Archive
Integrated visual magnitude (V)4.6 mag± 0.1 magWide-aperture integrated photometrydefinitiveRevised New General Catalogue and Index Catalogue (RNGC/IC)
Heliocentric distance2155 ly± 30 lyGaia DR3 astrometric parallax of cluster & U Sgr (661 ± 9 pc) (J2016.0)definitiveESA Gaia Archive — Data Release 3
Apparent diameter32 arcmin± 2.5 arcminWide-field member star countshigh-confidenceRevised New General Catalogue and Index Catalogue (RNGC/IC)
Main-sequence turnoff & Cepheid evolutionary age0.09 Gyr± 0.015 GyrGaia DR3 turnoff + U Sgr pulsation evolutionary modelinghigh-confidenceESA Gaia Archive — Data Release 3
Measurement Uncertainties & Competing Solutions: Differential interstellar reddening across M25 spans E(B−V) = 0.29 to 0.60 mag; Gaia DR3 trigonometric parallaxes bypass this dust uncertainty to anchor the cluster and its Cepheid U Sagittarii at 661 ± 9 pc.

J2000 Equatorial Sky Chart & Culmination Altitude

The chart below plots Messier 25 (IC 4725 Cepheid Calibrator Cluster) at RA 18h 31m 47.00s, Dec -19° 06′ 51.0″ within Sagittarius, 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 (Sagittarius) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 31° 40°N Lat 79° 30°S Lat RA: 18h 31m 47.00s Dec: -19° 06′ 51.0″ Epoch: J2000.0 · V = 4.6 mag First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Astrophysical Context & Structure

Messier 25 (IC 4725) is a bright, sprawling open cluster located 2,155 light-years (661 ± 9 parsecs) away in northern Sagittarius, sitting just east of the Small Sagittarius Star Cloud (M24). Discovered by Swiss astronomer Jean-Philippe Loys de Chéseaux in 1745 and cataloged by Charles Messier on June 20, 1764, M25 is the only Messier object that was accidentally omitted from John Dreyer's original 1888 New General Catalogue—receiving instead the designation IC 4725 in the Second Index Catalogue of 1908 after Solon Bailey photographed it.

In observational cosmology, M25 is one of the most important open clusters in the sky because it hosts the classical Cepheid variable star U Sagittarii (U Sgr) as a confirmed gravitational member. Discovered to be variable by Johann Julius Schmidt in 1866 and proven to belong to M25 by John Irwin in 1955, U Sagittarii pulsates between visual magnitude 6.28 and 7.15 with a period of 6.74523 days. Because Gaia DR3 measures the parallax of dozens of surrounding M25 member stars Simultaneously, U Sagittarii provides a gold-standard geometric anchor for the Leavitt period-luminosity law used to measure the Hubble constant.

Spanning 32 arcminutes (~20 light-years physically) with an age of ~90 million years, M25 also contains two luminous M-type red giants and several rapid-rotator Be stars.

Practical Visual Observing Guide

Minimum InstrumentNaked eye / 7×50 binoculars
Recommended Magnification20×–50× wide-field
Best Evening SeasonSummer (June–August)
  • Located 6° north-northeast of Mu Sagittarii and 3.5° east of the Small Sagittarius Star Cloud (M24), forming a summer binocular triad with M23 and M24.
  • Visible to the naked eye under dark skies at magnitude 4.6, and resolves immediately in 7×50 or 10×50 binoculars into two dozen 6th-to-9th magnitude stars.
  • In a 70–130 mm telescope at 25×–50×, look near the cluster center for golden-yellow U Sagittarii and compare its brightness from night to night against nearby 6.7- and 7.3-magnitude reference stars.

Catalog Cross-Identifications & Aliases

Common Aliases: M25 · IC 4725

Cross-Catalog Identifiers for Messier 25 (IC 4725)
Catalog AuthorityIdentifier
MESSIERM 25
ICIC 4725
SIMBADM 25
WIKIDATAQ11371
Related Object · Sagittarius

Messier 22 (NGC 6656 Sagittarius Cluster)

Coordinates, Gaia DR3 distance (3.30 kpc), magnitude 5.10, stellar-mass black holes, and observing guide for Messier 22 in Sagittarius.

Related Object · Sagittarius

Lagoon Nebula (Messier 8 / NGC 6523)

Coordinates, Gaia DR3 distance (1.25 kpc), NGC 6530 cluster, Hourglass core, and observing guide for the Lagoon Nebula (M8) in Sagittarius.

Reference Guide

Trigonometric Parallax, Parsecs & Gaia DR3

How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.

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

  • 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