Ophiuchius, the Serpent Bearer occupies a large piece of the sky. The 11th-largest constellation by area, its shape is like a box with a roof and legs.
Sabik (Eta (η) Ophiuchi) is the star that marks the box’s southeast corner (lower left as seen from the UK), where it connects with the eastern leg.
In traditional depictions, Sabik marks the position near to one of Ophiuchus’s knees.
Sabik is a mid-brightness star shining at mag. +2.4, easily visible to the naked eye.
The name Sabik is Arabic for ‘the preceding one’, but why it has this name isn’t completely clear.
Sabik is a binary star with two similarly bright, white components in a mutual 88-year orbit.
The stars’ brightness and spectral classifications are mag. +3.0, A2 and +3.5, A3.
The orbit is quite small, producing a maximum separation of 1.7 arcseconds, and is highly eccentric, bringing the components to within 2 AU when closest and out as far as 65 AU when farthest apart.
To have any chance of splitting the pair, observations need to be made when they are farthest apart (called apastron).
The Sabik system is 88 lightyears from the Sun. The two binary components have estimated masses 3 and 3.5 times larger than the Sun.
The globular cluster M9 (pictured above) is located 3.5° to the southeast of Sabik, 1° east of the midpoint of the line from Sabik towards mag. +4.4 Xi (ξ) Ophiuchi.
This guide originally appeared in the August 2023 issue of BBC Sky at Night Magazine.
