Image: Constellation Hercules, cluster M13
Starry Night Software /
The constellation Hercules and the cluster M13 high in the overhead sky in summer 2010.
updated 7/16/2010 2:05:41 PM ET 2010-07-16T18:05:41

A fascinating deep sky object called the Great Globular Cluster can currently be found in telescopes looking nearly overhead on clear summer nights, and there's good story behind how it and other objects are classified.

How often in looking through books on astronomy have you noticed star clusters and nebulae designated by the letter M, followed by some number? For example: the famous Andromeda Galaxy is known as M31 and the Great Orion Nebula is M42.

The Great Globular Cluster is known as M13 and sits the constellation Hercules. The 'M' stands for the initial of the famed 18th century comet observer, Charles Messier (1730-1817).

Messier was deeply interested in discovering comets but he was plagued by the same trouble that besets all comet hunters. He kept finding comets that were not comets at all but only star clusters and nebulae. His hopes were dashed so often that for his own convenience he kept a list of these deceiving objects, which he published in a catalogue. (Photos: Messier Objects in space)

We will come back to Messier in a moment. First, a few words about the M-object number 13 (or Messier 13 as the Great Globular Cluster is known) in his catalogue.

Celestial chrysanthemum
To locate Messier 13, 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. Its between the two western stars of the keystone that we can find the Great Globular Cluster of Hercules. Its about a third of the way along a line drawn from the stars Eta to Zeta.

Actually, it was not Messier, but 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, Messier 13 is transformed into a spectacular celestial chrysanthemum.

In his Celestial Handbook, the late Robert Burnham described 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.

He couldn't care less
Many of the objects listed in Messiers catalogue have turned out to be beautiful star clusters like that in Hercules. Others are great clouds of nebulosity, while still others are galaxies similar to our own.

Yet, there is absolutely no reason to believe that Messier had the slightest interest in these objects. They were all merely a nuisance to him. The 13 comets that Messier discovered and of which he was so proud of are long gone and forgotten now.

But his catalogue, the by-product of his main work, has turned out to be amazingly useful to astronomers. His catalogue numbers have been retained and are the principal reason Messier is still remembered today.

Messiers original list contained 45 such objects and was published in 1774. By 1781, the list had grown to 103.

Historians have since added seven more objects that were seen, but never catalogued by Messier. It has been written by many that Messier had consummate skill in making astronomical observations, even though by contemporary standards the telescopes he utilized were inefficient and crude (that fact is made obvious by his description of the Great Cluster in Hercules).

Nonetheless, he was highly ambitious and always sought recognition of his observing skills and comet discoveries.

One oft-repeated anecdote demonstrates this single-minded and zealous pursuit for finding comets. On the occasion that a rival, Jacques Leibax Montaigne (1716-1785?) discovered a brand new comet, Messier was holding watch at his wifes deathbed.

When a friend later offered condolences for his loss, Messier thinking only of the comet answered, I had discovered twelve, alas, to be robbed of the thirteenth by that Montaigne! His eyes filled with tears. Then, realizing his friend was commenting about his dead wife, Messier quickly added Ah! The poor woman

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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