Close-up on the Hercules Region

Close-up on the Hercules Region

“Dim but interesting” is an accurate description of Hercules, just past the meridian on August evenings. Although it contains no star brighter than third magnitude, this constellation holds within its boundaries a vast store of celestial bounty of the telescopic hunter. Altogether, Hercules encompasses 1,225 square degrees of the sky, ranking it fifth among the constellations in order of size. The heart of the of the figure is the well-known keystone, marked by the stars Eta, Zeta, Pi and Epsilon. Lines of stars outlining the arms and legs of the mighty hero radiate from this distinctive grouping.

Barns tells us that Hercules “discloses many rarities in radiant fields to the south and southeast”. The constellation contains a seemingly inexhaustible supply of the fine double stars, displaying every imaginable combination of magnitudes, separations and colors. Only some of the more attractive ones could be listed in our Table of Selected Objects. Indeed, it is somewhat of a mystery why this area of sky should harbor so many fair pairs while other regions are almost devoid of them.

Hercules also is home of several fine globular star clusters, as detailed in Eyepiece Impressions. One of these – M – 13, the famed Hercules cluster – is the finest representative of its class in the northern sky. Located 1/3 of the way from Eta to Zeta in the keystone, M – 13 glows at magnitude 5,7 and can be glimpsed with the unaided eye on a dark transparent night. Binoculars clearly reveal its fuzzy, non-stellar appearance. Halley, who discovered it in 1716, referred to it as a “little patch”.

Another naked eye feature of this constellation is the star Alpha, known as Ras Algethi (the kneeler’s head) and located south of the keystone. The reddish-orange giant of spectral class M is a semi-regular variable, slumbering fitfully between third and fourth magnitudes with no certain period. A good reference star to use in checking on Alpha’s variations is Delta Herculis, 10 degrees due north and shining at a constant 3.2 magnitude. The two stars nearly match in brightness on some nights, while on others Alpha is noticeable fainter.

M-13 Hercules Cluster. Photo: Bareket.astro.com

Hercules Region: Eyepiece Impressions

Zeta Hercules

We begin our telescopic survey of Hercules with a selection of four distinctly different double stars. The first of these is Zeta, marking the lower right corner of the keystone.

This binary star consists of 3.1 and 5.6 magnitude components, presently a tight 1.2 seconds of arc apart. With a period of just 34 years, this pair has completed more than five revolutions since its discovery by Sir William Herschel in 1782. As a result Zeta’s orbit is one of the best determined in the sky. At closest approach (as in March 1967), the star’s separation drops to just 0.5 seconds. The double is now opening fairly rapidly and will reach a maximum of 1.6 seconds in November 1990.

At least a six inch telescope is needed to clearly split the pair now, although tantalizing glimpses of elongation can be had even in a three inch refractor at 150 to 200 power in good seing. The components are yellow and red, and even when the stars are unresolved, their blended hues can be seen at high magnifications.

Try separating this tightly bound duo with your telescope the next clear tranquil night. Even you should fail now, keep watching Zeta from season to season as the stars move apart. Some night, when least expected, the pair will reach your resolution threshold and suddenly reveal itself.

Delta Herculis

Our second offering presents an interesting test of magnitude and color, rather than of resolution. This is Delta Herculis, southeast of the keystone and consisting of a 3.2 magnitude primary attended by an 8.8 magnitude companion about 10 seconds away.

Delta is an example of an usual class of objects known as optical doubles (two unrelated stars at different distances from us). The “companion” happens to lie in the some line of sight but is actually much more distant than the primary, a mere 100 hundred-years away. The gap between them is closing as the two stars appear to cross paths with each other, making Delta look more than ever like a real (gravitationally coupled) double star.

The faint star can be glimpsed in a 2.4 inch refractor at 35 power or more on a good night. At least a four inch at 90 power, however, is needed to detect this object’s color. With larger aperture, Delta becomes increasingly lovely as a magnitude contrast pair.

Barns called this double “a study in color” and rightly so, for countless hues have been reported. Most observers see the pair as cream and lilac. Nut green and purple, pale yellow and bluish green, green and ashy white, yellow white and azure, and even greenish white and grape red have been recorded! The actual spectral types of the stars are A0 and G2. What colors do you find for this “fake” but interesting duo?

Alpha Herculis

Those seeking a spectacular double to show visitors at the telescope are sure to be pleased with third magnitude (sometimes fourth) Alpha Herculis. Located south of the keystone, it is not far from second magnitude Alpha Opiuchi.

Alpha Herculis consists of stunning combination of third magnitude orange and fifth magnitude blue green suns, a comfortable 4.6 second apart. Unlike Delta, Alpha’s tints are strong and easy to see without guessing. A 2.4 inch refractor at 100 power nicely resolves Alpha, while three and four inch scopes give the kind of view we can appreciate. Larger telescopes simply add to already beautiful scene.

Smyth called this pair “a lovely object, one of the finest in the heavens”. Webb gives the primary as very yellow or orange, and the companion as blue, bluish-green or emerald on different occasions. Olcott simple states “orange and green” while Barns strangely makes no comment on the colors. The spectral types are M5 for the primary and G for the companion.

This stellar couple lies 540 light-ears from us; the stars are apparently in very slow and orbital motion about each other. The primary is a pulsating giant star, whose tenuous outer atmosphere has been found to actually engulf the companion sun. This small is known to be a spectrosonic binary with a period of 52 days, making Alpha at least a triple system. Quite a varied and beautiful spectacle must grace the sky of any planet there.

95 Herculis

Our forth pair provides one of the subtlest, yet most beautiful, combinations of celestial color anywhere in the sky. Found by drawing a line from Epsilon to Lambda and extending it its own length, the double 95 Herculis is one of a group of naked eye stars in Eastern Hercules.

The components of 95 Herculis are nearly matched fifth magnitude suns, 6.5 seconds apart. A 2.4 inch refractor at 35 power splits them nicely; 75 to 100 power makes the color contrast apparent. Larger instruments enhance the tints greatly.

The finest color notation for this pair is that given long ago by Smyth: “apple green and cherry red” – and so they look to most eyes. Other less charming terms such as pale (or yellow) red and pale (or yellowish) green have been recorded, amounting to the same thing. Not to be outdone, Barns christened the duo as “beryl and sardonyx”. These tints are delicate, but persistent in all apertures.

Webb agreed with Smyth’s colors and added that 95 Herculis “is extremely pretty”. He called attention to the strange discrepancies in color reported among early observers, when comments such” red the larger, green the brighter” were made. We should try to confirm such descriptions by actual observation at the telescope.

NGC-6210

For a change of pace, we next offer a little planetary nebula. Like most of its kind, location of NGC-6210 presents a test of observing skill. Lying slightly more than one degree southwest of fifth magnitude 51 Herculis, a line from Gamma to Beta also points to its neighbourhood. Sweeping with a low power, wide piece eyepiece will reveal a triangle of stars – one of which is actually the nebula.

NGC appears as an eerie, 9.7 magnitude blue-green egg, 20 by 13 seconds of arc in size. (Some observers make it as small as eight seconds). It is fun to see how low a magnification will distinguish the planetary from a star; 45 power on a three inch refractor will do for a trained eye in this case. A definite disk can be made out at about 75 power on three and four inch scopes. A six-inch reflector at 100 power gives a fine view, while eight inch and larger instruments at 150 power or more make this a real show piece.

NGC-6210 is one of a handful of small nebulae picked up by F.G.W. Struve during his double star searches. This planetary is one of the few notable objects overlooked by Herschel in his great sweeps of the heavens. Many observers have likened this “cosmic pea” to a star out of focus. Some owners of large reflectors report its disk as an intense blue in color, but most see green with only a tinge of blue. Large apertures at high magnification will also reveal a faint outer hale surrounding the bright disk itself.

NGC-6229

We complete our survey of Hercules with three globular star clusters. The first of these is the “misidentified” object known as NGC-6229, situated midway between two fifth magnitude stars – 42 and 32 Herculis.