The Great Cluster in the constellation Hercules – also known as Messier 13, or M13 – is considered to be the finest globular cluster in the northern half of the heavens. It’s found in a star pattern called the Keystone – a lopsided square within the constellation Hercules – between the two brightest stars of northern spring and summer, Vega and Arcturus. Follow the links below to learn more about M13.
How to find M13. M13 is not the easiest of sky objects to spot, but once you find it, you’ll be able to go back to it again and again. It’s located in the constellation Hercules, between summertime’s two brightest stars, Vega and Arcturus.
About one-third the way from Vega to Arcturus, locate the four modestly bright stars forming the Keystone of Hercules. On the Arcturus side of the Keystone, M13 is found between the stars Eta Herculis and Zeta Herculis. On a dark, clear night, the unaided eye barely perceives the Hercules cluster as a faint and possibly fuzzy point of light. This “fuzzy” star is much easier to make out in binoculars. A typical binocular field is about 5 to 6 degrees in diameter, and the Hercules cluster is found about 2.5 degrees south of Eta Herculis.
Globular star clusters are huge globe-shaped stellar cities teeming with tens to hundreds of thousands of stars. Globular clusters orbit the Milky Way galaxy outside the galactic disk at tens of thousands of light-years away. In contrast, the relatively nearby Pleiades and Hyades open star clusters reside within the galactic disk, and usually harbor a few hundred to a thousand stars.
The best way to see this beautiful cluster. Like all globular clusters, the Great Cluster in Hercules is best viewed through telescopes with large apertures (light-gathering capability). Otherwise, the stars in this cluster at 25,000 light-years away are hard to resolve.
M13 looks like a dim, fuzzy star to the unaided eye. In binoculars and low-power telescopes, this cluster looks somewhat like a hazy mothball, resembling a comet in appearance. In fact, the famous comet hunter Charles Messier (1730-1817) listed M13 in his Messier catalog, to steer comet seekers away from this masquerade comet.
If you don’t have a telescope, or are unfamiliar with how to use one, how about attending a public star party? That way you can view M13 through an assortment of telescopes. This bejeweled ball of perhaps a half million suns is well worth checking out on a clear, dark night.
At mid-northern latitudes, the M13 cluster can be found in the sky for at least part of the night all year round. It’s up all night long in May, June and July. In August and September the Hercules cluster is still very much a night owl, staying up till after midnight.
Brighter globular star clusters. The Southern Hemisphere sky can boast of two globular star clusters that are more massive and brighter than the Great Hercules cluster: Omega Centauri and 47 Tucanae.
Omega Centauri can be seen – but not very well, only low in the southern sky – from the southern half of the United States and similar northern latitudes at certain times of the year. You have to travel farther south than Hawaii to catch even a glimpse of 47 Tucanae.
And that leaves us in the north with the Great Cluster in Hercules. A fine consolation prize it is! Of the Milky Way’s 250 or so globular clusters, the Great Hercules cluster reigns supreme in the northern hemisphere, or north of the celestial equator. If you get a taste for globular clusters, be assured that many more await for you in the northern skies. There are 29 globular clusters listed in the Messier catalog alone.
Globular star clusters, unlike open star clusters such as the Pleiades, are tightly held together by gravity. Whereas open clusters break up after hundreds of millions of years, globular clusters remain intact after billions of years.
When you gaze at M13 or other globulars, you are looking at stars that are thought to be 12 to 13 billion years old. That’s almost as old as the universe.
Hercules cluster’s position is Right Ascension: 16h 41.7m; Declination: 36 degrees 28′ north
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.