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M13, The Great Hercules Cluster

TargetTypeR. A.Dec.ConstellationMagnitudeSize
M13 (NGC 6205)Globular Cluster16h 41m 41s+36° 27′ 36″Hercules5.820.0 arcmins

Overview

The Hercules cluster, the 13th object in the Messier catalog, is an easy to locate, rich cluster in the constellation of Hercules.

It’s located in a sparsely populated area of sky and rides high overhead in the summer evening sky. The Great Hercules Cluster is, for many astronomers, the best cluster in the northern hemisphere. It’s also a well-signposted object with easily recognizable stars pointing our way towards it.

It was discovered by Edmond Halley (of Halley’s Comet fame) in 1714, who saw it with his naked eye – this being a time with significantly less light pollution than we modern astronomers have to contend with. Messier didn’t catalog it until fifty years later, in 1764.

We’re not certain of the exact number of stars in this colossal cluster, but it’s in the order of 300,000 to half a million, and all of them in an area approximately 135-150 light-years across. The Hercules Cluster is relatively nearby for such a huge object, being only 24,000 light-years away from Earth, which is another reason it is so visually stunning.

This stunning cluster is thought to be about 12 billion years old and shines at magnitude +5.8, making it just about visible to the naked-eye under dark skies. The cluster is large too, at 20 arcminutes across, it is wider than the first quarter moon. In reality, however, nearly all of us will need binoculars or a low-power telescope view to enjoy it.

Let’s take a look at it for ourselves.

M13 Cluster in Hercules, as imaged by NASA (source)

Finder Charts

This first sky chart, from SkySafari 6, shows the sky looking south at 10:00 p.m. in mid-August. M13 is found in what’s known as the ‘Keystone Asterism’ in the constellation of Hercules. This is circled in orange in the chart below and easily identified above (east) of the bright star Arcturus, in Boötes. Alternatively, the handle of the Big Dipper (Plough) points in the direction of the Keystone – you can see the Big Dipper in the middle of the star chart below, on the right hand side.

Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.

[M13-1] Begin by identifying the Keystone asterism using Arcturus to point the way. Click for full-screen.

Blowing up the trapezoid shape of the Keystone asterism in our second chart, which also shows stars to magnitude 5.0, you can see that it lies on the line connecting Eta Herculis to Zeta Herculis. Both of these are bright stars, easy to identify under all except the brightest skies.

M13 lies slightly closer to Eta Herculis and shows up in as an unmistakable blurry patch in binoculars and magnifying finderscopes.

[M13-2] This bright cluster has its home between the two bright stars on the western side of the Keystone asterism. Click for full-screen.

Our third sky chart shows stars to magnitude 8. We’ve added a blue circle to show a 1° field of view as well If you click on the image to make it full screen, you’ll note two stars to the left of the green circle, one above and one below. These are magnitude 7 stars and will look very different to the cluster itself when you spy them with any magnification.

[M13-3] M13 is easy to locate and given away by its distinctly ‘fuzzy’ nature in the eyepiece. Click for full-screen or download the pdf below.

Now you have this beautiful cluster in your eyepiece, what should you expect to see?

Individual Telescope Views

The following views will help you find M13 in different telescope types by presenting the images as your telescope will show them. The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Stars are shown to magnitude 13.0 and the larger circle is a 1° field of view, as shown in the third star chart above.

Each image can be clicked on for a full-screen version.

Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope

[M13-4]
[M13-5]

Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal

[M13-6]
[M13-7]

Mirrored View – Refractors and Cassegrain models with a star diagonal show this view

[M13-8]
[M13-9]

Observation

M13 is arguably the most impressive globular cluster north of the celestial equator, though a good case can be made for M5 in the constellation of Serpens.

The question of which is better comes down largely to personal taste; looking back at my notes over the years I can see that I’ve vacillated between the two. 

For newcomers to the hobby, M13 has the edge over M5 for two reasons: It’s easier to find; and, due to its more northerly declination, it remains visible for a longer stretch of the year.

SMALL TELESCOPES

As mentioned earlier, the brightest stars in M13 are around 12th magnitude, which puts them within reach of a 4-inch scope under a dark sky.

In my 4-inch refractor, from a suburban location, M13 was always on the threshold of being resolved, but never quite to my satisfaction.

At 111x it appeared as a grainy snowball of congealed light, prickling with faint stars that popped in and out of view depending on which side of the cluster I was looking at.

My notes from this time describe it as “spidery” and “loose”, with particular reference to two chains of stars that curl out from the cluster like wings.

MEDIUM-TO-LARGE TELESCOPES

That frustrating “almost-but-not-quite-resolved” aspect of globular clusters was one of the major factors that swayed me into buying a 10-inch reflector. When I took my new scope outside for the first time, M13 was the obvious initial target – and I wasn’t disappointed.

At medium powers (90x – 150x) I was able to resolve pinpoint stars (in both direct and averted vision) right across the face of the cluster, giving an impressive sense of scale and distance.

Overall, the Great Cluster looked like a sprawling sea monster compared to the more condensed M5, with prominent chains of stars curving out from the center.

Again, as mentioned earlier, if your scope can reach as deep as 15th magnitude then M13 will show you hundreds of stars. Use higher magnifications (from 150x to 250x) and averted vision to draw out these fainter stars from the cluster.

Now look more closely and you should be able to see a dark Y-shaped feature just southeast of the core. This is popularly known as the “propeller”, a contrast effect caused by chance alignments of regions containing bright and not-so-bright stars. It took me several attempts before I spotted it for the first time, but once I did I wondered how I’d missed it.

Finally, just because guides like this one recommend using particular magnifications to get the best views of particular objects, that doesn’t mean you shouldn’t occasionally experiment with higher ones.

I recently used a Powermate to observe M13 at a whopping 428x. The stars were a little mushy because of the seeing, and at this magnification, it looked less like a sea monster and more like a hanging basket brimming over with unruly flowers. But for a few fleeting moments when the seeing did stabilize, I was treated to the unforgettable sight of an entire field of view sprinkled with ancient starlight.


PDFs for Printing

Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [M13-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.

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