November 2026 - Double Star of the Month
The binary star alpha Psc (02 02 02.80 +02 45 49.4) is a fine sight in a small telescopes. The components have magnitudes 4.1 (A) and 5.2 (B) and at present are separated by 1".85 in PA 257 degrees. The stars were first seen by William Herschel on Oct 19 1779 - he recorded that the separation and PA were respectively 4".35 and 337 degrees and that both stars were white.
The orbital period is thought to be over 3,000 years and at present the closest approach has just occurred. I have measured AB regularly with the Cambridge 8-inch OG but the angular motion is very slow, amounting to 18 degrees in 34 years. This is a complex multiple star system. Using the CHARA ground-based interferometer which uses six one-metre mirrors with separations up to 331 metres, the B component was resolved and found to have a period of 25 days. The separation between the stars is a few milli-arcseconds so they are a thousand times closer than the AB pair.
A star of magnitude 8.2 at a distance of 404" is also thought to be physically connected. AB can be well seen in 10-cm but the relatively low declination of the star often results in poor seeing.
Located just below the middle of the line joining zeta and eta Ceti is BU 399 (01 27 46.53 -10 54 05.6), another discovery by S. W. Burnham with his 6-inch refractor in 1876 when he estimated the magnitudes as 6.3 and 10.0.
The Washington Double Star Catalog (WDS) lists the primary as a K4 giant of magnitude 6.4 and the companion at magnitude 8.8. They are currently separated by 1".3 away so at least 15-cm and a night of good seeing would be needed to divide them but there is very little motion.
Brian Skiff at Lowell Observatory found that a 15th magnitude M star at a distance of 196" had common proper motion with the bright stars and the system is called SKF 118. This star, and the binary pair, lie about 770 light years away.
Bob Argyle - Double Star Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
