August 2021 - Double Star of the Month

I have been following STT358 in Hercules (18 35 33.22 +16 58 32.5) for almost 50 years. In 1970, with the 28-inch refractor at Herstmonceux, a measure gave the values of 168 degrees, 1".7. In 2018 using the Cambridge 8-inch Cooke, the separation had remained virtually unchanged and the position angle reduced to 149 degrees. This fine binary has a period of 380 years according to Wulff Heintz in 1995 and it is currently at 142 degrees and 1".5. The pair will remain within range of 15-cm or so for decades to come, only reducing to 1".3 by 2100 before widening again.

Gaia EDR3 shows that both stars have the same trigonometrical parallax to within the stated errors (±0.1 light-year in each case) and the mean distance of the system is 111.2 light-years. The pair can be found 3 degrees WSW of 111 Her. In 2007 I. N. Reid found a magnitude 12.6 star 349 degrees and 35" from A which is 0.3 light-year further out. This turns out to be an M3 dwarf.

One of the brightest triples in the sky is β Sgr which sits in the extreme south-west corner of the constellation and is unfortunately not visible from the UK. The two brightest components, called β¹ (19 22 38.30 -44 27 32, V = 4.0) and β² (19 23 13.14 -44 47 59.2, V = 4.3) are separated by 21' on the sky and thus easily visible to the naked-eye.

β¹ has a magnitude 7.1 companion at a distance of 28" and position angle 76 degrees, and was first noted as double by James Dunlop in 1826. Ernst Hartung thought these stars physically connected and indeed Gaia EDR3 seems to indicate that they lie at a similar distance, but the parallax for A has a very large quoted error, which may be partly due to the star's brightness. Hartung also noted the colours of the components of β¹ as bright pale yellow and ashy-white, while Ross Gould found them to be white and yellowish, more in line with the spectral types of B9V and A5V.

Draw a line from β¹ through β², extend it by about 1.5 degrees and you will come upon I 116, a fine triple star which will be well seen with 20-cm. The close pair has magnitudes of 8.6 and 9.4 and they are currently at 24 degrees and 2".7, whilst 16" from A is another 8.6 magnitude. star at PA 190 degrees. The discoverer, Robert Innes, re-visited the system some years later and added a magnitude 13 star only 2".4 from C. This star was last measured in 1960.

Bob Argyle - Double Star Section Director