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Challenge #1 – M13, The Great Hercules Cluster

TargetTypeR. A.Dec.ConstellationMagnitudeSize
M13Globular Cluster16h 42m 25s+36° 25′ 19″Hercules+5.820 arcminutes

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 at this time of year. Furthermore, it is a well signposted deep sky object, there are easily recognizable stars which point our way towards it.

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

Tying down how many stars the cluster contains is not easy, different sources have it anywhere from 100,000 to a million, or more! All of them occupying an area approximately 135-150 light-years across. The Hercules Cluster is believed to be in the order of 23,000 light-years away from Earth.

In our skies it shines at magnitude +5.8 – making it visible to the naked-eye under dark skies – and is 20 arcminutes across, which is wider than the first quarter moon. In reality, 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.

Finder Charts

To begin, we need to locate the constellation of Hercules. The image below shows the sky from mid-US latitudes at 10:30pm mid-month, Hercules is high above the eastern horizon, almost overhead. We need to locate the asterism (like a mini-constellation) known as the keystone, which is the not-quite-square shape in the middle of Hercules.

I’ve shown it with an orange line in the SkySafari 6 image below, which you can click on for a full-screen version.

Locate the Keystone asterism in Hercules over the eastern horizon in June evenings.

The next image zooms in to the Keystone and shows the four stars which mark its corners, Eta (magnitude 3.5), Zeta (2.8), Epsilon (3.9) and Pi Herculis (3.1). These are all bright stars, making them easy to pick out in this part of the sky, even under moderate light pollution.

There are two images below, the one on the left is color, the one on the right is inverse monochrome. Some observers prefer to use the inverse monochrome outside at their telescope (it’s also easier on the printer ink!). Click on the images for full-screen versions.

Color chart for the Keystone stars.
Inverse monochrome for the Keystone stars.

The final step to locate M13 is moving roughly one third of the way along the imaginary line from Eta Herculis to Zeta Herculis. A magnifying finderscope will reveal M13 when you look through it, looking like a poorly-focussed star. For red dots and Telrads, move along the line until you are roughly one-third of the way along, then move to your telescope’s eyepiece to observe this wonderful cluster.

Observation

Use a low magnification eyepiece to begin with. This is a bright cluster occupying a large area of sky, so a low magnification and large field of view are in order to get the most pleasing observations.

The cluster appears more dense at its center and thins as you move to its edges. As you spend time with the cluster, you’ll notice that there are more- and less-dense regions of stars.

Swapping to higher magnification, especially in a larger scope, reveals some of this cluster’s brightest stars as individuals. Bright is a relative term, for the members we can pick out shine no brighter than magnitude 12, which is already at the limit of smaller scopes. There are maybe 20 or more shining to magnitude 13, and a hunt for them with the right equipment is incredibly absorbing and rewarding.

The rest of the stars cannot be resolved individually, rather they form a haze, almost like nebulosity. It is quite transfixing and can cause you to lose large chunks of the evening staring into it.

Whether you pick M13 out with a small refractor or a light-bucket Dobsonian, you are in for a visual treat!