Popular Astronomy

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Hercules and Corona Borealis

Considering all the legends about Hercules, the great hero of many an amazing exploit, he gets a rather raw deal when it comes to the stars. None of his stars are particularly bright, and he is kept well away from the other macho types, such as Perseus and Orion. But he does attract attention, because of two star clusters, about which more below. You can find him between the distinctive semicircle of Corona Borealis, the Northern Crown, and the bright star Vega,in Lyra.

The most distinctive feature of Hercules is a group of four stars known as the Keystone. Once you've spotted this, the rest of the constellation is easier to pick out.

Map of Hercules
For hints on understanding the star map, please click here

Things to look for in Hercules

M13 and M92 compared, credit Robin Scagell

At one time, the globular cluster M13 was known as 'The Great Hercules Cluster'. That was before the days of jet travel, when most people didn't have the chance to go to the southern hemisphere and see for themselves a really great cluster such as Omega Centauri. But some of us have to make the best of what we've got, so maybe we should call it 'The Fairly Good Hercules Cluster', or just use its catalogue number of M13.

It is known as a globular cluster, because of its shape. In fact, globular clusters are really far more amazing than ones we can see better, such as the Pleiades, because they contain getting on for a million stars, whereas most other clusters have only a few hundred. But most globulars are much more distant, so we need a fairly good telescope to see the individual stars.

But you can see M13 with binoculars, and occasionally even with no optical aid at all, if you have good eyesight and really clear skies. Even with binoculars you may have to look careully, because M13 is quite small compared with most other star clusters. In fact, you might even mistake it for a star. Look about two thirds of the way up the right-hand side of the Keystone to find it. It looks like a circular misty patch with small telescopes, but larger ones start to show individual stars and with really big back-garden telescopes it can look spectacular.

Also in Hercules is another globular cluster, M92. If it weren't for M13, it would be more popular, but it's the poor relative. So you should feel sorry for it, and give it a look. Find it by making a slightly distorted triangle with the top two stars of the Keystone.

Text by Robin Scagell