Star profile
Castor B
Alpha Geminorum · Gemini
Binary systemWhat is Castor B
Castor B is the white companion of famous Castor, together 51.6 light-years away in Gemini. What the eye perceives as a single point is in fact a multiple system, and B is one of its main pieces: itself a binary star, gravitationally bound to its twin. That two stars called «brothers» should each split into pairs and trios feels like a joke of the sky: the Greek myth of the twins repeated at stellar scale.
Where it is and when to see it
Castor B lies in the northern celestial hemisphere. It is visible to the naked eye under a dark sky. Its best month of the year is January, when it culminates around midnight.
- From London it culminates about 70° above the horizon.
- From New York it culminates about 81° above the horizon.
- From Sydney it culminates about 24° above the horizon.
It can be seen from anywhere north of 58° S.
It ranks no. 4 in brightness among the 74 stars of Gemini in our catalogue. To find it, start from Pollux, the brightest: it lies about 5° away (a fist at arm's length covers about 10°).
The light you see today left it about 52 years ago.
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Astronomical data for Castor B
- Constellation
- Gemini
- Apparent magnitude
- 2.85
- Distance
- 51.6 light years
- Coordinates (J2000)
- RA 113.652° · Dec 31.891°
- Catalogue
- HYG 118485 · HD 60178
A symbolic commemorative gift: neither the signature nor the chosen name implies ownership of the star or recognition by the IAU.
Other stars in Gemini
Gemini has 74 stars in Stelvia's catalogue. Here are a few of them.
- PolluxBeta Geminorum1.16
- CastorAlpha Geminorum1.58
- AlhenaGamma Geminorum1.93
- TejatMu Geminorum2.87
- MebsutaEpsilon Geminorum3.06
- PropusEta Geminorum3.31
- AlzirrXi Geminorum3.35
- WasatDelta Geminorum3.50