Double Star of the Month in Capricorn

• August 2024 - Double Star of the Month

  33 Cygni lies in northern Cygnus, near the border with both Cepheus and Draco. It forms one apex of a triangle whose sides are 1.5 degrees long with two other stars, both of which are visual doubles. The star which is directly east of 33 is STF 2687 (20 26 23.47 +56 38 19.3). This is an easy pair for the small aperture with the stars of magnitudes 6.3 and 8.1, separated by 25".

  

  The other member of the triangle is STF 2671 (H I 95). This is a brighter but closer pair than STF2687. The main components have magnitudes 6.1 and 7.5 and the current position of the fainter component is 337 degrees and 3".8, coordinates which have changed very little over 200 years. There is a 12.4 magnitude star at 53 degrees and 90" and two further 13th magnitude field stars.

  The second edition of The Cambridge Double Star Atlas shows the pair HJ 2975 in Capricornus about 5 degrees south of, and slightly following, the bright triangle of visual double stars formed by omicron (ο) Cap, rho (ρ) Cap, and pi (π) Cap (see this column for August 2015, 2016 and 2017 respectively). It is a fairly difficult pair for small telescopes as the Washington Double Star catalog (WDS) gives magnitudes of 7.5 and 11.6 and a separation of 11".

  

  The Atlas also shows that the star immediately north preceding by about 30 minutes is also double. This is a brighter and easier object (magnitudes 7.8 and 8.1) which does not warrant a label because the stars are 39" apart, but it is HJ 2973 (20 32 13.27 -22 09 17.6), a pair of yellowish F stars, one of which was found to be a close double at a lunar occultation in 1983.

  These Herschel pairs were discovered using the 20-foot reflector from Slough in 1830/1, and John Herschel allocated magnitudes of 8 and 14 to HJ 2975 and 8.9, 8.9 to HJ 2973.

  Bob Argyle - Double Star Section Director

• August 2017 - Double Star of the Month

  Located about 1.5 degrees south-west of Albireo in the Cambridge Double Star Atlas (2nd edition) is a single point marked magnitude 7 to 7.5 according to the scale. This is the binary star STF 2525 (19 26 33.71 +27 19 21.9) which consists of stars of mag 8.2 and 8.4.

  Found by Struve at Dorpat at a separation of 1".3, over the next 60 years it gradually closed up until it was unresolved between the late 1880's and about 1895 when it began to widen again. It was therefore regarded as an optical pair undergoing a close approach until Thomas Lewis (1906) clarified the situation.

  In the previous decade the stars had made a very close approach and the similarity in magnitude disguised the fact that the companion was heading out back to where it came from.

  The stars were measured in 2016 with the Cambridge 20-cm refractor (291 degrees, 2".38) and are now visible in 15-cm. The uncertain period is 883 years and the maximum separation of 3".3 will be reached around 2320.

  Pi Capricorni (20 27 19.20 -18 12 42.1) is the easterly of three faint naked-eye stars which form a triangle some 4 degrees south and somewhat east of beta Capricorni (V = 3.1). The other two stars in the triangle, omicron Capricorni and rho Capricorni, have already been dealt with in this column in August 2015 and August 2016 respectively.

  Pi Cap. is one of S. W. Burnham's earliest discoveries using the 6-inch Clark, but it transpires that it was first seen by O. M. Mitchel in 1846 and not published until 5 years after Burnham noted it in print.

  From northern latitudes this is not a particularly easy pair. The stars are of magnitude 5.1 and 8.5 and the current position is 3".3 in PA 160 degrees; there has been very little motion since 1846. Hartung notes that the companion is white and the primary is a B8 star.

  The discovery of variable radial velocity in the A star later led to the discovery of a close companion which appears to have a period of the order of 40 years or so.

  For a real test, try the 14.1 mag star found by Burnham with the Washington 26-inch refractor. It is being left behind by AB and is currently 38" away in PA 40 degs.

  Bob Argyle - Double Star Section Director

• 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

• August 2015 - Double Star of the Month

  Lambda Cygni (20 47 24.563 +36 29 26.7) is easily found. Just move 3 degrees due north from epsilon Cygni, the left-hand star in the cross. One of Otto Struve's discoveries from Pulkova the seeing needs to be good to get a measure of the companion. The stars are magnitudes 4.7 and 6.3 but have always been separated by less than 1" although the angular distance between the stars is now roughly double that at discovery. Since 1990 the writer has observed the pair in 12 seasons and in that time the position angle has decreased by about 10 degrees. The current orbit has a period of 391 years but this is complicated by the fact that Hal McAlister and colleagues found the primary to be an interferometric binary with a period of 11 years and maximum separation of 0".05. There is some evidence that one of the three stars is also a single-lined spectroscopic binary. Lambda Cyg has a spectral type of B5Ve and is a rapid rotator surrounded by a circumstellar disk. Sir James South adds a faint companion, mag 9.7, at 106° and 83".

  The sprawling constellation of Capricornus sits near the meridian on a northern summer night but locating stars in it apart from the third magnitude alpha and beta needs the help of a star atlas. However, starting with the brightest star of all, beta, by moving 3 degrees south and slightly east a trio of stars is encountered, all enclosed by a 1 degree field. Each of these is a visual double star and the subject of this column is omicron Cap = SHJ 324 (20 29 53.91 -18 34 59.4) the most southerly of the three. Smyth calls it omicron2 and notes that both components are to be found in Piazzi's catalogue. The WDS gives magnitudes of 5.9 and 6.7 and the separation is currently 21".9, down from 25" when found by William Herschel with a small decrease in the position angle, currently 238°. The Hipparcos catalogue gives the distance as 216 light years but with a formal error of 27% this is an indicator that there could possibly be another star nestling in the system.

  Smyth calls both stars bluish, and whilst Sissy Haas regards them as almost equal, the report by Hartung notes that they are an 'unequal white pair'.

  Bob Argyle - Double Star Section Director