Alioth (Epsilon Ursae Majoris)
Coordinates, Hipparcos distance (82.6 ly), magnitude 1.77, Alpha2 CVn magnetic chromium spots, and observing guide for Alioth in Ursa Major.
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 visual magnitude (V) | 1.77 mag | ± 0.02 mag | Johnson UBV photometry (rotational modulation of 0.02 mag over 5.089 d) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 39.51 mas | ± 0.2 mas | Hipparcos New Reduction astrometric solution (25.31 ± 0.13 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 82.55 ly | ± 0.42 ly | Derived from Hipparcos parallax 39.51 ± 0.20 mas | definitive | Hipparcos New Reduction Catalog |
| Photospheric radius | 4.14 R☉ | ± 0.12 R☉ | Optical interferometry & TESS/BRITE rotational modeling | high-confidence | SIMBAD Astronomical Database (CDS) |
| Magnetic rotation & spectral modulation period | 5.0887 d | ± 0.0001 d | Doppler Zeeman imaging of chromium/iron abundance belts | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Alioth (Epsilon Ursae Majoris Ap Variable) at RA 12h 54m 01.75s, Dec +55° 57′ 35.4″ 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
Alioth (Epsilon Ursae Majoris) is the brightest star in the constellation Ursa Major and the thirty-second brightest star in the night sky at apparent visual magnitude 1.77, outshining Alpha Ursae Majoris (Dubhe) despite its 'Epsilon' Bayer letter because Johann Bayer lettered the seven stars of the Big Dipper in geographical order from west to east rather than strictly by brightness. Marking the base of the Big Dipper's handle closest to the bowl, Alioth is the most luminous core member of the ~400-million-year-old Ursa Major Moving Group at a distance of 25.31 ± 0.13 parsecs (82.55 light-years).
Weighing 2.91 solar masses with a radius of 4.14 solar radii and a luminosity of 102 solar luminosities (effective temperature 9,020 K), Alioth is the brightest chemically peculiar Ap star (Alpha² Canum Venaticorum variable) in the sky. Its ~100-Gauss dipolar magnetic field is inclined at ~54° to its 5.0887-day rotation axis, sorting heavy elements across its surface.
Doppler imaging reveals that depleted oxygen and calcium concentrate near Alioth's magnetic poles, while chromium and europium form a dense, patchy belt along its magnetic equator. As the star turns every 5.089 days, these chemical patches rotate across our line of sight, causing periodic changes in spectral line strengths and a 0.02-magnitude brightness oscillation.
Practical Visual Observing Guide
- Located in the Big Dipper's handle just 4.5° west-southwest of Mizar and Alcor, shining with a pure, crisp white light.
- Circumpolar for all observers north of +34°N latitude, Alioth is one of the 57 official navigational stars used in celestial navigation almanacs.
Catalog Cross-Identifications & Aliases
Common Aliases: ε Ursae Majoris · HD 112185 · HIP 62956 · 77 Ursae Majoris
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 62956 |
| HD | HD 112185 |
| SIMBAD | * eps UMa |
| WIKIDATA | Q13055 |
Related Objects, Comparisons & Guides
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.
Dubhe (Alpha Ursae Majoris Giant Binary)
Coordinates, Hipparcos distance (123 ly), magnitude 1.79, 44.4-year K0 III + F0 V binary orbit, and northern pointer guide for Dubhe.
Cor Caroli (Alpha Canum Venaticorum Double)
Coordinates, Hipparcos distance (115 ly), 19.3″ double star, 5.47-day magnetic Ap prototype α² CVn, and observing guide for Cor Caroli.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
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.
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)
- 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