Aldebaran (Alpha Tauri)
Coordinates, Hipparcos distance (66.6 ly), 45.1 R☉ radius, Hyades foreground alignment, and observing guide for Aldebaran in Taurus.
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) | 0.86 mag | 0.75 to 0.95 mag | Johnson UBV photometry (slow irregular LB micro-variability) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 48.94 mas | ± 0.77 mas | Hipparcos New Reduction astrometric solution (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 66.64 ly | ± 1.05 ly | Derived from parallax 48.94 ± 0.77 mas (20.43 ± 0.32 pc) | definitive | Hipparcos New Reduction Catalog |
| Photospheric radius | 45.1 R☉ | ± 0.1 R☉ | Lunar occultation & VLTI interferometric diameter (20.58 ± 0.03 mas) | definitive | SIMBAD Astronomical Database (CDS) |
| Effective temperature | 3910 K | ± 50 K | High-resolution spectroscopy & bolometric flux | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Aldebaran (Alpha Tauri Orange Giant) at RA 04h 35m 55.24s, Dec +16° 30′ 33.5″ within Taurus, 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
Aldebaran (Alpha Tauri, from the Arabic al-Dabarān meaning 'the Follower' because it rises just behind the Pleiades) is a K5+ III orange-red giant shining at magnitude 0.86 as the fiery eye of Taurus the Bull. Although it sits visually at the southeastern vertex of the V-shaped Hyades star cluster, Hipparcos parallax measurements (48.94 ± 0.77 mas) show Aldebaran lies only 20.43 parsecs (66.6 light-years) from Earth—less than half the 47-parsec distance to the Hyades.
Having exhausted its core hydrogen roughly a billion years ago, this 1.16-solar-mass star has ascended the red giant branch, inflating its convective envelope to 45.1 solar radii (nearly 63 million kilometers across) and radiating 439 times the bolometric luminosity of the Sun at a cool surface temperature of 3,910 K.
Because Aldebaran sits just 5.47° south of the ecliptic, it is the brightest star regularly occulted by the Moon during 18.6-year nodal series, allowing high-speed photometers to measure its 20.58-milliarcsecond disk directly as the lunar limb sweeps across it. NASA's Pioneer 10 spacecraft, launched in 1972, is coasting in the general direction of Aldebaran and will pass near its current position in roughly two million years.
Practical Visual Observing Guide
- Extend the three-star line of Orion's Belt ~20° northwest to hit orange-red Aldebaran and the V of the Hyades, then continue another 14° northwest to the Pleiades (M45).
- In 7×50 or 10×50 binoculars, Aldebaran's deep tangerine color contrasts vividly with the dozens of white and pale yellow 4th-to-7th magnitude Hyades cluster members filling the same field.
Catalog Cross-Identifications & Aliases
Common Aliases: α Tauri · HD 29139 · HIP 21421 · 87 Tauri
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 21421 |
| HD | HD 29139 |
| SIMBAD | * alf Tau |
| WIKIDATA | Q12977 |
Related Objects, Comparisons & Guides
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.
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.
Betelgeuse (Alpha Orionis Red Supergiant)
Coordinates, competing optical vs radio distances (168 vs 222 pc), radius, Great Dimming physics, and observing guide for Betelgeuse.
Arcturus (Alpha Bootis Red Giant)
Coordinates, Hipparcos distance (36.7 ly), magnitude −0.05, halo-stream kinematics, and observing guide for Arcturus in Boötes.
Winter Circle, Orion & Taurus Observing Guide
Winter sky observing tour of Orion, Taurus, Canis Major, Auriga, and Perseus: bright OB/M supergiants, open clusters, M42, and the Crab Nebula (M1).
Binocular Deep-Sky Objects Observing Guide
How to observe open clusters, globular clusters, diffuse nebulae, and Local Group galaxies with 7x50, 10x50, and 15x70 astronomy binoculars.
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.
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.
Bright, Multiple & Variable Stars Catalog
Cross-reference catalog of solar-neighborhood and navigation stars across Bayer, Henry Draper (HD), Hipparcos (HIP), WDS, and Gaia DR3 designations.
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)
- 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
- 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