The Great Hercules Cluster

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6205

Type Globular cluster

Class V

Distance 25,890 ly (R2005) 24,530 ly (CMD, 1998)

Size 160 ly

Constellation Hercules

R.A. 16h 41.7min

Decl. +36° 28′

Magnitude 5.7

Surface brightness –

Apparent diameter 21′

Discoverer Halley, 1714

History In 1714, Edmond Halley noticed, more or less by accident, a “nebulous patch” in the constellation of Hercules. A year later, he wrote up a list of the six nebulae then known to him (M 42, M 31, M 22, ω Cen, M 11, and M 13) and commented about the latter: “This is but a little patch, but it shows itself to the naked eye, when the sky is serene and the moon absent.” Charles Messier observed M 13 on the 1st of June 1764, and noted: “Nebula without star, discovered in the girdle of Hercules; it is round & brilliant, the center brighter than the borders; it is near two stars, both of 8th magnitude, one of them above & the other below; 6′ diameter.”

It was left to William Herschel to recognize the true nature of this star cluster in 1784: “A most beautiful cluster of stars. It is exceedingly compressed in the middle and very rich. About 14,000 stars can be counted.” 40 years later, John Herschel commented: “Very rich cluster; irregular figure; very large; very gradually much brighter toward the middle; stars from 10th to 15th magnitude, of which there must be thousands; does not come up to a nucleus; has hairy-looking curvilinear branches.” This impression was shared by Lord Rosse, who pointed his large 72-inch mirror towards M 13: “singularly fringed appendages to the globular figure out into the surrounding space.” His drawing shows a Y-shaped dark structure over the whole central region of the cluster. D'Arrest, too, saw “arm-shaped outliers” with a magnification of 95×.