The Great Hercules star cluster is one of the true treasures of the summer and early autumn Marshall sky. It’s a dense cluster of stars all crammed together in a tight sphere that you’ll love to direct your telescope to it 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!
The Hercules Cluster, known formally as Messier Catalog Object number 13 or M-13, 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, especially in the generally darker skies of the suburbs or the countryside. At the end of evening twilight, M-13 can be found on the west side of the faint constellation Hercules. I think 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. Just make sure you don’t confuse the super bright planet Jupiter in the low southwestern sky for these stars. Draw a line between Vega and Arcturus. M-13 will be just short of the halfway point. Scan that area with your binoculars or telescope and see if you can spot what looks like a little fuzz ball.
That little fuzzball is M-13, the Hercules Cluster. It’s a gigantic city of about a million stars, jammed into a sphere less than 150 light- year across. Even though one light-year equals nearly 6 trillion miles, that’s still a small area for that many stars to be crammed 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, anytime of the year, there are hundreds and hundreds of star clusters. Just 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 up of groups of stars that 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 in stellar terms would make them just infant to toddler stars.
Globular clusters like M-13 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 made up 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 a heck of a long ways away. The Hercules cluster, M-13, is about 25,000 light-years away with just one light- year equaling nearly 6 trillion miles. That’s so far away that the light we see from M-13 late this summer left that cluster in the year 23,000 B.C.
There’s another beautiful globular cluster not all that far away from M-13 in the constellation Hercules the Hero that’s also fairly easy to find. It’s M-92, which you can see on the diagram is really close to M-13. Scan your telescopes or binoculars about eight degrees above M-13 and you should be able to spot M-92. Eight degrees is just less than the width of your fist held at arm’s length. M-92 is just about as bright as M-13 and a little farther away at a distance of 27,000 light years.
There’s many other globular clusters to find in the night sky. You can locate them with software like Stellarium and smart phone apps like Sky Guide.
CELESTIAL HUGGING THIS WEEKEND: This weekend the waxing gibbous moon will be passing by the bright planet Saturn just above the constellation Sagittarius. Look in the early low southwestern sky. You’ll love it!
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net.