Sometimes we have no clue what's over our heads. On July nights it's easy to find out. As soon as the sky gets dark (around 10:30-11 p.m. local daylight time) look high up in the southeastern sky for the bright, white star Vega. Vega totally overwhelms its home constellation Lyra the Lyre, a tiny harp made of five faint stars. On the plus side, this luminary makes the group easy to find.

Vega stands high in the southeastern sky at nightfall in early July. It's the brightest star in the giant asterism, the Summer Triangle. (Stellarium)
Vega stands high in the southeastern sky at nightfall in early July. It's the brightest star in the giant asterism, the Summer Triangle. (Stellarium)

Now make a fist, hold it horizontally and reach it to the sky. If you mark off two fists (equal to about 20°) to the right of Vega you'll see a small trapezoid outlined by four stars. This bent square is an asterism called the Keystone of Hercules and named for the central stone in the top of an arch that locks and holds it together.

Start at Vega in Lyra, and shoot a line about two fists long (20°) to the right to arrive at the Keystone of Hercules. The globular cluster M13 is located a short distance (2.3°) below the Keystone's upper right corner star. (Stellarium)
Start at Vega in Lyra, and shoot a line about two fists long (20°) to the right to arrive at the Keystone of Hercules. The globular cluster M13 is located a short distance (2.3°) below the Keystone's upper right corner star. (Stellarium)

On the right or western side of the Keystone we arrive at our target, the Great Globular Cluster in Hercules, also cataloged as M13. There are 110 "M" or Messier objects across the sky. Charles Messier, an 18th century French astronomer and comet hunter, gathered them into a catalog so he wouldn't accidentally mistake them for comets during his searches. Most look like small glowing clouds in a pair of binoculars.

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That's exactly what the Hercules globular looks like in my 8x40 and 10x50 binoculars— a small, circular fuzzy spot like it's depicted on the map above. It's quite easy to see even from moderately light-polluted locations.

From mid-northern latitudes M13 is nearly overhead, far above the horizon muck. If you observe from a rural location it's faintly visible without optical aid at magnitude 5.8 — try looking out of the side of your eye, a technique called averted vision. This will expose a more sensitive part of your retina to the cluster.

This photo accurately captures the appearance of the Great Hercules Globular in a 10-inch or larger telescope. The cluster's jammed with so many stars they seem to spill over the sides.  (Mike Brown)
This photo accurately captures the appearance of the Great Hercules Globular in a 10-inch or larger telescope. The cluster's jammed with so many stars they seem to spill over the sides. (Mike Brown)

For a truly good view a moderate-sized telescope is essential. A 6-inch will show the cluster's outer halo of stars, but if you're looking for breathtaking, a 10-inch or larger telescope delivers. Lacking a telescope, we can revel instead in beautiful imagery.

M13 is called a globular cluster because it's a compact, globe-shaped mass of stars. Best estimates put M13's stellar population at around 300,000. Inside the core of M13 roughly 1,000 stars are packed into each cubic light-year, with the average separation of about 0.1 light-year. Compare that to nearest star system beyond the sun, Alpha Centauri, which is 4.4 light-years distant.

In this closer view of the cluster many red stars are visible. These once sun-like stars have evolved over billions of years into red giants. (Sid Leach / Adam Block / Mount Lemmon Sky Center  / CC BY-SA 4.0)
In this closer view of the cluster many red stars are visible. These once sun-like stars have evolved over billions of years into red giants. (Sid Leach / Adam Block / Mount Lemmon Sky Center / CC BY-SA 4.0)

There are only a couple dozen stars of 1st magnitude or brighter in the entire night sky. But if the Earth sat at the center of the cluster we would see thousands of stars brighter than 1st magnitude and hundreds brighter than Sirius. The brightest of them would outshine Venus by 100 times.

In this fanciful scene, where the Earth has been moved to the center of the Hercules Cluster, a sled dog team crosses a landscape lit by starlight. (Bob King / background stars: NASA)
In this fanciful scene, where the Earth has been moved to the center of the Hercules Cluster, a sled dog team crosses a landscape lit by starlight. (Bob King / background stars: NASA)

On a very dark night a few thousand stars spangle the heavens, but from the core of Hercules more than 100,000 stars would spill across the sky. So much starlight would not only make observing fainter objects difficult, but you'd walk around in a dim twilight all night. The only true darkness would come under overcast skies.

There are about 150 known globular clusters in the Milky Way. They're extremely ancient objects with stars aged between 11 and 13 billion years, as old as the galaxy itself. Other star clusters like the Pleiades, called open clusters, are babies in comparison, with ages typically measured in the tens of millions of years.

Knowing a little bit about the "blurry spot" in the Keystone helps us to appreciate its magnificence. And July's the best month to see it.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.