Double Star of the Month - May 2009

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.

At this time of year the constellation of Lupus is nearing the meridian in southern latitudes and offers a number of beautiful and sometimes difficult pairs for the small telescope user. Pi Lupi (15 05 08.16 -47 03 04.3) was found by John Herschel in South Africa and since that time (1836) it has been slowly widening so that it is now within range of a 75-mm aperture. It seems to have given early observers no little difficulty and values for the observed separations vary wildly which in a bright equal pair is difficult to explain. The current position is 67°, 1".6 and as this value of separation has not changed significantly for about 30 years, we may conclude that this is a binary in a highly inclined orbit which is close to apastron.

The WDS gives the magnitudes as 4.56 and 4.60 and the negative value of (B-V) for both stars confirm that these are hot blue dwarf stars probably around spectral type B5. The revised parallax is 7.33 mas putting them at a distance of about 445 light years. A subsidiary note in the WDS says that both stars are spectroscopic binaries.

Eta CrB (15 23 12.23 +30 17 17.7) is one of William Herschel's most important discoveries and one of the shortest period visual binaries visible in a small telescope. A recent orbit gives the period as 41.556 years with an error of 5 days so the pair has made five revolutions since discovery. The tilt of the apparent orbit means that the pair will stay rather close for a number of years. From the current separation of 0".56 the stars widen to 0".67 in 2014 before closing to 0".38 in 2020 and then widening to 1".0 in 2032.

Eta CrB is also a relatively nearby system with a revised Hipparcos parallax of 55.72 mas which equates to 58.5 light years. The WDS gives the magnitudes as 5.64 and 5.95 and spectral types as FOV and GOV so that the stars appear yellow to the visual observer. There are three comites listed in the WDS, the first two, called C and D are field stars, but star E is a physically connected L8 dwarf some196 arc seconds distant from AB (equivalent to 3600 Astronomical units) and apparent visual magnitude 17. Imaging this star would be an interesting project for the well-equipped CCD astronomer.

Bob Argyle - Double Star Section Director