Messier 13: The Great Hercules Cluster

Hey, sky fans! Happy Friday! I hope you’ve had a great week, and here’s to you for making it through again. I hope you have some great plans, maybe a pizza, maybe a book, ahead of you this weekend. Me, from here, I’ll be taking some time to take care of some non-sky things over the next couple of weeks, so I might be quiet during that time, but I’ll be back soon.

Anyhow, we haven’t had a Friday photo in a while, so, why not?

From Hubble, here’s one of the best and brightest star clusters in the night sky, Messier 13, the Great Hercules Cluster.

This one’s about 22,000 light years away, and has about 300,000 stars pushed into a space about 145 light years (LY) in diameter. Some rough and bumpy high school math that’s farther and farther in the rearview all the time, tells me that’s an average of about about 5 stars per cubic light year within the cluster (I’ll put the math below). For comparison, the next nearest star to your chair, Proxima Centauri, is over four light years away! Things are pretty crowded inside globular clusters, you’re right.

Star clusters generally come in two main flavors: open clusters, which are widespread groups of stars that are pretty loosely bound to each other. The Pleiades and Hyades clusters are two of these, both nearby and easily seen during the north’s winter months.

Open clusters, like M13, are tightly bound groups of stars. They’re mostly older stars, and are far off, orbiting the core of galaxies. The core of the Milky Way is over 20,000 light years away.

I try to stick to mostly naked-eye things around here, but if you have a pair of binoculars, this is a great one. Gorgeous, isn’t it? I love seeing things like these, and I think if the skies hold, I’m going to take out my binoculars this weekend and try to track it down.

Clear skies, everyone. Have a great weekend!

Heart of M13 Hercules Globular Cluster.jpg
By ESA/Hubble and NASA –, Public Domain,

Here’s the math I used above. I hope I got close:

The volume of a circle is 4/3*π*(r^3). The diameter of this cluster is 145 LY, which means its radius is 72.5 LY. This makes its volume about 1,600,000 cubic LY, if we just roughly assume the cluster’s a sphere. 1,600,000 LY divided by 300,000 stars is, on average, a little more than 5 stars per cubic LY.


2 thoughts on “Messier 13: The Great Hercules Cluster

    1. Yeah. I know it would mean instant death, but that aside, I like to imagine what the night sky must look like inside a globular. Or, even, the daytime sky!

      Thanks for reading. I really like your astronomy posts. I have some open in tabs, waiting for me to take the time to read.

      Liked by 1 person

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