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Saturday, August 13, 2022

The Great Globular Cluster in Hercules

Sketch showing the six tails of the comet in the pre-dawn sky

There are comets, and then there are great comets. The blaze that went across the skies in 1743 and 1744 was definitely one of the later. 

As it hurtled past the Earth toward the Sun, the comet was reportedly bright enough to be seen in daylight and outshone Venus in the evening sky. It also developed a lengthy and clearly visible double-tail, which was already extremely unusual. Then, as it reached perihelion and swung around the Sun, the comet’s tail split into six clearly defined rays. In the morning, when the head of the comet was still hidden below the horizon, these six tails were bright and visible, reaching up into the sky as a kind if “fan” that seemed to be coming from the Sun.

Just why the comet gave this appearance is still something of a mystery. It may be that there was actually only one or two much broader tails, but they had areas darkened by heavy dust. In any case, it was recorded by astronomers around the world, including in China, where court astronomers claimed the comet actually made a crackling noise. This was a very odd comet.

A young Catherine the not-yet Great observed the comet as she was traveling to Russia to be wed. She seemed to take it as being all about proclaiming her future greatness because … of course she did. 

Back in France, young Messier seems to have also seen the comet, and it seems to have gone a long way in shoving him toward a future in astronomy, rather than the surely fascinating career of walking 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 (charting a course, etc.) and perhaps more importantly, filthy rich.

Delisle had a freshly-built observatory, and the young Messier settled into quickly. Over the next decade, he made a number of significant discoveries, earning himself a senior position with the government as well as a string of honorifics and scientific society memberships. As might be expected, comets remained a particular interest of Messier, and he seemed to be good at winkling out a distant comet before other astronomers managed to get their name on the approaching snowball. King Louis XV even gave Messier the absolutely delightful nickname of “the Ferret of Comets” which, if you’re going to have one title carved on your tombstone, that should be the one.

But it was Messier’s later work with deep sky objects for which he is best remembered today. Starting in 1771, Messier began assembling a catalog of some of those fuzzy spots in the night sky—things that we recognize today as nebula, galaxies, and star clusters. The first list included 45 such objects. The final list, which including some objects pulled from Messier’s footnotes and marginalia, totaled 110. These became known as the Messier objects. 

Ever since, finding these Messier objects has been something of a right of passage for astronomers. Something like climbing the seven summits in mountain climbing. 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 Cluster in Hercules, or the Hercules Globular Cluster. Messier wasn’t actually the first to spot M13. That credit goes to the other comet guy, Edmund Halley, who came across it in 1714. But Messier put it in the catalog, 

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

M13 is about 100x more dense with stars than the neighborhood around Earth. There are only about 135 stars within 50 light years of Earth. It’s interesting to contemplate what a sky with a couple of orders of magnitude more close neighbors might look like on a clear night. The stars in M13 are close enough together than every now and then a couple end up merging into a short-lived blue-white giant.

Something about M13 has made the Hercules globular cluster a frequent subject of science fiction novels. That may be why when the SETI folks at the gone-but-not-forgotten Arecibo telescope were looking for a target for a test message in 1974, they picked M13. Somewhere between here and there is a message containing basic information about math, then expands on that to describe the structure of atoms, then elements, then DNA, then some basic facts about human life.

If someone is out there, and they have a very 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-clever Vespera telescope. And, as usual with this feature, I expect some of you have done a lot better. But probably not better than this…

Hubble telescope image of M13.

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

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