November 2024 - Double Star of the Month

George Knott was a Victorian amateur astronomer with an interest in double stars. Using a 7.3-inch Clark refractor which once belonged to W. R. Dawes at his observatory in Cuckfield, West Sussex, he made a number of discoveries, five of which can be found in the current Washington Double Star catalog.

The primary star of KNT 1 (01 02 18.34 +81 52 32.1) is the variable star U Cephei. It is an eclipsing binary with a period of 2.5 days and in mid-eclipse the star drops by almost three magnitudes. At maximum, it is V = 6.7 and has a faint companion of magnitude 11.8 some 14" distant in PA 63 degrees.

Finder chart for the double star KNT 1 in Cepheus
A finder chart for the double star KNT 1 in Cepheus created with Cartes du Ciel.

Rather surprisingly both stars lie at the same distance (632 light-years) and have very similar proper motions. A slighter fainter, and definitely unconnected, third star can be found at PA 315 degrees (increasing) and 24" (decreasing).

Another pair of stars with a variable primary is HJ 3476 (02 00 26.77 -08 31 25.8). This is AR Cet which is probably a semi-regular variable. John Herschel swept them up from Feldhausen and noted Large star very yellow. The primary is an M3 giant and its catalogue magnitude is given as 5.7 with the companion 61" away in PA 202 degrees. Herschel noted magnitudes 6 and 10.

Finder chart for the double star HJ 3476 in Cetus
A finder chart for the double star HJ 3476 in Cetus created with Cartes du Ciel.

The position angle has reduced 20 degrees since discovery but this is due to the proper motion of the primary star which is 536 light-years away whilst its faint companion lies 2160 light-years distant.

HJ 3476 lies about 3 degrees NE of zeta Cet which is also known as Baten Kaitos. This, too, is a very unequal and wide double with the components of 3.8 and 10.5 being divided by 106".

Bob Argyle - Double Star Section Director