White main-sequence star · Gemini
Castor
Also known as: α Geminorum (α Gem), HIP 36850
mag 1.651 light-years
About
Castor (α Geminorum) is a white main-sequence star of magnitude 1.6 in Gemini, about 51 light-years away.
How to find it
- Castor is in the constellation Gemini.
- Best evenings: around late February it is highest at about 9 pm (10 pm in summer time); a month earlier, two hours later.
- Where on Earth: it is seen well, at least 10° up, from north of 48° S.
- Star patterns: Winter Hexagon.
When it rises and sets tonight, how high it climbs and how much the Moon gets in the way are worked out in your browser for your place.
What you will see
- EyesEasy, even from town.
- Binoculars (10×50)A sharp point; binoculars bring out its colour and the fainter stars around it.
- Telescope (6–8 inch)Still a point of light at any magnification — stars are too far away to show a disc. Low in the sky, bright stars twinkle and flash colours.
Stars look the same in any instrument: brighter and sharper, but never bigger.
Facts
| Proper name | Castor |
|---|---|
| Designation | α Geminorum (α Gem) |
| Hipparcos number | HIP 36850 |
| Type | White main-sequence star |
| Spectral type | A2Vm |
| Colour | WhiteB−V colour index 0.03 |
| Magnitude | 1.6The smaller the number, the brighter. |
| Distance | 51 light-yearsThe light you see left it around 1975. |
| Proper motion | 0.25″ per year across the sky |
| Constellation | Gemini |
| Right ascension (J2000) | 07h 34m 35.9s |
| Declination (J2000) | +31° 53′ 18″ |
More in Gemini
Sources and credits
Star data: HYG Database by David Nash, CC BY-SA 4.0. All data sources →