Messier 68 (NGC 4590)
Coordinates, Gaia DR3 distance (10.40 kpc), magnitude 7.84, Fimbulthul tidal stream, and observing guide for Messier 68 in Hydra.
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) | 7.84 mag | ± 0.05 mag | Integrated photoelectric & CCD photometry | definitive | Catalog of Parameters for Milky Way Globular Clusters |
| Heliocentric distance | 33900 ly | ± 400 ly | Gaia EDR3 parallax + RR Lyrae calibration (10.40 ± 0.12 kpc) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Apparent diameter | 11 arcmin | ± 1 arcmin | Outer surface-density profile fitting | high-confidence | Revised New General Catalogue and Index Catalogue (RNGC/IC) |
| Isochrone turnoff age | 12.5 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 68 (NGC 4590 Hydra Globular Cluster) at RA 12h 39m 27.98s, Dec -26° 44′ 38.6″ within Hydra, 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 68 (NGC 4590) is a loose, extremely metal-poor ([Fe/H] = −2.23 dex) globular cluster located 33,900 light-years (10.40 ± 0.12 kiloparsecs) away in eastern Hydra, south of the quadrilateral of Corvus. Discovered by Charles Messier on April 9, 1780, M68 spans about 106 light-years physically and binds roughly 120,000 solar masses of ancient Population II stars in an open Shapley-Sawyer Class X profile.
Using data from ESA's Gaia space astrometry mission, astronomers discovered that M68 is actively shedding stars into a gigantic 180-degree tidal stream ('Fjorm' / M68 stream) that arches across the northern Galactic hemisphere and passes within 16,000 light-years of the Sun. Its prograde, eccentric halo orbit (reaching 100,000 light-years at apogalacticon) indicates that M68 was accreted into the Milky Way during an early dwarf-galaxy merger.
M68 is also notable for its rich collection of more than 42 variable stars—mostly RR Lyrae stars, including many double-mode RRd pulsators vibrating simultaneously in their fundamental and first-overtone radial modes, providing sensitive seismic probes of horizontal-branch stellar interiors.
Practical Visual Observing Guide
- Easy to find by drawing a line from Delta Corvi (Algorab) through Beta Corvi (Kraz) and extending 3.5° further south-southeast into Hydra, just 25′ northeast of a 5.4-magnitude orange field star (HD 110458).
- In 10×50 binoculars M68 is a soft, ghostly 7.8-magnitude patch next to its bright orange companion star.
- Because its Class X halo is fairly loose and its brightest red giants shine at magnitude 12.6, a 130–200 mm telescope at 120×–180× resolves dozens of glittering stars across a 4-arcminute inner disk.
Catalog Cross-Identifications & Aliases
Common Aliases: M68 · NGC 4590
| Catalog Authority | Identifier |
|---|---|
| MESSIER | M 68 |
| NGC | NGC 4590 |
| SIMBAD | M 68 |
| WIKIDATA | Q11484 |
Related Objects, Comparisons & Guides
Messier 53 (NGC 5024 Outer Halo Globular)
Coordinates, Gaia DR3 distance (18.50 kpc), magnitude 7.61, Helmi Stream origin, and observing guide for Messier 53 in Coma Berenices.
Messier 3 (NGC 5272 Globular Cluster)
Coordinates, Gaia DR3 distance (10.18 kpc), magnitude 6.19, RR Lyrae variable benchmark data, and observing guide for Messier 3 in Canes Venatici.
Messier 79 (NGC 1904 Winter Globular Cluster)
Coordinates, Gaia DR3 distance (13.08 kpc), magnitude 7.73, Gaia-Enceladus retrograde orbit, and observing guide for Messier 79 in Lepus.
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.
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
- 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