Castor, alpha Geminorum (Visual triple star)

Diameter in '
Number of stars
Brightest star (magnitude)
Distance in parsecs (3.26Ly)

Details Sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: Canes Venatici
: 12:56:02 / +38.19
: 2.9 /5.5
: 19.3”
: 229°
: A0pSiEuHg / F0V
: White / White-yellowish

: 17/02/10 / 0:05
: Landgraaf
: 5 / 3
: Orion Optics UK 300mm
: 22mm Nagler Type 4
: 78 / 68
image cor caroli

Observing report

In the 35mm Panoptic Cor Caroli is very hard to split. However, in the 22mm Nagler at 78x the double is split easily. The bright A component is definitely white. Between 10 and 11 o'clock, to the southwest, I see the fainter B component. At first sight it looks white as well, but the longer I observe this beautiful double, the more yellowish the B-component looks. After taking a short break and observing the double again, I still find the B-component yellowish, when compared to the brighter A-component.

The 68' field looks almost empty, just some fainter field stars. There is however an interesting double visible between 3 and 4 o'clock, to the east. The double lies very close to the edge of the field of view. Both stars look white. I do not see any other interesting stars, asterisms or background nebulosity.


Other observers have described this beautiful double with very different colors, especially the fainter star. It is described as pale lilac (Allen), pale yellow (Clerke), pale olive blue (Dembowski) and pale copper (T.W. Webb). So it probably comes down to which contrast you see, very subjective. I saw Cor Caroli as white and white with a hint of yellow.

The bright white star at the centre is alpha 2 Canum Venaticorum (the fainter, westernmost, companion is alpha 1). Alpha 2 is a magnetically peculiar star. The magnetic field is found by the Zeeman effect: the splitting of a spectral line into several components in the presence of a static magnetic field.

The fainter double star to the east is formed by the 8th magnitude HD112733 and BD +39 2587.