November 2017 - Double Star of the Month

STF 163 (01 51 16.93 +64 51 17.9) is a colourful pair which can be easily found just over a degree north of epsilon Cas, the easternmost star of the 'W' of Cassiopeia.

The colours astonished F. G. W. Struve when he observed them. He recorded aurea (cuprea) golden (coppery) and caerulea (light or sky blue). The stars are magnitudes 6.8 and 9.1 and are currently 34".5 apart and PA 38 degrees.

There has been little movement over the past two centuries or so. Both stars are at great distances from us but seem to be unrelated. A is a mid-K supergiant and the Hipparcos parallax gives a distance of 2200 light years with an uncertainty of 770 light years. More recently the Gaia mission has measured B and gets 800 ± 130 light years.

A number of observers on the Cloudy Nights website find that companion to be pale blue or gray whilst most see orange in the primary.

One degree further east is a triangle of three 6th magnitude stars, the westerly of which, according to Mullaney and Tirion is HJ 1000, but which is, in fact, HJ1100 5.3 +11, 309°, 43" (distance increasing).

Reticulum is a kite-shaped grouping of stars about 1 degree north-west of the Large Magellanic Cloud. The brightest star is alpha (V = 3.4) which forms a wide naked-eye pair with HR 1340, 17' to the north, itself a telescopic double (HJ 3641, 5.6, 11.0, 215°, 13".3 - an optical system).

Moving about 1 degree south of alpha and slightly east brings you to theta Ret (04 17 40.27 -63 15 19.7). This bright pair is number 3 in the small catalogue of stars compiled by Rumker at Parramatta.

John Herschel estimated the stars to be magnitude 6 and 9 and gave distances of 5".53 and 6".85. The WDS catalogue gives 6.0 and 7.7 with a separation of 3".9.

There has been virtually no angular motion since discovery and as the stars seem to be slowly widening again from a minimum of 3".7 in 1907 it might be assumed that this is a very long period binary whose apparent orbit is highly inclined to the line of sight.

Notwithstanding the B9 spectral type of the primary, Hartung notes the stars are pale and deep yellow, and reports that they lie in an attractive field.

Bob Argyle - Double Star Section Director