THE GREAT GLOBULAR CLUSTER

WHEN: MID-LATE MAY

The bright orange star Arcturus is easy to locate from the Plough/Saucepan asterism (an asterism is an unofficial pattern). Extend the natural arc of the Saucepan’s handle from the pan and you’ll eventually reach Arcturus.

For this month’s guide, there’s another bright star you’ll need to find too. Vega is a steely blue-white star, fractionally brighter than Arcturus, poised roughly halfway up the sky above the eastern horizon as darkness falls in late May. You can find it by extending a line a little over two-and-a-half times the length of the Saucepan from Merak through the middle of the top edge of the pan.

Head one-third of the way from Vega towards Arcturus and you’ll reach a faint, but distinctive, pattern of four stars. They form the Keystone asterism, which is part of the constellation Hercules the Strongman. The Keystone gets its name from its resemblance to the wedgeshaped stones used to lock a stone arch in place.

If you have dark skies and keen eyes, binoculars, or a telescope, take a look at the point one-third of the way down the western (right) side of the Keystone. The fuzzy blob here is M13, the Great Globular Cluster in Hercules. It’s an ancient object around 11.65 billion years old; a collection of several hundred thousand stars all packed into a spherical volume around 160 light-years in diameter. A light-year is the distance that light travels in a vacuum, in one year, at the speed of 186,282 miles every second – so around 6 trillion miles or 9.5 trillion kilometres.

All the stars in M13 are in orbit around the common centre of mass of the globular cluster. M13 is estimated to lie at a distance of 22,200 light-years from Earth and is one of the best globulars visible from the Northern Hemisphere.