June 2017 - Double Star of the Month
beta Serpentis (15 46 11.21 +15 25 28.9) is a magnitude 3.7 star of spectral type A2IV some 155 light years away.
About 30" west of it is a magnitude 10 companion which is unrelated. This pairing was seen by William Herschel in 1781. The companion should be visible in 7.5-cm on a fair night as it is not too close to the glare of the bright primary.
About 25 arcminutes further west is an 8th magnitude star which is moving through space with the same motion as beta and it is almost certainly physically associated. This star, in turn, has a companion and the system is known as ROE 75.
E. D. Roe was an American astronomer who had access to the 12 and 40-inch refractors of Yerkes Observatory for double star measurement, but in this case he discovered number 75 on 1911 May 20 with his own 6.5-inch Clark refractor.The secondary is magnitude 10.7 at only 6" distance so a larger aperture may be needed to see this unequal pairing.
Ara lies below Scorpius in the southern sky and the north-east of the constellation is rich in open clusters.
NGC 6193 is part of the Ara OB1 association and is thought to be 4200 light years away. Buried in the cluster is the bright and wide pair DUN 206 (16 41 20.42 -48 45 46.7).
This is an easy pair for the small aperture with the white components of magnitudes 5.7 and 6.7 being 10" apart.
The brighter component is itself quadruple. Owners of 10-cm or more may be able to see the mag 8.4 star at 1".7 from A, but A itself is a massive triple consisting of three O stars, two of which form a close SB of 2,67 days period and they are in turn circled by a third star every 8.1 years. The total mass of these three stars is thought to be about 140 solar.
The whole region is spectacular for the small telescope and there are fainter and more distant stars to be seen with small apertures.
Bob Argyle - Double Star Section Director