Stephen Tonkin’s Binocular Tour

Famous globulars and lesser know binaries rub shoulders in Ophiuchus and Libra

ij Tick the box when you’ve seen each one

1 M5

M5 is a fine globular cluster adjacent to 10x 50 the mag +5.0 star 5 Serpentis (itself 8° west of mag. +5.2 Omega (t) Serpentis), and it may be just visible to the naked eye in a dark and transparent sky. The globular contains mostly Population II stars, which are deficient in heavy elements and are thus amongst the oldest stars that we can see; perhaps as old as 12 billion years. In 10 50 binoculars, you should notice that it brightens towards the centre, exactly like a comet does. Ĵ SEEN IT 2 ZUBENELAKRIBI

Variable star Zubenelakribi (Delta (b) 10x 50 Librae; mag. + 5.8 to mag. +4.4) is half-way between M5 and Zubenelgenubi (Alpha2 (_2) Librae). It is an eclipsing binary star, meaning a pair of stars in orbit around their common barycentre with the orbital plane approximately along our line of sight. The fall in brightness, which takes about six hours, occurs as the larger dimmer star occults the smaller brighter one. Zubenelakribi has a period of 2.3 days, so even with the short summer nights you should have several opportunities to spot the magnitude change. Ĵ SEEN IT 3 Xi1/Xi2 & 17/18 LIBRAE 10x Nearly mid-way from Zubenelakribi to 50 Zubenelgenubi are two easy-to-split stellar pairs. First are the orange stars Xi1 (j1) and Xi2 (j2) Librae (mag. +5.8 and mag. +5.4), separated by 0.75°. Only 0.5º northeast of Xi2 is the other pair, 17 and 18 Librae. The two components, which shine at mag. +6.6 and +5.8, are nearly 10 arcminutes apart. In both cases, the proximity is a line of sight illusion: there are about 50 lightyears between 17 and 18, and over 200 lightyears between Xi1 and Xi2. Ĵ SEEN IT 4 KX LIBRAE

From Zubenelgenubi, navigate 5.5º 15x 70 towards Brachium (Sigma (m) Librae). Here you will find a mag. +5.8 star, deep yellow in colour. Look closer and you will see that this is actually a double system, with an 8th-magnitude companion 26 arcseconds to the northwest. The brighter of these is KX Librae, which is a BY Draconis type variable – this means that its variability, which is less than 0.05 magnitudes, is due to either flares or starspots on its surface. This is a true multiple star, with an orbital period of 2,130 years. Ĵ SEEN IT 5 M12

Globular M12 is close to the northeast 10x 50 apex of an equilateral triangle that has mag. +2.7 Yed Prior (Delta (b) Ophiuchi) and mag. +2.5 Han (Zeta (c) Ophiuchi) as its other points. Shining at mag. +6.7, the 75 lightyearwide M12 is one of the larger southern globulars and is an easy object in 10x50 binoculars in moderately good skies. Its core is very indistinct for a globular, so much so it was once suspected of being a compact open cluster. It’s thought that many of its low-mass stars have been stripped away by the Milky Way’s gravity. Ĵ SEEN IT 6 M10

Place M12 near the northwest of the field 10x 50 of view and you should see another, fainter globular. Keeping both in view, mount your binoculars and examine them in turn using averted vision: when you direct your gaze to one of them, you should notice that the other one seems to both grow and brighten. This will also show you that M10 has a much more distinct core than M12. Ĵ SEEN IT