Double Star of the Month - May 2011

In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

STT 288 (14 53 23.35 +15 42 18.7) can be found about 3.5 degrees due south of xi Bootis as a star just below the usual limit of naked-eye visibility. The components are magnitudes 6.9 and 7.5 and revolve around one another in about 313 years. About 100 years ago, the Greenwich Observatory observer William Bowyer considered the two stars were optical in nature. To him they appeared to be steadily increasing in separation since discovery in 1845 when Otto Struve found them separated by 0".45. A few years after Bowyer's observation, the stars began to slowly close again. The orbit is fairly eccentric and the stars will continue to close to 0".51 in about 50 years time. Hipparcos places them at a distance of 155 light years. In the meantime at PA 160°, 1".06 they form an excellent test for the 10-cm aperture.

Lupus is full of fine pairs and this column regularly features stars in that southern constellation. This month is the turn of Kappa Lupi = Dunlop 177 (15 11 56.07 -48 44 16.2), a beautiful pair of pale yellow stars according to E J Hartung. More recently Richard Jaworski notes them as yellow-white and plain white whilst the WDS gives the spectral types as B9.5V and A5V whilst the brightnesses are listed as 3.8 and 5.8. To the small aperture this is one of the easiest and brightest pairs in the sky - the current separation is 26".2 and this has decreased about 3" since 1826. With very similar proper motions, it is highly likely that the stars form a binary system, but Hipparcos has had some trouble in deciding the distance of the fainter component.

Bob Argyle - Double Star Section Director