Castor (Alpha Geminorum)
Coordinates, Hipparcos distance (50.8 ly), 5.6″ A–B binary orbit, YY Geminorum eclipsing pair, and observing guide for Castor.
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 |
|---|---|---|---|---|---|
| Combined apparent visual magnitude (V) | 1.58 mag | ± 0.01 mag | Johnson UBV photometry (Castor A = 1.93, B = 2.97, C = 9.27) | definitive | Hipparcos New Reduction Catalog |
| Trigonometric parallax | 64.12 mas | ± 3.75 mas | Hipparcos + visual binary orbital parallax (~15.6 pc / 50.8 ly) (J2000.0) | high-confidence | Hipparcos New Reduction Catalog |
| Castor A–B angular separation (2026 epoch) | 0.093 arcmin | ± 0.002 arcmin | Visual binary ephemeris (~5.6 arcseconds at PA ~51°; widening toward apastron) | definitive | SIMBAD Astronomical Database (CDS) |
| Castor A–B visual orbital period | 162600 d | ± 1800 d | Keplerian visual orbit (~445 years, e = 0.343) | high-confidence | SIMBAD Astronomical Database (CDS) |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots Castor (Alpha Geminorum Sextuple System) at RA 07h 34m 35.86s, Dec +31° 53′ 17.8″ within Gemini, 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
Castor (Alpha Geminorum) is the second-brightest star in Gemini (shining at combined magnitude 1.58 just 4.5° northwest of orange Pollux) and one of the most celebrated multiple star systems in the sky. In 1803 William Herschel used 80 years of positional measurements of Castor A and B to prove for the first time that Newton's law of universal gravitation operates outside the Solar System, establishing the existence of true gravitationally bound binary stars.
Located 15.6 parsecs (50.8 light-years) from Earth, Castor is actually a gravitational ballet of six stars arranged in three pairs. The two brilliant white visual components—Castor A (magnitude 1.93, spectral class A1 V) and Castor B (magnitude 2.97, spectral class A2 Vm)—orbit each other every ~445 years along an eccentric path (currently ~5.6 arcseconds apart). Spectroscopic radial-velocity studies revealed that both Castor A (9.21-day period) and Castor B (2.93-day period) are single-lined spectroscopic binaries, each orbited closely by an unseen M-dwarf companion (Ab and Bb).
Seventy-one arcseconds (~1,100 AU) south-southeast of the main pair sits 9.3-magnitude Castor C, also cataloged as YY Geminorum: a double-lined eclipsing binary of two nearly identical 0.60-solar-mass M1 Ve red dwarfs that eclipse each other every 19.54 hours (0.81428 days), enabling astronomers to measure fundamental M-dwarf masses and radii directly.
Practical Visual Observing Guide
- In the 1960s and 1970s Castor A and B closed to 1.8″ and required high power, but in the 2020s their separation has widened to ~5.6 arcseconds—making Castor an easy, razor-sharp split in any 60–80 mm telescope at 80×–120×.
- Look 71 arcseconds south-southeast in the same eyepiece field to spot 9.3-magnitude reddish Castor C (YY Geminorum); all three visual components (representing six physical stars) are visible simultaneously.
Catalog Cross-Identifications & Aliases
Common Aliases: α Geminorum · HD 60179 · HIP 36850 · 66 Geminorum
| Catalog Authority | Identifier |
|---|---|
| HIP | HIP 36850 |
| HD | HD 60179 |
| SIMBAD | * alf Gem |
| WIKIDATA | Q13029 |
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.
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.
Capella (Alpha Aurigae Giant Binary)
Coordinates, Hipparcos distance (42.9 ly), twin G-giant 104-day interferometric binary orbit, and observing guide for Capella in Auriga.
Beehive Cluster (Messier 44 Praesepe)
Coordinates, Gaia DR3 distance (186.2 pc), magnitude 3.1, exoplanet hosts, and observing guide for the Beehive Cluster (M44 / NGC 2632).
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.
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).
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.
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.
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