Double Star of the Month in Bootes

• May 2025 - Double Star of the Month

  Lambda Bootis is the prototype of a class of stars with unusual chemical composition - the abundance of metals is unusually low and they also tend to be accompanied by accretion disks or planetary systems. Two degrees NE of lambda is STF1843 (14 24 38.91 +47 49 50.0) is a wide and rather unequal pair which lie 316 light-years away.

  

  The Washington Double Star catalog (WDS) gives magnitudes of 7.7 and 9.2 and a recent measure puts the companion at 186 degrees and 19".8 with the separation widening with time. A third star of magnitude 9.3 can be seen 101" away in PA 63 degrees but the parallax given by Gaia DR3 indicates that it is more than 800 light-years away. There is, however, another member of the system - a magnitude 17.8 star found by Andrei Tokovinin. An observation which I made with a 10-inch reflector at x96 showed colours of yellow and lilac.

  In eastern Virgo lies phi Virginis (14 28 12.22 -02 13 40.6) which presents a greater challenge to the observer with a small telescope. It is the magnitude of the companion and its relatively close separation to the A star which means that a 15-cm telescope is recommended. The two stars are given catalogue magnitudes 4.0 and 10.0 and the current separation is 5".2, although when first measured in 1827 they were only 4" apart.

  

  The pair was measured with the Cambridge 8-inch in late 2015 and given that the micrometer uses a red LED to illuminate the field and hence shows the wires in outline I would have said that the B star must be brighter than 10. Both stars lie at 121 light-years distance and are moving through space with almost identical proper motions of 0".139 per year. The relatively large error on the parallax of A as measured by Gaia chimes with the comment in the WDS that A may be a spectroscopic binary.

  Bob Argyle - Double Star Section Director

  If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

• April 2025 - Double Star of the Month

  With Arcturus riding high in the Spring sky, STF 1825 (14 16 32.84 +20 07 18.7) can be easily found as it is just 1.5 degrees north and a little west of the bright, orange giant star. The components have magnitudes 6.5 and 8.4 and the current separation is 4".4 so it is a relatively easy object for the small telescope.

  

  The very slow orbital motion is reflected in the fact that my two sets of measures made in 1993 and 2019 show a difference in position angle of just 2 degrees. This is a relatively nearby system as the parallax determined by Gaia DR3 shows, the distance is 108 light-years and both stars are moving across the sky at more than 0.1 arc-second per year.

  In Centaurus, near the border with Lupus, and about 9 degrees north and slightly west of Hadar (beta Centauri, V = 0.6) is HJ 4651 (14 09 35.03 -51 30 16.8), a wide and unequal pair of unassociated stars. The magnitudes are 6.0 and 8.9 and the current separation is 65". The primary star is the eclipsing binary V869 Cen and is almost four times nearer to the Sun than its fainter companion.

  

  Of more direct interest but certainly a more difficult object is SLR 19 which is to be found 1.5 degrees north and a little preceeding HJ 4651. It is an almost equally bright pair which was first observed by R. P. Sellors in Australia using the 11.5-inch refractor at Sydney Observatory. The motion to date amounts to 45 degrees and an orbit by Izmailov gives a period of 415 years which predicts a closest separation of 0".6 in 2054. The predicted position for 2026.0 is 337 degrees and 0".9 and the magnitudes are 7.1 and 7.4.

  Bob Argyle - Double Star Section Director