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July 30, 2026Short & citedBlog
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Field Notes • Astronomy

Castor

Castor is the traditional name of Alpha Geminorum, a bright multiple star system in the constellation Gemini.

Updated July 30, 20261 min readCited sources

It appears as a single star to the naked eye but is actually a six-star system arranged in three binary pairs.

What it is

Castor is the traditional name for Alpha Geminorum (α Geminorum), one of the brightest objects in the constellation Gemini. In modern astronomy it is classified as a multiple star system rather than a single star, consisting of six stars in three binary pairs: Castor A, Castor B, and Castor C.

How it is structured

The Castor system contains six stars organized into three close binary pairs. Each of the three visible components (Castor A, B, and C) is itself a spectroscopic binary, meaning the individual stars are too close to separate visually even with a telescope.

Key details

Castor has an apparent magnitude of about 1.58, making it easily visible without a telescope. It is the second-brightest star-like object in Gemini after Pollux, and lies roughly 51 light-years from Earth in the zodiac constellation.

In short

  • Castor is a multiple star system of six stars in three binary pairs.
  • It appears as a single star but is composed of three spectroscopic binaries.
  • It is the second-brightest object in Gemini after Pollux.
  • The system is about 51 light-years from Earth.

Quick questions

What is Castor?
Castor is the traditional name for Alpha Geminorum, a bright multiple star system in the constellation Gemini.
How many stars are in Castor?
Castor is made up of six stars arranged in three binary pairs.
Is Castor a single star?
No. It appears as one star to the naked eye, but it is actually a multiple star system.

Sources

  1. Wikipedia — https://en.wikipedia.org/wiki/Castor_(star)
    Supports: Traditional name, Bayer designation, brightness, and multiple-star nature of Castor.
  2. Space.com — https://www.space.com/21940-castor-star.html
    Supports: Six-star system, distance from Earth, constellation Gemini, and apparent brightness.
  3. Encyclopaedia Britannica — https://www.britannica.com/place/Castor-star
    Supports: Castor as a six-component multiple star system in Gemini and its combined magnitude.
  4. NASA HEASARC — https://heasarc.gsfc.nasa.gov/docs/objects/stars/castor.html
    Supports: Castor as the second-brightest star in Gemini and its six-star composition.
  5. Astropixels — https://astropixels.com/stars/Castor-01.html
    Supports: Magnitude, coordinates, distance, and basic stellar properties.