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A great glob under the radar

The cluster M5 lies in an interesting neighborhood.
Harrington
If you were asked to name the greatest visual observer of all time, whom would you choose? There are many worthy candidates, but my vote would have to go to the 18th-century astronomer William Herschel. He had quite a track record, including the discovery of the planet Uranus as well as thousands of deep-sky objects. That’s a pretty good run.

Herschel did more than just look at the sky, however. He also tried to understand what he was looking at. Based on his studies of how stars appear distributed along the hazy band of the Milky Way, Herschel concluded that the Milky Way is shaped like a flattened disk. We know today that he was exactly right.

When it came time to knowing his place in the Milky Way, however, Herschel was way off. He noticed how stars appeared to be evenly distributed along the Milky Way. From this, he reasoned that the Sun must be in the very center. Of course, that isn’t correct. That’s because, unbeknownst to him, cosmic dust blocked his view (as well as ours) along the plane of the galaxy beyond about 6,000 light-years.

ASYPH0518_02
The author describes this asterism near cluster M5 as the “Serpent’s Sailboat.” It’s an easy binocular target.
Tony Hallas
It would be another century and a half before Harlow Shapley discovered our solar system’s true position in the Milky Way. By studying the distances to globular star clusters, Shapley found that most seemed to be distributed in a spherical volume centered not on the Sun, but instead on a distant point in the direction of the constellation Sagittarius. He concluded that globular clusters must hover around the core of the Milky Way — a conclusion we know today to be correct.

While most of the Milky Way’s globular clusters are clumped in Sagittarius, Scorpius, and Ophiuchus, a few renegades are out there. This month, let’s examine a loner found in western Serpens. M5 may be off the beaten path, but this recluse is one of the most magnificent globulars anywhere in the sky.

Gottfried Kirch discovered M5 on May 5, 1702, from Berlin, Germany. His notes described it as a “nebulous star.” Although Kirch was Germany’s first Astronomer Royal at the time, those notes remained unpublished until after Charles Messier independently found M5 on May 23, 1764. Messier described it as a “beautiful nebula; it does not contain any star.” It was only after Herschel viewed it through a larger telescope 27 years later that its true nature became clear.

ASYPH0518_01copy
M5 in Serpens is one of the most brilliant globular clusters in the northern sky, and an attractive target for binoculars.
Adam Block/Mount Lemmon Skycenter/University of Arizona
To find M5, cast off from Arcturus and slide southeastward to 4th-magnitude Zeta (ζ) Boötis. Continue from there twice the distance along a slightly more easterly course until you come to a not-quite right triangle of stars. The triangle’s brightest star, positioned at the not-quite right angle, is 3rd-magnitude Unukalhai [pronounced oo-NOO-kul-hi and also known as Alpha (α) Serpentis]. Next, turn southwest toward the 5th-magnitude star 5 Serpentis. M5 will be just to its north-northwest. Through binoculars, it looks like a fuzzy star, just as Kirch described.

Estimates say that M5 is 24,500 light-years away and may contain as many as 500,000 stars crammed into a space about 165 light-years across. By contrast, M13 — summer’s Hercules Cluster, which we visited in this column in July 2016 — holds about 300,000 stars.

Before we bid farewell to this month’s binocular universe, let’s take a look at an asterism drawn among several stars to the south and east of M5. Added together, they remind me of a sailboat. The boat’s bow is marked by 5 Serpentis, while 6 and 8 Serpentis symbolize the deck. The hull extends to 4 Serpentis and SAO 140444, the faintest star in the pattern. Finally, the sails’ mast extends east-northeast from 6 to 10 Serpentis. The sailboat looks capsized in the evening sky, but if you happen to pull an all-nighter this month, come back just before the onset of morning twilight. That’s when the Serpent’s Sailboat asterism will be most apparent, as it approaches the western horizon.

If you have a favorite binocular target that you’d like to share with the rest of us, I’d love to feature it in a future column. Drop me a line through my website, philharrington.net. Until next time, don’t forget that two eyes are better than one!

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