STEPHEN TONKIN’S BINOCULAR TOUR
A pretend comet, a giant star, a celestial picture puzzle and more sights to see around Hercules
Tick the box when you’ve seen each one
1 M13 10x 50
About a third of the way down the western side of Hercules’s Keystone asterism, you’ll find a distinct ‘fuzzy blob’, which you might even be able to see with your naked eye in very transparent skies. In binoculars, the Great Globular Cluster in Hercules looks like a comet, brightening towards the core. This is why Charles Messier included it in his famous list of objects that comet hunters should not be fooled by. Even in urban skies, M13 should be detectable in 10x50 binoculars. This ball of about 300,000 stars is 22,200 lightyears away and has a diameter of 145 lightyears. SEEN IT
2 30 HERCULIS 10x 50
The distinctly orange 30 Herculis (also known as g Her), lies 1° west-southwest of Sigma (m) Herculis. This semi-regular variable star swings between about mag. +4.4 and mag. +5.5 with a period of 73 to 93 days between maxima. It has a radius of 230 Suns, larger than Earth’s orbit. Despite its size, its mass is similar to the Sun’s, and so it shows us how our star will evolve. It is preparing to lose its outer shell as a planetary nebula and leave its core to decay as a white dwarf. SEEN IT
3 TAU CORONAE BOREALIS GROUP 10x 50
From 30 Her, head 6.5° southwest to find mag. +4.7 Tau (o) Coronae Borealis, the brightest in a straight line of five stars running east-west for 2.6°. All but the central one shine brighter than mag. +6, and binoculars reveal their colours. Notice that the mag. +5.6 stars at each end of the chain are a deeper yellow than the others, while the star at the eastern end is almost white. Under dark skies you should find that the fainter central star easily resolves into a widely separated triple star. SEEN IT
4 DELTA BOÖTIS 10x 50
Our next stop is another multiple star, the mag. +3.5 Delta (b) Boötis. The primary in this pair is a giant star that shines a deep yellow and is nearly 60 times more luminous than the Sun. Its paler mag. +7.8 companion appears to be 105 arcseconds away. Like 30 Her, it is another Sun-like star, but this one is earlier in its evolutionary development. This binary pair is 117 lightyears away; they have a separation of at least 0.6 lightyears and an orbital period of at least 120,000 years. SEEN IT
5 H V 38 15x 70
We’ll switch up to larger binoculars for our final two objects. Midway between Xi (j) and Nu (i) Coronae Borealis, is an unremarkable mag. +6.4 white star, H V 38. The ‘H’ refers to Herschel’s double star catalogue; the ‘V’ tells us that the separation is between 30 and 60 arcseconds. The companion, which lies in the direction of Nu, is at the lower end of this range (32 arcseconds), and it is a mere mag. +9.7, which, unless you have good skies, could be near the limit of your binoculars. SEEN IT
6 HARRINGTON 7 15x 70
Identify Kajam (Omega (t) Herculis) and pan 2° west to a golden 8th magnitude star. This is part of a 1.3°-long chain of fainter stars that runs roughly north-south. Currently best seen in the early evening, it’s an interesting test of pareidolia – our inclination to find pictures in abstract images. It’s been variously described as a zigzag, a dragon, a tadpole and a flower. What do you see? SEEN IT