October 2026 - Double Star of the Month
About ten degrees north of eta pegasi (V = 3.0), also known as Matar, is a cloud of 5th and 6th magnitude stars in which there are some Struve double stars. STF 2942 (22 44 05.21 +39 27 55.5) is at the apex of the right-angled triangle which it forms with 10 and 12 Lac.
With 10 to 15-cm this will appear as a rather unequal and fairly close double. The Washington Double Star Catalog (WDS) lists the magnitudes as 6.2 and 8.9 whilst the companion was found 2".7 away in position angle 273 degrees using the 8-inch refractor at Cambridge in 2000 and these values have both slowly increased since then.
There are other components in this system - C is a magnitude 11.7 at 9" from A and was found by S. W. Burnham with his 6-inch Clark refractor. A single observation in 2010 indicated that A was a very close pair but there have been no confirming observations.
About 1.5 degrees NE is STF 2946 (8.1, 8.3, 262 degrees, 5" in 2022).
Psi1 Aquarii (23 15 53.50 -09 05 15.9) is in a tight triangular group with psi2 (V = 4.4) and psi3 (V = 5.0) Aquarii and is the westernmost star of the three.
F. G. W. Struve did not include it in the Dorpat Catalogue due to the large difference in magnitude and the wide separation but did later publish two appendix lists.
psi2 is also known as STF II 12 or STFB 12 as it appears in the WDS catalogue. It is a nice sight in smaller apertures. The stars have magnitudes 4.4 and 9.9 and the current position of B is 312 degrees from A at 49". An observation made with a 21-cm reflector showed colours of orange and blue. B is a close binary (BC) which was found by Burnham in 1890 with the Lick 36-inch. The stars are almost equally bright and are currently below 0".2 and closing quickly. The period is 83.6 years. A and BC have identical and large proper motions suggesting that they form a single physical system. There are also two faint field stars, magnitudes 13.5 and 14.3. In 2003 a planetary body was found orbiting A with a period of 181 days.
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.
