Pleiades (Messier 45 Open Cluster)
Coordinates, Gaia DR3 distance (136.2 pc), magnitude 1.6, reflection nebulosity, and observing guide for the Pleiades (M45) in Taurus.
Aliases: M45, Seven Sisters, Melotte 22
Open Star Clusters Catalog & Observing Guide
Loosely bound groups of dozens to several thousand young Population I stars born together from a single giant molecular cloud in the Milky Way's thin disk.
Cataloged Profiles4 Objects
ClassificationOpen Clusters
Machine-Readable DataJSON Shard (open-clusters.json)
Physical & Observational Overview
Unlike halo globular clusters that survive for Hubble times, galactic open clusters orbit within the gas-rich thin disk of the Milky Way and gradually disperse over 100 million to 1 billion years via internal evaporation and tidal encounters with giant molecular clouds.
With Gaia DR3 providing microarcsecond parallaxes and proper motions, nearby open clusters—anchored by the Hyades (47 pc) and Pleiades (136 pc)—serve as the geometric foundation of the cosmic distance ladder and empirical main-sequence isochrone calibration.
Classification Criteria: Classified using the Trumpler system by central concentration (I–IV), brightness range of member stars (1–3), and richness (p = poor, m = medium, r = rich), plus isochrone turnoff age (10 Myr to 1 Gyr).
Visual Observing Regime: Because nearest open clusters subtend 0.5° to 5.5° on the sky, they are premier targets for the unaided eye, 10×50 binoculars, and wide-field low-power (20×–50×) telescope eyepieces.
Open Clusters Comparison Table
| Object | Constellation | Classification | RA (J2000) | Dec (J2000) | Min. Instrument | Best Season |
|---|---|---|---|---|---|---|
| Pleiades (Messier 45 Open Cluster) | Taurus | Open star cluster with dusty reflection nebulosity | 03h 47m 00.00s | +24° 07′ 00.0″ | Naked eye; best in 7×50 or 10×50 binoculars | Late Autumn to Winter (November–February) |
| Hyades (Melotte 25 Nearest Cluster) | Taurus | Open star cluster (nearest open cluster to the Sun) | 04h 26m 54.00s | +15° 52′ 00.0″ | Naked eye and 7×50 binoculars | Late Autumn to Winter (November–February) |
| Beehive Cluster (Messier 44 Praesepe) | Cancer | Open star cluster | 08h 40m 24.00s | +19° 40′ 00.0″ | Naked eye; spectacular in 10×50 binoculars | Late Winter to Spring (February–May) |
| Double Cluster (NGC 869 & NGC 884) | Perseus | Twin young supergiant open clusters (Perseus OB1 core) | 02h 20m 00.00s | +57° 08′ 00.0″ | Naked eye; best in binoculars and wide-field telescopes | Autumn to Winter (September–January) |
Detailed Open Clusters Profiles
Hyades (Melotte 25 Nearest Cluster)
Coordinates, Gaia DR3 distance (47.0 pc), age 650 Myr, moving-group kinematics, and observing guide for the Hyades cluster in Taurus.
Aliases: Melotte 25, Collinder 50, Caldwell 41
Beehive Cluster (Messier 44 Praesepe)
Coordinates, Gaia DR3 distance (186.2 pc), magnitude 3.1, exoplanet hosts, and observing guide for the Beehive Cluster (M44 / NGC 2632).
Aliases: M44, Praesepe, NGC 2632, Manger Cluster
Double Cluster (NGC 869 & NGC 884)
Coordinates, Gaia DR3 distance (2.29 kpc), age 14 Myr, supergiant population, and observing guide for NGC 869 and NGC 884 in Perseus.
Aliases: NGC 869, NGC 884, h and Chi Persei, Caldwell 14
Related Astrometric & Optical Reference Guides
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.
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.