Targeting the Hercules Star Cluster

Finding Hercules and seeing what it looks like
The constellation Hercules contains the globular cluster M13, the Great Hercules Cluster. This chart gives hints on how to find it and what it looks like through good binoculars or a small telescope. Carolyn Collins Petersen/Rawastrodata CC-by-.4.0

In 1974, astronomers using the Arecibo radio telescope beamed a coded message to a star cluster that lies just over 25,000 light-years from Earth. The message contained information about the human race, such an image of our DNA, atomic numbers, Earth's position in space, a graphic figure of what humans look like, and a graphic of the telescope used to send the radio message out to space. The idea of sending this information, and other data, was to celebrate the remodeling of the telescope.

It was an evocative idea, and although the message won't arrive for 25,000 years yet (and a reply wouldn't come back for at least 50,000 years), it still served as a reminder that humans are exploring the stars, even if only with telescopes. 

Targeting the Cluster From Your Backyard

The cluster the scientists sent the message to is called M13, or more familiarly as the Hercules Cluster. It can be spotted from a reasonably good dark-sky viewing site, but is pretty dim for naked-eye viewers. The best way to look for it is with binoculars or a small telescope. Once you spot it, you'll be seeing the light of hundreds of thousands of stars all held together in a roughly globe-shaped region of space. Some astronomers estimate there could be a million stars in M13, making it incredibly dense. 

The Hercules Cluster is one of 150 known globular clusters that orbit the core of the Milky Way. It's visible in the evenings during thelate winter months of the northern hemisphere and well into spring and early summer, making it a favorite of amateur observers.


To find the Hercules Cluster, locate the Keystone of Hercules (see the star chart).  The cluster lies along one side of the Keystone. There's also another globular cluster in nearby, called M92. It's considerably dimmer and a bit tougher to find. 

The Specs on Hercules

The Hercules Cluster's hundreds of thousands of stars are all packed into a region of space only 145 light-years across.

Its stars are mainly older ones, ranging from coolish red supergiants to blue-white, superhot giants. Hercules, like the other globulars that orbit the Milky Way, has some of the oldest stars around. Chances are these stars formed before the Milky Way did, some 10 or so billion years ago.

The Hubble Space Telescope has studied the Hercules Cluster in detail. It peered into the densely packed central core of the cluster, which has stars packed together so tightly that any planets (if they exist) would have extremely starry skies. The stars in the core actually are so close to each other that occasionally they collide with each other. When that happens, a "blue straggler" is formed, the name astronomers give to a star that's incredibly old, but looks young due to its blue-white color.

When stars are crowded together as they are in M13, they're hard to tell apart. Hubble was able to discern many individual stars, but even it had trouble picking out individual stars in the very densest part of the cluster's central region. 

Science Fiction/Science Fact

Globular clusters such as the Hercules Cluster were the inspiration for Dr. Isaac Asimov to write a famous science-fiction story called Nightfall.

Asimov was challenged to write a story illustrating a line by Ralph Waldo Emerson, who wrote: "If the stars should appear one night in a thousand years, how would men believe and adore, and preserve for many generations the remembrance of the city of God!"  

Asimov took the story one step further and invented a world in the center of a six-star system in a globular cluster where the skies were dark only one night every thousand years or so.  When that happened, the inhabitants of the planet would see the stars of the cluster. Since generations passed between the appearance of the stars, Asimov's story had people going mad because of things they'd never seen. Civilization would fall, and then take a thousand years to pick up the pieces.

It turns out that planets CAN exist in globular clusters.

Astronomers found one in the cluster M4, and it's possible that M13 also contains worlds circling among the starry regions. If they do exist, the next question would be whether planets in globulars could support life. There are many obstacles to the formation of planets around stars in a globular cluster,  so the barriers to life could be quite high. But, if planets DO exist in the Hercules Cluster, and if they bear life, then perhaps 25,000 years from now, somebody will get our 1974 message about humans on Earth and conditions in our neck of the galaxy. Think about THAT as you head out to gaze at the Hercules Cluster some night!