by Shaphan Shank | Jul 1, 2023 | 0 comments

M13 globular star cluster
M13. Photo © Reinhold Wittich/Dreamstime.com.

Hercules, the Strongman, is a large constellation that lies straight east of Bootes, the constellation we focused on last month. If you imagine a line connecting Arcturus and Vega, the two brightest stars that are currently high in the evening sky, you will find that Hercules straddles this line, just over halfway from Arcturus to Vega. The most noticeable part of Hercules is not any individual bright star, but rather a keystone-shaped foursome of stars (among astronomers, it’s widely known as the Keystone) that lies in the middle of the constellation. A chain of stars extends from each corner of the Keystone, apparently representing the arms and legs of the celestial strongman.

Hercules is best known for containing what many consider the greatest globular star cluster in the northern half of the sky. This cluster, which is commonly known as the Hercules Cluster, or M13, contains several hundred thousand stars in a sphere about 145 light-years across. M13 lies on the western edge of the Keystone, about one-third of the way from Eta Herculis (in the northwestern corner) to Zeta Herculis (in the southwestern corner).

M13 is bright enough that you may be able to glimpse it with the unaided eye in a dark sky; it will look a lot like a faint star. Binoculars will make the cluster much easier to see, revealing a round fuzzy spot that is clearly not a star. To really appreciate this cluster though, you will need to see it through a good telescope, the bigger the better. Of course, that’s not to say that small telescopes can’t give good views of M13; it’s a great deep-sky target for any good-quality telescope. You’ll just be able to resolve more of its stars with larger instruments, particularly those with a diameter of 8” or more.

Once you’ve found the cluster, try using higher magnifications to resolve more stars and see more detail in this cluster. Although it may look like a homogenous ball of stars at first glance, careful observation should reveal numerous chains of stars extending from the core of the cluster. With a large scope, you may also be able to see a three-pronged shape that is slightly darker than the surrounding parts of the cluster; this feature is known as the Propeller.

M92 is another fine globular cluster that often gets overlooked because of its proximity to M13, which is a little larger and brighter. To find M92, imagine a line connecting Eta Herculis to Iota Herculis, the star at the end of the chain extending from the northeastern corner of the Keystone. M92 is about two-thirds of the way from Eta Her to Iota Her. The first difference you will probably notice between M92 and M13 is the smaller size of M92. M92 also has a more concentrated core than M13, but a 6”–8” telescope will be sufficient to begin resolving some of its stars.

Hercules is home to a number of double stars. One of the best of these doubles is Rasalgethi, which lies in the southern part of Hercules near the neighboring constellation Ophiuchus. Rasalgethi consists of a bright orange primary star separated by almost 5” from a slightly fainter secondary star.

One of the fascinating things about observing double stars is that different people often perceive them as being different colors. Rasalgethi is a case in point. Some observers see the secondary star as yellowish, while others see a variety of colors ranging from white to blue or even green. Try moderately high magnification (100–175×) to split this double and see its colors.

Hercules star map

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