Regulus (Alpha Leonis)
Coordinates, Hipparcos distance (79.3 ly), magnitude 1.36, 15.9-hour rapid oblate rotation, wide companions, and observing guide for Regulus.
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) | 1.36 mag | ± 0.01 mag | Johnson UBV photoelectric photometry | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 41.13 mas | ± 0.35 mas | Hipparcos New Reduction astrometric solution (24.31 ± 0.21 pc) (J2000.0) | definitive | Hipparcos New Reduction Catalog |
| Distance in light-years | 79.3 ly | ± 0.7 ly | Derived from Hipparcos parallax 41.13 ± 0.35 mas | definitive | Hipparcos New Reduction Catalog |
| Equatorial radius (oblate rotator) | 4.16 R☉ | ± 0.08 R☉ | CHARA optical interferometry (polar radius = 3.14 ± 0.06 R☉) | definitive | SIMBAD Astronomical Database (CDS) |
| Primary stellar mass (Regulus A) | 3.8 M☉ | ± 0.2 M☉ | CHARA gravity-darkened model + 40.11-day pre-white-dwarf binary orbit | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Regulus (Alpha Leonis Quadruple System) at RA 10h 08m 22.31s, Dec +11° 58′ 02.0″ within Leo, 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
Regulus (Alpha Leonis, Latin for 'Little King' and traditionally called Cor Leonis, the Heart of the Lion) is the brightest star in Leo at apparent visual magnitude 1.36, lying 24.31 ± 0.21 parsecs (79.3 light-years) from Earth. Sitting just 0.46° (27.6 arcminutes) north of the ecliptic—closer to the Sun's apparent path than any other first-magnitude star—Regulus is regularly occulted by the Moon, Venus, and Mercury, and was revered by ancient Babylonian and Persian astronomers as one of the four 'Royal Stars' guarding the heavens.
Optical interferometry with the CHARA Array revealed that Regulus A (spectral type B8 IVn, 3.8 solar masses) spins on its axis once every 15.9 hours at an equatorial velocity of 317 ± 3 km/s—96.5% of the speed at which centrifugal force would tear the star apart! That extreme spin flattens its equatorial radius to 4.16 solar radii compared to 3.14 solar radii at its poles, while gravity darkening makes its poles five times brighter per unit area than its equator.
In 2008 spectroscopic radial-velocity measurements explained why Regulus spins so fast: Regulus A is orbited every 40.11 days at a separation of 0.35 AU by a faint ~0.30-solar-mass pre-white-dwarf companion that transferred mass and angular momentum onto Regulus A hundreds of millions of years ago. At an angular separation of 176.9 arcseconds (~4,200 AU) northwest orbits the co-moving binary Regulus BC, a pair of K2 V and M4 V dwarfs.
Practical Visual Observing Guide
- Anchors the base of the famous 'Sickle of Leo' (a backward question-mark asterism), with Regulus serving as the dot at the bottom of the question mark.
- Any 50–60 mm telescope or 10×50 binocular easily shows 8.13-magnitude orange companion Regulus B located 176.9 arcseconds (nearly 3 arcminutes) northwest of dazzling blue-white Regulus A.
- In a 150–200 mm telescope at 180×–250×, Regulus B itself splits into a tight 2.5-arcsecond binary with its 13.5-magnitude red dwarf partner Regulus C.
Catalog Cross-Identifications & Aliases
Common Aliases: α Leonis · Cor Leonis · HD 87901 · HIP 49669 · 32 Leonis
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 49669 |
| HD | HD 87901 |
| SIMBAD | * alf Leo |
| WIKIDATA | Q13022 |
Related Objects, Comparisons & Guides
Spica (Alpha Virginis Ellipsoidal Binary)
Coordinates, Hipparcos distance (250 ly), 4.01-day close B-star binary, precession discovery history, and observing guide for Spica.
Arcturus (Alpha Bootis Red Giant)
Coordinates, Hipparcos distance (36.7 ly), magnitude −0.05, halo-stream kinematics, and observing guide for Arcturus in Boötes.
Altair (Alpha Aquilae Rapid Rotator)
Coordinates, Hipparcos distance (16.73 ly), magnitude 0.76, 8.9-hour oblate spin imaged by CHARA, and observing guide for Altair in Aquila.
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.
Celestial Coordinates (RA & Dec) Explained
Practical reference to J2000 equatorial coordinates: Right Ascension (RA in hours, minutes, seconds), Declination (Dec in degrees), and sexagesimal conversion.
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