Messier 28 (NGC 6626)
Coordinates, Gaia DR3 distance (5.37 kpc), magnitude 6.79, first globular millisecond pulsar PSR B1821-24, and observing guide for M28 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) | 6.79 mag | ± 0.06 mag | Integrated photoelectric & CCD photometry | definitive | Catalog of Parameters for Milky Way Globular Clusters |
| Heliocentric distance | 17500 ly | ± 320 ly | Gaia EDR3 parallax + HST dynamical modeling (5.37 ± 0.10 kpc) (J2016.0) | high-confidence | ESA Gaia Archive — Data Release 3 |
| Apparent tidal diameter | 11.2 arcmin | ± 1 arcmin | Surface-brightness profile fitting | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Isochrone turnoff age | 12.5 Gyr | ± 0.5 Gyr | HST ACS color-magnitude turnoff fitting | model-dependent | Catalog of Parameters for Milky Way Globular Clusters |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Messier 28 (NGC 6626 Pulsar Cluster) at RA 18h 24m 32.89s, Dec -24° 52′ 11.4″ 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 28 (NGC 6626) is a tightly concentrated Shapley-Sawyer Class IV globular cluster located 17,500 light-years (5.37 ± 0.10 kiloparsecs) away in Sagittarius, less than a degree northwest of the bright Teapot lid star Kaus Borealis. Discovered by Charles Messier on July 27, 1764, M28 looks much smaller on the sky (11.2 arcminutes) than nearby M22 because it lies 60% farther away, yet both clusters have comparable physical diameters near 60–90 light-years.
In July 1987, a team using the Jodrell Bank Lovell Telescope discovered PSR B1821−24 inside M28—a neutron star spinning once every 3.05 milliseconds (327.4 rotations per second). That landmark discovery was the very first millisecond pulsar ever found inside a globular cluster, confirming the theoretical prediction that dense cluster cores recycle aged neutron stars through mass accretion in low-mass X-ray binary systems.
Since then, Green Bank Telescope and MeerKAT radio surveys have uncovered at least 14 millisecond pulsars in M28, including the extraordinary transitional pulsar IGR J18245−2452 (M28I), which was caught switching back and forth between an accretion-powered X-ray pulsar state and a rotation-powered radio millisecond pulsar state over timescales of weeks.
Practical Visual Observing Guide
- Point just 0.9° northwest of 2.8-magnitude Kaus Borealis (λ Sagittarii); both fit easily inside the same finder or low-power telescope field.
- In 10×50 binoculars M28 is a compact, round 6.8-magnitude knot contrasting sharply with the huge, loose sprawl of M22 3° to the northeast.
- A 150–200 mm telescope at 140×–200× resolves the outer 5-arcminute halo into fine 13.5-magnitude stardust surrounding a compressed, blazing Class IV core.
Catalog Cross-Identifications & Aliases
Common Aliases: M28 · NGC 6626
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 28 |
| NGC | NGC 6626 |
| SIMBAD | M 28 |
| WIKIDATA | Q11373 |
Related Objects, Comparisons & Guides
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.
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.
47 Tucanae (NGC 104 Globular Cluster)
Coordinates, Gaia distance (4.52 kpc), magnitude 4.09, millisecond pulsar core, and observing guide for 47 Tucanae (NGC 104).
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.
Angular Size, Separation & Field of View
How angular diameter (degrees, arcminutes, arcseconds), spherical angular separation, and small-angle physical diameter relate to telescope field of view.
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