Castor, alpha Geminorum (Visual triple star)

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Gemini
: 07:35:14 / +31 52
: 1.9 / 2.9 / 9.1
: 4.4" / 71"
: 61° / 164°
: A1V / A1V / M1Ve
: White / White / Yellow-orange
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 16/02/10 / 22:24
: Landgraaf
: 5 / 3
: Orion Optics UK 300mm
: 7mm Pentax XW
: 229x / 18

Observing Report

With the 22mm Nagler and a magnification of 72x Castor shows a hint of being a double. In The 17mm Nagler, magnification 94x, alpha Geminorum is easily split. The best views however of the two brightest members of Castor are achieved with the 7mm Pentax XW and a magnification of 229x. The view of the two bright white components is simply stunning. Two sparkling white diamonds of almost equal brightness. The southernmost of the two seems a little brighter than the northern component.

Two bright sets of diffraction spikes add to the beauty of this bright double. Amazing how beautiful bright stars look in my 12-inch telescope. This again proves that you should not only use a large telescope for hunting faint fuzzies from exotic catalogues. You should definitely use your large telescope to (re-)visit well-known bright objects like bright stars and bright, well-known deepsky objects, like the ones from the Messier- or Caldwell catalogue.

To the southeast of the bright Castor A and B components, lies a trio of fainter stars, from which the middle star looks deep yellow or even orange. This is the C-component of Castor, the variable YY Geminorum. To the northwest I detect another faint star. The six stars in the field of view remember me of a big airplane or jet, maybe even a Concord, with the two bright stars at the centre.


Each of the three components of Castor are spectroscopic binaries itself, so in fact Castor is a sextuple star system. Component C, with the variable designation YY Geminorum is an eclipsing binary, consisting of two cool M-type stars.