Boötes the herdsman is low in the north May to August and quite unremarkable apart from its star Arcturus, which is the fourth brightest star in the night sky overall and the brightest in the northern sky. Arcturus is only slightly more massive than our Sun, but its advanced evolution has taken it off the Main Sequence to become a bloated giant 25 times bigger and 170 times brighter, which gives it a golden orange hue at about 37 light years.
Centaurus the half man half horse represents the scholarly Chiron, tutor of many of the Greek gods and heroes including Heracles, Achilles and Jason. Its two brightest stars of Alpha – the closest star system to the Sun and Beta Centauri – make up the front legs of the centaur. When used with Crux, they help find south. It also contains the most spectacular globular cluster of all, Omega Centauri NGC 5139. Centaurus was one of the original 48 constellations as mapped by Ptolemy and originally included Crux before the latter was identified separately in the 17th century CE.
Crux or the Southern Cross is the smallest of all 88 western constellations and consists of four bright stars. Along with the nearby pointers of Alpha and Beta Centauri, Crux can be used to find south. Draw an imaginary line from the top of the cross shape though the bottom and across the sky. Midway between the pointers draw a line perpendicular to the line that joins them. Where these two longer lines intersect is close to the southern celestial pole. From this point drop to the horizon to locate south. This technique works at any time of the night, any time of the year. The superb open cluster NGC 4755 (Jewel Box) appears very close to the second brightest star, Beta Crucis.
Libra has been recognised historically as scales or a part of Scorpius, forming the scorpion’s claws, however the Romans returned it the separate set of scales we are familiar with today. The former association of Libra and Scorpius is reflected in the names of the two brightest stars in Libra: Zubenelgenubi and Zubeneschamali, meaning ‘the southern claw’ and ‘the northern claw’ respectively. Libra is host to the star Gliese 581, the first star identified with an Earth-like exoplanet.
Ophiuchus the serpent bearer is an ancient constellation and one of the original 48 as mapped by Ptolemy. It is said to represent the mythical healer Aesculapius. It sits close to the galactic centre in Sagittarius. It is now the thirteenth Zodiac sign with the Sun, Moon and planets passing through it from 30 November to 17 December.
