The constellation of Hercules is one of the original 48 constellations plotted by 2nd century astronomer Ptolemy and remains today as one of the modern 88 constellations. As constellations go, Hercules is large and ranks as the 5th largest of all, spanning an impressive 1225 square degrees of sky, but despite its large size it is rather faint. Although easily traceable under dark skies, it can become elusive with just a hint of light pollution or when viewed under a moonlit sky.
The centrepiece of Hercules is a trapezoid shaped asterism commonly known as the “Keystone”. The four stars of the “Keystone”, Eta, Zeta, Epsilon and Pi Herculis, are either 3rd or 4th mag. stars in brightness. The brightest of the four, Zeta Herculis, is also the second brightest star in the entire constellation, with a mag. of 2.81. Along with the marginally brighter Kornephoros (Beta Herculis), these are the only two stars in Hercules above mag. 3.0.
Variable Star, Double Star Deep Sky
Rasalgethi (alpha Her) – is a red supergiant star, located at a distance of 380 light-years. With a diameter of 560 million kilometres (350 million miles) it is one of the largest stars known. It is so enormous, that were Rasalgethi located at the centre of our solar system it would extend for 1.87AU and swallow up the Earth and even Mars. Like most red giants it varies erratically, between mag. 3.1 and 3.9 with an average brightness of about mag. 3.5. The period is of the order of 100 days, although there are much longer variations that may last for about 6 years.
Rasalgethi is also a beautiful double star, with a mag. 5.4 blue-green companion separated by 4.7 arc seconds and at a position angle of 103 degrees. Telescopically, even a small 60mm scope at medium magnification will split the components, with the primary star displaying a very prominent orange colour, sometimes showing a hint of red. In contrast, the colour of the secondary star is much more difficult to ascertain. Often it looks just white, but on occasions it may seem blue-green or even blue-turquoise. Regardless of what colours you see, it is a truly striking combination.
Marsic (Kappa Her) – is a very nice double star easily visible in small telescopes. Both stars are remarkably similar. Kappa Her A at mag. 5.1 is the brighter of the two and is a class G8 yellow giant star, separated by 27 arc seconds (PA of 15 degrees) from its companion, mag. 6.2 K1 orange giant star Kappa Her B. However, it is not that simple as there is a certain amount of doubt about the authenticity of this double star. Recent age and distance measurements hint that this may just be an amazing chance alignment and not a true double star at all. Nether the less, it is a wonderful sight in telescopes of all sizes.
The distance of Kappa A Her from Earth is about 390 light-years while Kappa B Her may be up to 470 light-years away.
Zeta Herculis – 35 light-years distance, is a testing double for large apertures. It is a rapid rotating binary with a period of 34.5 years, consisting of a yellow primary and an orange companion of mag. 2.9 and 5.4 respectively. The separation is currently 1.1 arc seconds with a PA of 165 degrees and requires a recommended aperture of at least 150mm (6-inch) under exceptional seeing conditions to split the pair. Manage to do this and you will join a select group of observers.
Rho Herculis - at 402 light-years away, is a pretty double consisting of two white stars, easily resolvable in a small telescope at high magnification. The components are of mag. 4.5 and 5.4, with a PA of 326 degrees and a separation 4.5 arc seconds.
95 Herculis / Struve 2264 – 470 light-years distance, is another beautiful double star consisting of two 5th mag. suns. With a separation of 6.3 arc seconds (PA of 256 degrees) they are easily split in a small 60mm (2.4 inch) telescope at medium/high power. What is outstanding about this system is that it offers one of the most stunning colour contrasts of all pairs of stars. They have been described as gold and silver, yellow and blue and even apple green and cherry red by 19th century observer Admiral Symth. Do you agree?
OP Herculis – is a giant red M class star that varies between mag. 5.9 and 6.7 over a period of 120 days. It is located about 1000 light-years from Earth.
DQ Herculis - or Nova Herculis 1934 was an extremely bright slow nova that reached mag. 1.3 in Hercules in December 1934 and remained at this brightness for almost 2 months. It is the prototype for a category of cataclysmic variable stars known as intermediate polars.
Currently it is a speck of light at mag. 15.2, barely perceivable with amateur instruments but it is worth looking in this area of the sky. You never know, you may be one of first to witness the next explosion!
For deep sky enthusiasts Hercules contains a sprinkle of globular clusters, challenging planetary nebulas and a host of faint galaxies.
M13 (NGC 6205) - Of all of the deep sky objects in Hercules, one of them stands out above all and that is the spectacular showpiece globular cluster M13. It is universally acclaimed as the finest globular in the northern hemisphere and is easily found on the western side of the "Keystone" asterism, 2.5 degrees south of Eta Herculis along a line connecting Eta Herculis with Zeta Herculis. At mag. 5.8, with an apparent size of 20 arc minutes, M13 is just about visible to the naked eye under a perfectly black sky and is an easy object in 10 x 50 binoculars where it forms a right-angled triangle with two 7th mag. stars. Looking like a fuzzy out of focus star it appears obviously non-stellar but without resolution. An 80 mm telescope (3.1-inch) shows M13 as a uniform extended hazy disk. But it’s with larger instruments, that M13’s really shows its true glory. The clusters outer stars are easily resolved with a 150mm (6-inch) scope at medium to high magnification. When viewed with even larger scopes M13 is a breathtaking sight. A 250mm (10-inch) scope under a dark sky with good transparency shows thousands of pinpoints across the whole face of M13. Push up the power, and look out for a couple of curious affects. First, many of the outer stars seemed to be arranged in long arcs weaving their way across the cluster face and secondly the distribution of the bright stars is not even across the surface. This can result in an optical illusion of apparent voids or relatively barren areas interspersed across the cluster. All in all, M13 is a very compact cluster of at least 300,000 stars, located at 25,100 light-years distance with a spatial diameter of about 145 light-years. Like all globulars it is extremely old, the estimated age been of the order of 10 billion years.
M92 (NGC 6341) – Located nearly 10 degrees NE of M13, in a relatively blank area of the sky is M92, another fine globular cluster. This superb object is often over-shadowed due to its proximity to its much more illustrious neighbour and would almost certainly rate higher on observation lists if it were located in a different area of the sky. Shining at mag. 6.4 and spanning some 14 arc minutes across, M92 appears in telescopes as a slightly fainter and smaller version of M13. A 150mm (6-inch) telescope at high power will show a number of stars, a 250mm (10-inch) scope considerably more. Despite its smaller size, M92 is noticeably compact, especially the core region which is much more difficult to resolve than M13.
M92 is located 26,700 light-years from Earth and has a spatial diameter of 108 light-years. Certainly a worthy globular cluster for observation, even if considerably overshadowed by M13.
NGC 6229 – About 7 degrees to the NW of M92 and 1.5 degrees north of mag. 4.8 star 52 Herculis, is NGC 6229 our 3rd and final globular cluster in Hercules. At mag. 9.4 and with an angular size of 4.5 arc minutes, this ball of stars is much fainter and smaller compared to the other two. What is amazing about NGC 6229 is its vast distance from Earth. Estimated to lie at almost 100,000 light-years it is one of the furthest globular clusters that’s readily visible with an amateur telescope. It can be glimpsed with a 100mm (4-inch) aperture, is much easier with a 200mm (8-inch) scope but unfortunately un-resolvable even with the largest amateur instruments. However, do take time to marvel at a cluster of stars that is as many light-years distant as the diameter of our own Milky Way galaxy.
Hercules contains dozens of faint galaxies around mag. 12 and 13 that are challenging objects to view visually but of course much easier to image or photograph. The following is a small selection of some of these galaxies.
NGC 6207 - is an unspectacular mag. 11.6 compact spiral galaxy with a long axis of about 3 arc minutes and titled at about 45 degrees to our line of site. Nevertheless, this galaxy is imaged and observed much more than many galaxies of similar mag. and size since it lies only 1/2 degree to the NNE of the magnificent globular cluster M13.
Visually, good skies and a 200mm (8-inch) telescope will pick out NGC 6207 but it isn’t terrible impressive, the only detail visible is a star like nucleus surrounded by a misty envelope of light. What is remarkable though, is that together with M13, NGC 6207 offers one of the sky’s best extreme depths of view. The distance of NGC 6207 is estimated to range from 20 to 120 Million light-years and for calculation purposes if we assume a distance of 40 Million light-years, then NGC 6207 is approx. 1600 times more distance than M13. Undoubtedly this galaxy also contains its own globular clusters, and if M13 surrounded NGC 6207 instead of our own galaxy, it would appear as an extremely dim, almost invisible cluster at mag. 22 when viewed from Earth!
NGC 6181 – is a faint compact mag. 11.9 spiral galaxy located just less than 2 degrees south of Kornephoros (Beta Hercules). It is very similar to NGC 6207 only slightly fainter and smaller.
Viewed through a 200mm (8-inch) telescope at medium power, NGC 6181 appears as an extremely faint, elongated, diffuse patch of light with a central core that is best seen using averted vision.
NGC 6239 – is a very faint peculiar barred spiral galaxy without a distinct core. At mag. 12.3 with dimensions of only 2.5 x 1.1 arc minutes, it is a challenging object for visual observers. A 250mm (10-inch) or preferably larger scope is recommended. Even then, this galaxy appears as an elongated unspectacular diffuse streak of light, surrounded by an arc of 6th mag. stars.
NGC 6482 – at mag. 11.4 is marginally the brightest of all Hercules faint galaxies. Unlike the previous galaxies we have looked at, this one is different; it is elliptical, appearing as a uniform halo that brightens towards the centre. A minimum aperture of 200mm (8-inch) is recommended for viewing NGC 6482. Larger apertures still, show the galaxy more clearly without additional detail. It measures 2.1 x 1.8 arc minutes.
Hercules contains 3 bright planetary nebulae that are observable with backyard telescopes, NGC 6210, NGC 6058 and IC 4593.
NGC 6210 – is the brightest and the best of Hercules planetary nebulae. It is bright, elongated, quite small at about 20 x 16 arc seconds but very pretty with an easy to view central star. NGC 6210 shines at mag. 8.8 and is an excellent target for small scopes due to its high surface brightness and ability to take high magnifications. With a 80mm (3.1-inch) scope it is easy discernible from the background star field, appearing as an out of focus star that forms a triangle with two other stars of almost equal brightness. Bump up the magnification and this lovely planetary can appear blue, blue-green or even aqua in colour. Like many planetary nebulae it can exhibit a blinking effect especially when switching between direct and averted vision.
Larger instruments reveal more detail including a faint outer shell. For some planetary nebulas the central star is elusive, however this one is different and it is possible under nights of good seeing to observe the mag. 12.7 central star. For this purpose, a minimum 150mm (6-inch) scope is recommended.
IC 4593 – sometimes known as the White Eyed Pea, IC 4953 is a mag. 10.7 planetary nebula. It lies at the SW section of Hercules near the border with Serpens Caput. It is easily visible with a 200mm (8-inch) telescope, appearing as a small, slightly elongated out of focus star. Like NGC 6210, IC 4593 can also appear to noticeably “blink” when switching between direct vision and averted vision. At mag. 11.3, the central star is relatively bright and therefore easy to see. In larger apertures and/or times of good seeing, IC 4953 may appear as a halo of green nebulosity surrounding the white as white central star, hence the name “White Eyed Pea”.
The diameter of IC 4953 is about 12 arc seconds.
NGC 6058 – at mag. 12.9 is the faintest of our 3 planetary nebulae in Hercules. Since it also lies in a rather barren patch of sky near the Hercules / Corona Borealis border, NGC 6058 is a challenging object to located and observe. This planetary is certainly one for the larger scopes, with a minimum 300mm (12 inch) aperture recommended. Even in larger instruments, NGC 6058 only appears as a small pale ghostly patch of light about 25 arc seconds across, slightly elongated, predominantly grey although a bluish tint may be observable. The central star is mag. 13.5 and visible in a 300mm (12 inch) telescope.
NGC 6058 is tucked in the middle of an isosceles triangle of 9th and 10th mag. stars.
Hercules Star Data Table
Henry Draper Catalogue
|Bayer ||Flamsteed ||Name||
|Visual Mag.||Var.||Var. Mag. Range||
|145001||79043||Kappa Herculis||7||Marsic||16h 08m 05s||17d 02m 49s||5||---||---||---||Y||27||15||5.1,6.2|
|150680||81693||Zeta Herculis||40||Zeta Herculis||16h 41m 17s||31d 36m 07s||2.81||---||---||---||Y||1.1||165||2.9,5.4|
|156014||84345||Alpha Herculis||64||Rasalgethi||17h 14m 39s||14d 23m 25s||3.48||Y||3.1->3.9||100(var)||Y||4.7||103||3.5(var),5.4|
|157778||85112||Rho Herculis||75||Rho Herculis||17h 23m 41s||37d 08m 45s||4.15||---||---||---||Y||4.5||326||4.5,5.4|
|163990||87850||---||---||OP Herculis||17h 56m 49s||45d 21m 03s||6.22||Y||5.85->6.73||120||---||---||---||---|
|164668||88267||---||95||95 Herculis||18h 01m 30s||21d 35m 45s||4.26||---||---||---||Y||6.3||256||4.9,5.2|
|NOVA Her 1934||---||---||---||DQ Herculis||18h 07m 30s||45d 51m 33s||15.16||Y(Nova)||1.3->15.2||---||---||---||---||---|
Hercules Deep Sky Data Table
|13||6205||---||Great Hercules Globular Cluster||Globular Cluster||16h 41m 42s||36d 27m 41s||5.8||20'||25,100||145|
|92||6341||---||---||Globular Cluster||17h 17m 07s||43d 08m 10s||6.4||14'||26,700||108|
|---||6229||---||---||Globular Cluster||16h 46m 59s||47d 31m 38s||9.4||4.5'||99,100||130|
|---||6207||---||---||Spiral Galaxy (Sc)||16h 43m 04s||36d 49m 57s||11.6||3.0' x 1.3'||40,000,000||35,000 x 15,000|
|---||6181||---||---||Spiral Galaxy (Sc)||16h 32m 21s||19d 49m 36s||11.9||2.6' x 1.3'||115,000,000||87,000 x 43,500|
|---||6239||---||---||Barred Spiral Galaxy (Sb)||16h 50m 05s||42d 44m 23s||12.3||2.5' x 1.1'||40,000,000||29,000 x 13,000|
|---||6482||---||---||Elliptical Galaxy (E)||17h 51m 49s||23d 04m 19s||11.4||2.1' x 1.8'||170,000,000||105,000 x 90,000|
|---||6210||---||---||Planetary Nebula||16h 44m 29s||23d 48m 00s||8.8||20" x 16"||6,500||0.6 x 0.5|
|---||---||4593||White Eyed Pea||Planetary Nebula||16h 11m 45s||12d 04m 17s||10.7||12"||6,800||0.4|
|---||6058||---||---||Planetary Nebula||16h 04m 27s||40d 40m 56s||12.9||25" x 20"||9,500||1.1 x 0.9|