August 2016 - Double Star of the Month
STF 2690 (20 31 11.94 +11 15 37.7) was found by William Herschel in 1779 and he called it H III 16. Since that time the position angle has reduced 26° to 255° and the separation has increased from 15" to 17".6.
Located in Delphinus it can be found just 30 arc mins preceding epsilon Del. Also in a low power field is 1 Del (BU 63) an unequal close pair which is a test for 15-cm.
The components of STF 2690 are mags 7.1 and 7.4 and Sissy Haas called them both 'peach-white'. Whilst observing at George Bishop's observatory, at South Villa in Regent's Park, London, W. R. Dawes noted
.. on the first night on which I measured this object with Mr. BISHOP's 7-inch refractor (1840, Oct 27) I perceived that B was unquestionably wedge-shaped, and succeeded in obtaining moderately good measures of it. Dawes also speculated on why Struve has missed this pair during the Dorpat survey, and suggested that pairs such as STF 2690, being wide and easy, were only measured on poor nights and hence the real nature of star B would have eluded him.
DA 1, as the close pair became known, has a highly eccentric orbit and the separation ranges from 0".55 to 0".02, a distance which it attained in the last years of the previous century. It is now widening and in mid-2016, the separation is expected to be 0".34. The stars are mags 7.9 and 8.0 so this will be a severe test for 30-cm. A is also a very close pair being found in speckle survey of B stars in 1983. Motion appears slow and the period is likely to be a century or more.
Rho Capricorni (SHJ 323 - 20 28 52.19 -17 48 49.2) is the northernmost of a bright triangle of naked-eye stars about 5 degrees south-south-east of beta Capricorni. Of the other two, omicron featured in this column in 2015, and pi will appear in 2017.
Although discovered by the elder Herschel the pair now has the moniker SHJ attached to it - the name for stars catalogued by James South and Sir John Herschel.
This is a binary of period 278 years which is now widening but will remain quite a difficult object for the northern observer. The writer has not yet observed it but with a magnitude difference of 2 between the brightest components and a separation of 1".8 then it requires a good night to be seen.
There is a 13.3 star at 55" (distance increasing) and a 6.7 mag star some 259" away (and increasing) has, in turn, a companion of mag 10.6 at 54" and is catalogued as DOB 13DE.
Sissy Haas notes that A and D are pretty -
Bright Sun-yellow and pale rose-red.
Bob Argyle - Double Star Section Director