Constellation of Hercules

Venus, 3° above and left of Regulus in Leo, is very bright but low down in the W at nightfall, edging away from the star in the direction of the fainter Mars and Saturn. Our next brightest night-time object after Venus is Jupiter which rises in the E at midnight. The wide swath of sky between Venus and Jupiter is dominated by Arcturus in Bootes, higher in our W evening sky, and by the Summer Triangle of Vega, Deneb and Altair which stands high in the SE at nightfall and crosses the meridian in the middle of the night. Our star chart looks S at 23:00 BST and features the relatively dim region sandwiched between Arcturus and the Triangle. It reaches from the zenith at the top to the horizon at its base, and includes as its two brightest stars Vega in Lyra near the upper-left and Antares, the red supergiant in Scorpius, towards the lower-right. Starwatch regulars will note the reappearance of the Teapot of Sagittarius (SGR) at the lower-left and Northern Crown, Corona Borealis (CrB), in the upper-right. Much of the chart though is filled by the constellations of the hero Hercules and serpent-bearer Ophiuchus whose Alpha stars, Rasalgethi and Rasalhague, stand only 5° apart in the centre. A white star of mag 2.1, Rasalhague lies 47 light years (ly) distant and has a name derived from the Arabic for "Head of the Serpent-charmer". The constellation of the serpent, Serpens, is the only one to be divided in the sky with its head lying to the W (right) of Ophiuchus, while the tail slots between Ophiuchus and Scutum the Shield to the E. Rasalgethi, "Head of the Kneeler", is a red supergiant at some 380 ly. Always fainter than Rasalhague, it shines near mag 3.5 but pulsates semi-regularly over a magnitude or so with a period of about four months. Further N, and on the W edge of the quadrilateral of the Keystone, is the globular star cluster M13. A round smudge to the unaided eye on good nights, and easy through binoculars, this is the brightest globular in the N half of the sky. About half as wide as the Moon, its hundreds of thousands of stars are packed in a region 145 ly across and 25,100 ly away. M92, further N is only slightly fainter, smaller and more distant, but is often overlooked. M5 in Serpens is fainter still, but a little closer than M13, while M22, hovering above the Teapot, outshines even M13 and lies "only" 10,600 ly from us.