The Great Hercules Star Cluster is one of the true treasures of the summer and early autumn Butler sky. It’s a dense cluster of stars all crammed together in a tight sphere that you’ll love directing your telescope to again and again. I sure do!

As with many celestial treasures, you’ll have to dig for it a bit, but this treasure hunt is certainly worth it. This cluster is one of the true jewels of the heavens.

Mike Lynch

The Hercules Cluster, known formally as Messier Catalog Object No. 13 or M13, is not visible to the naked eye. You should be able to hunt it down with a decent pair of binoculars or a small telescope.

It’s best seen in the generally darker skies of the suburbs or countryside. At the end of evening twilight, M-13 is on the west side of the faint constellation Hercules.

The easiest way to find it is to face west and look up for the two brightest stars you can see in the western sky, Vega and Arcturus. Vega will be the higher of the two. Draw a line between Vega and Arcturus. M13 will be about two-thirds of the way from Arcturus to Vega on that line. Scan that area with your binoculars or a telescope and see if you can spot what looks like a little fuzz ball.

That spot of fuzz is M13, the Hercules Cluster. It’s a gigantic city of at least a half million stars, jammed into a sphere less than 150 light-years across. Even though one light-year equals nearly six trillion miles, it’s still a small area for that many stars to squeeze into. That’s real celestial congestion! With enough magnification and light gathering, you may see some individual stars at the cluster’s edge.

The Hercules Cluster is a prime example of a globular star cluster. All around the night sky, there are hundreds and hundreds of star clusters at any time of the year. Slowly scan the heavens with any old pair of binoculars, and you can’t help but find them. Most of these are open star clusters made of groups of stars recently formed out of the same hydrogen gas cloud. The stars in these open star clusters are generally anywhere from 50 to 500 million years old, which makes them just infant to toddler stars in stellar terms.

Globular clusters like M13 are different. They are spherical swarms of thousands or even hundreds of thousands of stars packed in a small area, usually less than 300 light years in diameter. Globular clusters are comprised of old stars, more than 11 to 12 billion years old.

More than 140 globular clusters form a giant halo around our Milky Way Galaxy. In a way, they are part of the outer structure of our home galaxy, or what some astronomers call satellites of our Milky Way. Because of this, globular clusters are one heck of a long way away.

The Hercules Cluster, M13, is about 25,000 light years away, with just one light year equaling nearly six trillion miles. That’s so far away that the light we see from M13 late this summer left that cluster in the year 23,000 BC!

Another beautiful globular cluster near M13 in the constellation Hercules the Hero is also fairly easy to find. It’s M92, which you can see on the diagram is really close to M13. Scan your telescopes or binoculars about eight degrees above M13, and you should be able to spot M92. Eight degrees is just less than the width of your fist held at arm’s length. M92 is just about as bright as M13, and a little farther away at 27,000 light years.

There are many other globular clusters to find in the night sky. You can locate them with software like Stellarium and smartphone apps like Sky Guide.

Celestial Happening this week: The first quarter moon will be eclipsing the bright star Antares in the southwestern sky around 11 pm. Astronomically this call and occultation. Even with binoculars or a small telescope you’ll be Antares pop behind the darkened eastern limb of the moon. Don’t miss the show!

Mike Lynch is an amateur astronomer and retired broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is the author of "Stars: a Month by Month Tour of the Constellations," published by Adventure Publications and available at bookstores and adventurepublications.net. Contact him at mikewlynch@comcast.net.