June 2015 - Double Star of the Month
Two bright double stars, each with its own observational difficulties, feature this month.
In the constellation of Draco, eta Draconis (16 23 59.51 +61 30 50.7) is circumpolar for observers in the UK but as a classical refractor user I find it is easiest to observe when in the north-west in the early evening. The primary is a G8 giant (V = 2.8) which is 92 light years distant and as a relatively nearby object it has attracted the attention of the astronomers wishing to determine its diameter. The result is that it is a shade under 10 million miles in diameter or about 11 solar diameters. The mag. 8.2 companion was first noted by Otto Struve at Pulkova in 1843 and he found it at PA 150° and separation 4".4. Since then it has moved retrograde by 11 degrees and the separation has slightly increased. I measured the pair on 4 nights in 1994 but in recent years have not been able to see the companion. The primary is also recorded as being variable so here is a pair to keep an eye on.
epsilon Lupi (15 22 40.89 -44 41 22.5) first attracted the attention of James Dunlop from Paramatta as a fine unequal and wide pair (DUN 182 AC). The stars are magnitudes 3.6 and 9.1 and today are separated by more than 26", a considerable increase on Dunlop's figure of 19". When Ralph Copeland, who later became Astronomer Royal for Scotland, visited the Andes with a 6-inch refractor for site-testing purposes, he found the primary star was a close and unequal pair which has since turned out to be a binary of period 319 years. Star B, mag 5.1, ranges in distance from A between 1".2 (as in 1834) to 0".18 (in 2026); in 2015 the pair are separated by only 0".26 and needs a large aperture. More recently, in a survey of stars in the Sco-Cen-Lup-Cru association, Rizzuto and colleagues, using the Sydney University Stellar Interferometer (SUSI) found another star of magnitude 5.1 but closer in at a distance of 0".05. It is not clear whether this is the known spectroscopic binary component of A (which has a period of 4.55 days), but epsilon appears to be at least a massive triple system.
Bob Argyle - Double Star Section Director