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The Great Globular Cluster of Hercules

Great Six-Tailed Comet of 1744
Sketch showing the six tails of the comet in the pre-dawn sky

There are comets, and then there are the big comets. The fire that crossed the sky in 1743 and 1744 was certainly one of the last.

As it passed Earth toward the Sun, the comet would have been bright enough to be seen in daylight and eclipsed Venus in the evening sky. He also developed a long and clearly visible double tail, which was already extremely unusual. Then, as it reached perihelion and orbited the Sun, the comet’s tail split into six clearly defined rays. In the morning, when the comet’s head was still hidden below the horizon, these six tails were bright and visible, reaching into the sky like a kind of “fan” that seemed to come from the Sun.

Why the comet gave this appearance remains a mystery. There may actually only be one or two much larger tails, but they had areas darkened by thick dust. In any case, it was recorded by astronomers around the world, including in China, where court astronomers claimed the comet had in fact made a crackling sound. It was a very strange comet.

A young, not-yet-grown-up Catherine observed the comet as she traveled to Russia to get married. She seemed to consider it a matter of proclaiming her future greatness because… of course she did.

Back in France, young Messier also appears to have seen the comet, and that seems to have done much to push him towards a future in astronomy, rather than the surely fascinating career of bringing people into a courtroom. Messier was able to secure a position as an assistant to Joseph-Nicolas Delisle who was the official astronomer of the French Navy (plotting a course, etc.) and perhaps more importantly, very wealthy.

Delisle had a newly built observatory, and the young Messier quickly settled there. Over the next decade he made a number of important discoveries, earning himself a senior position in government as well as a series of honors and memberships in scientific societies. Predictably, comets remained of particular interest to Messier, and he seemed to be good at spotting a distant comet before other astronomers managed to get their name on the approaching snowball. King Louis XV even gave Messier the absolutely delicious nickname of “the Ferret of Comets” which, if you want to have a title engraved on your tombstone, this should be the one.

But it is Messier’s later work with deep sky objects that is best remembered today. Beginning in 1771, Messier began assembling a catalog of some of these fuzzy spots in the night sky – things we recognize today as nebulae, galaxies and star clusters. The first list included 45 such objects. The final list, which included some objects taken from Messier’s footnotes and marginalia, totaled 110. These became known as Messier objects.

Since then, the discovery of these Messier objects has been something of a right of passage for astronomers. Something like climbing the seven peaks in mountaineering. Except with significantly less chance of dying in an avalanche.

And… okay, Messier 13 turns out to be something variously known as the Hercules Star Cluster, the Great Globular Hercules Cluster, or the Hercules Globular Cluster. Messier was not the first to spot M13. That credit goes to the other comet guy, Edmund Halley, who met him in 1714. But Messier put him in the catalog,

M13 is a group of several hundred thousand stars, but it is not a galaxy. In fact, it’s one of many such blob orbiting our good old Milky Way. It is located approximately 22,500 light years from Earth. If you want to find it, look where its name suggests – in the constellation of Hercules. But bring a telescope. Despite the number of stars in this cluster, it has a visual magnitude greater than 11, too faint to be seen with the naked eye.

M13 is about 100 times denser in stars than the neighborhood around Earth. There are only about 135 stars within 50 light years of Earth. It is interesting to contemplate what a sky with a few orders of magnitude nearest neighbor might look like on a clear night. The stars of M13 are close enough together that every once in a while a couple ends up merging into a short-lived blue-white giant.

Something about M13 has made the globular cluster of Hercules a frequent subject of science fiction novels. Perhaps that’s why, when the SETI folks at the Vanished But Not Forgotten Arecibo Telescope were looking for a target for a test message in 1974, they chose M13. Somewhere between here and there is a post with some basic math info, then expands that to describe the structure of atoms, then elements, then DNA, then some basic facts about human life .

If somebody’s out there and they have a really good receiver, they’ll have mail in about 22,450 years.

As with most of the images I run in this feature, the top image was taken on my tiny but smart Vespera telescope. And, as usual with this feature, I expect some of you have done much better. But probably not better than that…

Hubble Telescope image of M13.

Countdown to Webb“NASA, in partnership with ESA and CSA, will release the first color images and spectroscopic data from the James Webb Space Telescope during a television broadcast beginning at 10:30 a.m. ET on Tuesday, July 12.” And we’ll cover it live.

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