Mizar & Alcor (Zeta & 80 Ursae Majoris)
Coordinates, Hipparcos/VLBI distance (82.8 ly), 11.8′ naked-eye pair, 14.4″ Mizar A–B split, and observing guide for Mizar & Alcor.
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 |
|---|---|---|---|---|---|
| Apparent magnitude (Mizar A+B combined) | 2.04 mag | ± 0.01 mag | Johnson UBV photometry (Mizar A = 2.23, Mizar B = 3.88, Alcor = 3.99) | definitive | Hipparcos New Reduction Catalog |
| Distance (NPOI interferometric orbital parallax of Mizar A) | 25.4 pc | ± 0.2 pc | Navy Precision Optical Interferometer (NPOI) resolved orbit (82.8 ± 0.6 ly) | definitive | SIMBAD Astronomical Database (CDS) |
| Mizar A–B telescopic separation | 0.24 arcmin | ± 0.002 arcmin | Astrometric separation 14.4 arcseconds at PA 153° (Alcor is 11.8 arcminutes away) | definitive | SIMBAD Astronomical Database (CDS) |
| Mizar Aa–Ab spectroscopic orbital period | 20.53835 d | ± 0.00005 d | Double-lined spectroscopic + NPOI interferometric orbit (e = 0.537) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Mizar & Alcor (Zeta Ursae Majoris System) at RA 13h 23m 55.54s, Dec +54° 55′ 31.3″ within Ursa Major, 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
Mizar (Zeta Ursae Majoris, magnitude 2.04) and Alcor (80 Ursae Majoris, magnitude 3.99), sitting at the bend of the Big Dipper's handle in Ursa Major, form the most famous naked-eye double star in history (`the Horse and Rider'), separated by 11.8 arcminutes (11′ 49″). Beyond being a classic test of unaided human visual acuity, Mizar is a cornerstone of modern observational astronomy with four historic firsts.
In 1617 Galileo Galilei and Benedetto Castelli resolved Mizar into two stars separated by 14.4 arcseconds (Mizar A at magnitude 2.23 and Mizar B at magnitude 3.88)—the first telescopic double star ever discovered. In 1857 George Phillips Bond photographed Mizar A and B on a wet collodion plate at Harvard (the first double star ever photographed). Then in 1889 Edward C. Pickering and Antonia Maury discovered that the spectral lines of Mizar A split and merge every 20.54 days, making Mizar A the very first spectroscopic binary ever identified.
Soon afterward, Mizar B was also found to be a single-lined spectroscopic binary (175.6-day period). Finally, in 2009 coronagraphic adaptive optics revealed that Alcor itself is orbited at 1.1 arcseconds by a 0.25-solar-mass M3–M4 red dwarf (Alcor B) and co-moves gravitationally with Mizar at a distance of ~82.8 light-years, making the Mizar–Alcor complex a hierarchical sextuple system.
Practical Visual Observing Guide
- Look at the middle star of the Big Dipper's handle with the naked eye: 4.0-magnitude Alcor sits 11.8 arcminutes (just over a third of the Moon's diameter) northeast of 2.0-magnitude Mizar.
- In any small telescope at 40×–80×, Mizar splits cleanly into two brilliant white diamonds (Mizar A and B) separated by 14.4 arcseconds, while Alcor shines in the same low-power field.
- Between Mizar and Alcor sits an 8th-magnitude background field star, Sidus Ludoviciana (HD 116798), once mistaken in 1722 for a new planet.
Catalog Cross-Identifications & Aliases
Common Aliases: ζ Ursae Majoris · 80 Ursae Majoris · Horse and Rider · HIP 65378
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 65378 |
| HD | HD 116656 |
| SIMBAD | * zet UMa |
| WIKIDATA | Q13059 |
Related Objects, Comparisons & Guides
Epsilon Lyrae (The Double Double Star)
Coordinates, Gaia DR3 distance (160 ly), 208″ wide pair and 2.3″/2.4″ tight sub-pairs, and telescope resolution test guide for Epsilon Lyrae.
Albireo (Beta Cygni Color Double Star)
Coordinates, Gaia DR3 parallaxes of Beta Cygni A & B, 34.6″ angular separation, color contrast, and observing guide for Albireo in Cygnus.
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.
Bode's Galaxy (Messier 81 / NGC 3031)
Coordinates, Cepheid/TRGB distance (3.63 Mpc / 11.8 Mly), magnitude 6.94, M81–M82 tidal pair, and observing guide for Bode's Galaxy in Ursa Major.
Splitting Bright Double & Multiple Stars Guide
How to resolve showcase binary and multiple star systems—Albireo, Mizar & Alcor, Epsilon Lyrae, Castor, and Sirius B—and test telescope Dawes limit.
Spring Galaxy Season: Ursa Major to Virgo
Spring galaxy observing guide from Ursa Major (M81, M82, Whirlpool M51) across Coma/Virgo to giant elliptical M87, the Sombrero Galaxy (M104), and 3C 273.
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.
Scientific Sources & Dataset Provenance
- Hipparcos New Reduction Catalog (2007 New Reduction): van Leeuwen, F. 2007, A&A, 474, 653 (Validation of the new Hipparcos reduction) — ESA / van Leeuwen (2007)
- 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
- IAU Working Group on Star Names (WGSN) (2026 IAU WGSN Bulletin): Mamajek, E., et al. (IAU WGSN) 2016–2026, IAU Catalog of Star Names — International Astronomical Union