January 2018 - Double Star of the Month
7 Cam (04 57 17.2 +53 45 07.5) is a fairly unprepossessing system in the F G. W. Struve Dorpat catalogue and appears as number 610 in that list.
The stars are magnitudes 4.9 and 11.3 with the current position 242° and 26 arc-seconds, and there is no evidence that the stars are in anyway connected. It is 7 degrees north of Capella and about 4 degrees preceding, and is part of a group of 4th and 5th mag stars which also contain the splendid pairs 1 and 2 Cam.
In 1864, Baron Ercole Dembowski discovered that A was an unequal, close pair, the new component being magnitude 7.9 at 307° and 1".2. Since then the stars have closed in, but are now slowly widening, although by 2020 the separation will still only be 0".62 in PA 196°.
A recent orbit with a period of 2733 years has since been replaced in 2014 by a linear ephemeris by Drummond, although this has now, in turn, been updated by Hartkopf in 2017.
The two recent linear solutions give 315° 0".88 and 196° 0".62 respectively for epoch 2020 but it is difficult to understand why there is such confusion about the quadrant in which the companion is located as there is such a large difference in magnitude. Suffice to say it will take sustantial aperture to see Dembowksi's companion but visual observations will help to confirm which of the two predictions is right.
With Canis Major skimming along the southern horizon at present there is a brief window of opportunity to delve into its treasures.
Mu (μ) CMa (06 56 06.59 -14 02 34) is a pretty pair but rather close and unequal and tends to be more difficult than it really is due to its low altitude from the UK.
The stars are magnitudes 5.3 and 7.1 with a current separation of 3".1, showing a very slow closing since discovery almost 200 years ago. Struve gave colours of orange and reddish although the Washington Double Star Catalog (WDS) gives spectral types of G5III and A2.
By the way, don't forget to take a look at Sirius, just 3.5 degrees south-west of μ CMa - the companion is now fully 10 arc-seconds distant.
Bob Argyle - Double Star Section Director