Messier 55 (NGC 6809)
Coordinates, Gaia DR3 distance (5.35 kpc), magnitude 6.32, loose Class XI structure, and observing guide for Messier 55 in Sagittarius.
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) | 6.32 mag | ± 0.05 mag | Wide-aperture integrated photometry | definitive | Catalog of Parameters for Milky Way Globular Clusters |
| Heliocentric distance | 17450 ly | ± 200 ly | Gaia EDR3 parallax + HST proper motions (5.35 ± 0.06 kpc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 19 arcmin | ± 1.5 arcmin | Outer surface-brightness profile fitting | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Main-sequence turnoff age | 12.9 Gyr | ± 0.5 Gyr | HST ACS metal-poor turnoff isochrone fitting | model-dependent | Catalog of Parameters for Milky Way Globular Clusters |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Messier 55 (NGC 6809 Summer Rose Star) at RA 19h 39m 59.71s, Dec -30° 57′ 53.1″ 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.
Astrophysical Context & Structure
Messier 55 (NGC 6809) is a large, remarkably loose globular cluster located 17,450 light-years (5.35 ± 0.06 kiloparsecs) away in eastern Sagittarius. First discovered by Nicolas-Louis de Lacaille from the Cape of Good Hope in June 1752, M55 frustrated Charles Messier from his Paris observatory for fourteen years due to its −31° southern declination and low central surface brightness before he finally logged it on July 24, 1778.
Classified as Shapley-Sawyer Class XI (one of the least concentrated globulars in the Messier catalog), M55 spans 19 arcminutes on the sky—nearly two-thirds the width of the full Moon—and about 98 light-years in physical diameter. Its relatively low total mass (~190,000 solar masses) and huge core radius (over 2.8 arcminutes) leave its stars loosely bound, allowing astronomers to resolve individual main-sequence stars and blue stragglers right across the geometrical center without stellar crowding.
Spectroscopic measurements show that M55 is very metal-poor ([Fe/H] = −1.94 dex, ~1.1% solar iron abundance) and nearly 13 billion years old. Unlike variable-rich M3, M55 contains only about a dozen RR Lyrae variables alongside a prominent population of SX Phoenicis pulsating blue stragglers.
Practical Visual Observing Guide
- Located 7.5° east-southeast of Zeta Sagittarii (Ascella) in a barren eastern region of Sagittarius devoid of bright pointer stars.
- Under dark skies M55 is a large, ghostly 6.3-magnitude puff in 7×50 or 10×50 binoculars, but light pollution or atmospheric haze low on the northern horizon easily washes out its uncondensed disk.
- Because it lacks a tight core and its brightest red giants reach magnitude 11.2, even a 90–130 mm telescope at 60×–120× resolves M55 completely to the center into uniform, granular stardust.
Catalog Cross-Identifications & Aliases
Common Aliases: M55 · NGC 6809 · Summer Rose Star · Specter Cluster
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 55 |
| NGC | NGC 6809 |
| SIMBAD | M 55 |
| WIKIDATA | Q11467 |
Related Objects, Comparisons & Guides
Messier 54 (NGC 6715 Extragalactic Globular)
Coordinates, distance (26.28 kpc / 85,700 ly), Sagittarius Dwarf Spheroidal Galaxy nucleus data, and observing guide for Messier 54.
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.
Messier 4 (NGC 6121 Nearest Globular)
Coordinates, Gaia DR3 distance (1.85 kpc), magnitude 5.63, central stellar bar, and Antares finder guide for Messier 4 in Scorpius.
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
- Catalog of Parameters for Milky Way Globular Clusters (2010 / 2021 Gaia EDR3 revision): Harris, W. E. 1996 (2010 ed.), AJ, 112, 1487; Vasiliev, E. & Baumgardt, H. 2021, MNRAS, 505, 5978 — McMaster University / Gaia EDR3 Cluster Survey
- 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
- 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