Arcturus (Alpha Bootis)
Coordinates, Hipparcos distance (36.7 ly), magnitude −0.05, halo-stream kinematics, and observing guide for Arcturus in Boötes.
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) | -0.05 mag | ± 0.01 mag | Johnson UBV photoelectric photometry | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 88.83 mas | ± 0.54 mas | Hipparcos New Reduction astrometric solution (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 36.72 ly | ± 0.22 ly | Derived from parallax 88.83 ± 0.54 mas (11.26 ± 0.07 pc) | definitive | Hipparcos New Reduction Catalog |
| Linear photospheric radius | 25.4 R☉ | ± 0.2 R☉ | Interferometric angular diameter (21.06 ± 0.17 mas) at 11.26 pc | definitive | SIMBAD Astronomical Database (CDS) |
| Effective temperature | 4286 K | ± 30 K | Bolometric flux + limb-darkened interferometric diameter | definitive | SIMBAD Astronomical Database (CDS) |
| Metallicity [Fe/H] | -0.52 dex | ± 0.04 dex | High-resolution optical/NIR spectroscopy (Ramírez & Allende Prieto 2011) | definitive | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Arcturus (Alpha Bootis Red Giant) at RA 14h 15m 39.67s, Dec +19° 10′ 56.7″ within Boötes, 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
Arcturus (Alpha Bootis) is the brightest star in the Northern Celestial Hemisphere and the fourth-brightest star in the entire night sky at apparent visual magnitude −0.05. Located 11.26 ± 0.07 parsecs (36.72 light-years) away in Boötes, it is an evolved K1.5 III red giant with roughly the same mass as our Sun (1.08 solar masses) that has exhausted its core hydrogen and swollen to 25.4 solar radii, shining with 170 times the Sun's bolometric luminosity.
In 1718 Edmond Halley compared contemporary astrometric positions of Arcturus, Sirius, and Aldebaran against Ptolemy's ancient Almagest catalog and proved for the first time in history that the 'fixed stars' move through space. Arcturus exhibits an enormous proper motion of 2.28 arcseconds per year toward Virgo, corresponding to a space velocity of ~122 km/s relative to the Sun.
Spectroscopically, Arcturus is iron-poor ([Fe/H] = −0.52 dex, about 30% of the Sun's iron abundance) but enriched in alpha elements (oxygen, magnesium, silicon, titanium) forged by ancient core-collapse supernovae. It leads the 'Arcturus moving group,' a kinematic stream of 50+ old thick-disk stars either perturbed by galactic bar resonances or stripped from an ancient satellite merger.
Practical Visual Observing Guide
- Follow the curved handle of the Big Dipper in Ursa Major ('arc to Arcturus') to land immediately on its golden-amber beacon.
- At magnitude −0.05, Arcturus slightly outshines blue-white Vega (+0.03) when both sit high in the June evening sky, offering a vivid naked-eye comparison between a 4,286 K K-giant and a 10,000 K A-dwarf.
- Moving across the sky by the width of the full Moon every 800 years, Arcturus is currently near its closest approach to the Solar System and will fade below naked-eye visibility in ~500,000 years.
Catalog Cross-Identifications & Aliases
Common Aliases: α Bootis · HD 124897 · HIP 69673 · 16 Bootis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 69673 |
| HD | HD 124897 |
| SIMBAD | * alf Boo |
| WIKIDATA | Q12985 |
Related Objects, Comparisons & Guides
Vega (Alpha Lyrae Zero-Point Star)
Coordinates, distance (25.04 ly), magnitude 0.03 photometric zero point, rapid oblate rotation, and observing guide for Vega in Lyra.
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Aldebaran (Alpha Tauri Orange Giant)
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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.
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.
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.
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.
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