Hercules

A big constellation with modest stars

Hercules covers more sky than all but four other constellations, but none of its stars reaches first magnitude. From a bright city it can be hard to see at all, and even under a dark sky it looks like a scatter of medium stars rather than a striking figure.

That is why observers learn Hercules through one small pattern, the Keystone, and through the two famous star clusters that sit beside it. Once you know where to look, it becomes one of the most rewarding parts of the summer sky.

How to find it

On July evenings at mid-northern latitudes, look high overhead between two bright stars: blue-white Vega in Lyra to the east and orange Arcturus in Boötes to the west. Hercules fills much of the gap between them.

Inside that gap, look for the Keystone, a slightly lopsided square of four stars named Pi, Eta, Zeta and Epsilon Herculis. It forms the hero's body. Fainter lines of stars run out from its corners as arms and legs, so the whole figure looks a bit like a pinwheel.

From the Southern Hemisphere, Hercules stays lower in the northern sky and is highest around July. There the figure appears the right way up. From Sydney, at about 32°S, its brightest star climbs to about 35 degrees above the northern horizon.

The main stars and their names

Kornephoros (Beta Herculis) is a yellow giant of magnitude about 2.8, roughly 140 to 150 light-years away, and the brightest star in the constellation. Zeta Herculis, at one corner of the Keystone, is almost as bright and much closer, about 35 light-years away.

Rasalgethi (Alpha Herculis), about 360 light-years away, marks the hero's head. Its Arabic name means 'head of the Kneeler'. It is a variable star whose light rises and falls irregularly, roughly between magnitude 2.7 and 4, and it belongs to a system of three stars.

Rasalgethi lies not far from Rasalhague, the brightest star of neighbouring Ophiuchus, whose name means 'head of the snake handler'. Beginners often mix up the two heads. Hercules also marks the general direction of the Sun's own motion: William Herschel found that the Solar System is moving toward a point near the star Lambda Herculis.

M13 and M92: ancient balls of stars

A globular cluster is a tightly packed ball of hundreds of thousands of very old stars that orbits the centre of our galaxy. M13, the Great Cluster in Hercules, shines at magnitude about 5.8. It sits on the western side of the Keystone, about two-thirds of the way from Zeta to Eta and about 2.5 degrees from Eta.

M13 lies about 22,000 to 25,000 light-years away (estimates differ) and holds about 300,000 stars packed into a ball roughly 150 light-years across. Its stars are about 12 to 13 billion years old. Edmond Halley found it in 1714, and Charles Messier added it to his catalogue in 1764.

M92 is often overlooked only because M13 is so close. It forms a triangle with the two northern stars of the Keystone, about 6.5 degrees north of Pi Herculis. At magnitude about 6.3 and roughly 26,000 to 27,000 light-years away, it is one of the oldest star clusters in the Milky Way. Johann Elert Bode discovered it in 1777.

Earth's axis slowly wobbles, a motion called precession, so the pole star changes over thousands of years. In about 14,000 years M92 will lie less than 1 degree from the north celestial pole.

A message to M13

On November 16, 1974, during a ceremony at the Arecibo Observatory in Puerto Rico, the giant radio dish sent a short message toward M13. It was 1,679 bits long, arranged in 73 lines of 23 characters to form a simple picture. A team led by Frank Drake, including Carl Sagan, designed it.

The signal travels at the speed of light, so it will need about 25,000 years to reach the cluster, and any reply would need just as long to come back.

Mythology

The oldest Greek star lists, starting with the poet Aratus in the 4th century BCE, describe a nameless figure on one knee, called Engonasin, 'the kneeling one'. Later writers named him Heracles, the hero the Romans called Hercules, son of Zeus (Jupiter) and famous for his twelve labours.

One ancient reading, given by Eratosthenes, shows Heracles standing over the dragon that guarded the golden apples of the Hesperides. On star charts the hero's foot rests near the head of Draco, the Dragon, his neighbour in the sky. The crab of Cancer, another constellation, also appears in his stories.

Practical observing tips

Start soon after dark between June and August, when Hercules is high and away from horizon haze. Give your eyes about 20 minutes to adapt, then find Vega and Arcturus first and look for the Keystone between them.

With binoculars, M13 looks like a small, round, fuzzy ball, clearly different from a star. A small telescope shows a brighter glow with a grainy edge, but breaking it into separate stars needs a larger telescope and higher magnification. Look at M92 the same night: comparing the two clusters side by side is the best way to appreciate both.