Star lore
Castor
Direct answer
Castor is Alpha Geminorum, a remarkable multiple-star system that appears to unaided eyes as one of Gemini's two bright heads.
Castor is Gemini's white head star and a six-star system in modern analysis.
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At a glance
- Found in
- Constellations & Star Lore
- Related
- Deneb, Pollux, Mimosa, Acrux
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The story behind Castor
Castor’s proper name identifies one of the classical Twins and has long shared the pair with Pollux.
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The Road of the Sun
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Why Castor matters
Castor demonstrates hierarchical multiplicity: several close binaries move together as one wider stellar family.
How to read Castor
Find Castor just north of brighter Pollux and resolve its brightest components with a small telescope.
Castor in practice: an example
A telescope splits Castor A and B, while spectroscopy and wider companions reveal the fuller six-star structure.
Where readings of Castor go wrong
Calling Castor a double star is observationally useful but physically incomplete.
Sources and editorial method
Research references used for this entry include:
- IAU Division C Working Group on Star Names - Official IAU working group responsible for standardizing star names used in international astronomy.
- CDS SIMBAD Astronomical Database - Professional astronomical database for stellar identifiers, positions, classifications, and bibliography.
- Source
- The Great Almanac Dictionary
- Category
- Constellations & Star Lore
- Last updated
- 2026-09-26
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Common questions
What is Castor?
Castor is Alpha Geminorum, a remarkable multiple-star system that appears to unaided eyes as one of Gemini's two bright heads.
What do people get wrong about Castor?
Calling Castor a double star is observationally useful but physically incomplete.
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