Double Star of the Month:
Sigma Orionis
AKA: STF 762 (C, D, E), SHJ 65 (H, I) and STF 3135 (F).
Position: 05 hr 38.7 min -2 degrees 36 min 00 sec
Due south at 23:59 (GMT) on 15 December 2020.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
For the most part, I stuck to doubles or occasionally triple star systems. But Sigma Orionis is really complicated! First let us establish where Sigma Orionis is. It is just under Zeta Orionis (Alnitak), the star at the left-hand end of Orion’s belt and close to the Horsehead Nebula. Sigma Orionis is part of the Orion’s Belt OB stellar association which lies about 1,260 light years away. The main star is actually a close double (AB), but with a separation of 0.3 arc second it is practically impossible to split. The next component is C, which is rather faint with a magnitude of 8.8, but relatively easy to split with a separation of 11.6 arc seconds. On the other side of AB is D which is much brighter (6.6) and a similar separation (12.9 arc sec). Moving out in more or less the same direction as D, we come to E which is as bright as D (6.3) and with a separation of 41.5 arc seconds; it is also a variable star. The last three easily visible members of this system are dimmer and much further out. F is magnitude 7.9 and 208 arc seconds, it is roughly halfway between C and E. H is about the same brightness of F (8.1) and 307 arc seconds, it is the same direction as F (more or less). Finally I is slightly dimmer than F and H (8.4); it is the furthest out at 525 arc seconds and is in the same direction as E. They are all basically white or blue: the B-V values suggest that AB is blueish and the others are white. The easy double AB-E was discovered by the Moravian astronomer Christian Mayer in 1777 and William Herschel recorded AB-D on 7 October 1779. The British astronomers James South and John Herschel discovered F, H and I in 1823. Wilhelm Struve added C in 1831. If you use a low power eyepiece, you can see STF 761 to the north-west. This is triple with three stars of similar brightness (7.9, 8.4 and 8.6). A and B are 68 arc seconds apart, but B and C are much closer at 8.5 arc seconds, so you will need to crank up the magnification to split BC: this is probably a good occasion to use a zoom eyepiece. The main star is yellowish and the others are blueish.