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Summer Observing Targets
Eye on the Sky - Hercules

By Tom Campbell

In Roman mythology, Hercules was the son of the god Jupiter and the mortal human Alcmene. Jupiter's wife, Juno, learned of the affair and did everything she could to make life difficult for the boy. Through Juno's machinations, Hercules found himself under the servitude of King Eurystheus, who ordered him to perform twelve labors if he wanted to earn his freedom. Each of these labors involved killing ferocious beasts that correspond to twelve constellations.

Hercules' first task was to defeat the Nemean lion (seen as Leo in the nighttime sky). The lion's skin was impenetrable, so arrows were useless. Hercules began wrestling the lion and after 30 days of battle, was finally able to strangle it. Hercules removed its skin and made a cloak which rendered him invulnerable.

His other labors involved killing Hydra the 9-headed snake, a giant crab (Cancer), capturing a large deer (the antlers are formed with the stars from Cassiopeia), killing a Cretan bull (Taurus), and several other creatures.

The constellation of Hercules lies north of the plane of the Milky Way and is home to a variety of celestial objects, including globular clusters, planetary nebulae and galaxies. There are actually two different asterisms located within Hercules. The most famous is the Keystone, consisting of the stars zeta (ζ), epsilon (ε), pi (π) and eta (η). The other asterism is the Butterfly, one wing of which is the Keystone and the other wing consisting of beta (β) and delta (δ).

Click image to see larger photo

Exploring Hercules

In honor of Hercules' feat, I offer my version of the Twelve Labors of Hercules. The dozen objects listed below are representative of a wide array of celestial objects with varying degrees of difficulty. How many of these labors can you complete this summer?

The most famous object in Hercules is probably Messier 13, the Great Hercules Cluster. This globular cluster was first discovered in 1714 by Edmond Halley (of Halley's Comet fame) and was catalogued 50 years later by Charles Messier. M13 contains about 300,000 stars and lies about 22,000 light years from Earth.

Although appearing somewhat small in the eyepiece, the cluster is actually about 145 light years across. From dark skies, it is visible with the naked eye as a small smudge of light. In a telescope, it becomes a glorious ball of stars.

The other Messier object in Hercules is another globular cluster, Messier 92. It is about 27,000 light years from Earth and is a fine cluster in its own right but is often overlooked because of the striking beauty of nearby M13. This cluster is very old, dating back to almost the beginning of the Universe.

A third globular cluster is NGC 6229. It was discovered by William Herschel in 1787 and is approximately 100,000 light years away. Because of its distance, this cluster is a lot fainter than the other two.

Alpha (α) Herculis, also known as Rasalgethi, is a nice double star, with the primary being yellow and the companion blue-white. With my 8-inch dob, it was a bit difficult to split due to the closeness of the stars and their magnitude difference.

Delta (δ) Herculis, or Sarin, is another fine double. The white primary is very bright but its blue companion is much fainter and can get lost in the glare at low magnifications. I noted several field stars that form an irregular-shaped box around the pair.

Rho (ρ) Herculis is a fairly tight double, but both stars have similar magnitudes which makes them a little easier to separate. Both stars are white and a lack of field stars in this area make them look lonely.

NGC 6207 is a small spiral galaxy just northeast of Messier 13. It appears fainter than the field stars so you may need to use averted vision to pick it out of the darkness.

The Turtle Nebula (NGC 6210) is a planetary nebula that is easy to see in small telescopes but requires a larger telescope to bring out details. In my 8-inch dob from a rural site, it appeared as a very small blue rectangle.

The elliptical galaxy NGC 6166 is the brightest member of the Abell 2199 cluster. This galaxy has an active nucleus, which emits two strong radio jets caused by the infall of gas into a supermassive black hole at the center.

Another nice galaxy is the barred spiral NGC 6181. The outer arms are very loose, giving the galaxy an “S” shape. In 1996, a Hydrogen-alpha observation of this galaxy revealed a ring of gas with a diameter of about 6,500 light years was in motion about the galaxy's center, rotating at a rate of 100 km/sec.

NGC 6239 is another barred spiral galaxy, but a little fainter than NGC 6181, even though it is much closer. The spiral arms on this galaxy are extremely faint, but the large dusty core is distinct and interesting. It is about 42,000,000 light years from us and was discovered by William Herschel in 1788.

The White-eyed Pea Nebula (IC 4593) is a planetary nebula near the border of Serpens. It is bipolar, with two small nodes projecting out from opposite sides of a nearly circular disk. The bright central star should be easily seen as well.