Mizar & Alcor vs Castor Sextuple Systems
Compare the two most famous sextuple star systems in the northern sky—Mizar/Alcor in Ursa Major and Castor in Gemini—in hierarchy and separation.
Compared Celestial Objects
Mizar & Alcor (Zeta Ursae Majoris System)
Coordinates, Hipparcos/VLBI distance (82.8 ly), 11.8′ naked-eye pair, 14.4″ Mizar A–B split, and observing guide for Mizar & Alcor.
Constellation: Ursa Major · Coords: 13h 23m 55.54s, +54° 55′ 31.3″
Castor (Alpha Geminorum Sextuple System)
Coordinates, Hipparcos distance (50.8 ly), 5.6″ A–B binary orbit, YY Geminorum eclipsing pair, and observing guide for Castor.
Constellation: Gemini · Coords: 07h 34m 35.86s, +31° 53′ 17.8″
Normalized Shared Parameter Table
| Parameter | Unit / Scale | Mizar & Alcor (Zeta Ursae Majoris System) | Castor (Alpha Geminorum Sextuple System) |
|---|---|---|---|
| Combined Visual Magnitude (V) | mag | +2.04 (Mizar AB: +2.23/+3.88; Alcor: +3.99) | +1.58 (Castor A: +1.93, B: +2.97, C: +9.27) |
| Heliocentric Distance | pc | 25.3 ± 0.2 pc (82.6 ly) | 15.60 ± 0.09 pc (50.9 ly) |
| Primary Telescopic Pair Separation | arcsec | 14.4″ (Mizar A to Mizar B) | 5.5″ (Castor A to Castor B, widening) |
| Third Visual Component Separation | arcsec | 708″ (11.8′ to Alcor AB) | 71.0″ (to eclipsing M-dwarf Castor C) |
| Total Confirmed Stellar Components | stars | 6 stars (Aa+Ab, Ba+Bb, Alcor A+B) | 6 stars (Aa+Ab, Ba+Bb, Ca+Cb) |
Astrophysical & Observational Synthesis
Mizar and Alcor hold three foundational firsts in observational astronomy: they form the classic 11.8-arcminute naked-eye acuity test in the Big Dipper handle; Mizar A and B (14.4″ separation) became the first telescopic binary discovered (by Benedetto Castelli and Galileo in 1617); and in 1889 Edward C. Pickering discovered that Mizar A's spectral lines double every 20.54 days, making it the first spectroscopic binary ever found. Subsequent spectroscopy proved Mizar B is also a 175.6-day binary, and adaptive optics in 2009 revealed Alcor A is orbited by an M3.5 V red dwarf (Alcor B), completing a six-star hierarchy.
Castor (Alpha Geminorum, 15.60 pc away) replicates that exact sextuple architecture on a tighter scale. In 1803, William Herschel used 40 years of position-angle measurements of Castor A (+1.93, A1 V) and Castor B (+2.97, A2 Vm) to prove for the first time that Newton's law of universal gravitation governs stars outside the Solar System.
Orbiting each other with a 445-year period (currently separated by 5.5 arcseconds), both Castor A (9.21-day period) and Castor B (2.93-day period) carry faint red-dwarf spectroscopic companions, while 71 arcseconds south sits Castor C (YY Geminorum)—an 0.814-day eclipsing binary of two nearly identical M1 Ve red dwarfs.
Related Reference Guides
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.
Proper Motion, Radial Velocity & Redshift
How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.
Primary Catalogs & Scientific Sources
- 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
- 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