Quite possibly the most celebrated object in the summertime skies is the Great Cluster in Hercules, known also as M13. Anyone who has visited the summer gathering of amateur astronomers near Springfield, Vermont known as Stellafane, knows that this famous cluster is often on display in the observatory that houses the famed Porter turret telescope.

To locate M13, look toward the four stars, known as the "Keystone" which supposedly forms the body of Hercules. A keystone is the stone atop an arch, and has this shape, narrower at one end.

It's between the two western stars of the keystone that we can find the Great Globular Cluster of Hercules. It's about a third of the way along a line drawn from the stars Eta to Zeta. Though the object carries an "M" designation from Charles Messier's catalogue of fuzzy objects masquerading as comets, it was not Messier, but Sir Edmund Halley of comet fame who first mentioned it in 1715, having discovered it the previous year: "This is but a little Patch," he wrote, "but it shows itself to the naked Eye, when the Sky is serene and the Moon absent."

Located at a distance of about 25,000 light-years, the Hercules Cluster has been estimated to be a ball of tens of thousands of stars roughly 160 light-years in diameter.

Messier first saw the cluster in June 1764 and described it as a "round and brilliant nebula with a brighter center, which I am sure contains no stars."

Today, if you use good binoculars and look toward that spot in the sky where M13 is you likely will see a similar view: a roundish glow or patch of light. Moving up to a telescope, the view dramatically improves. With a 4 to 6-inch telescope, the "patch" starts to become resolved into hundreds of tiny pinpoints of light. In larger instruments, M13 is transformed into a spectacular celestial chrysanthemum.

In his "Celestial Handbook," Robert Burnham describes the view of the cluster in a 12-inch or larger telescope as "…an incredibly wonderful sight; the vast swarm of thousands of glittering stars, when seen for the first time or the hundredth, is an absolutely amazing spectacle."

Truly a grand cosmic chrysanthemum!

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