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Hercules Cluster

NGC 6205, C 1639+365, GCl 45, Messier 13, Hercules Cluster, h 1968, GC 4230

RA: 16h 41m 41.44s
Dec: +36° 27′ 36.9″

Con: Hercules
Ch: MSA:1159, U2:114, SA:8

Ref: SIMBAD, SEDS

(reference key)

Type: globular cluster

Mag: B=?, V=5.8

Size: 20′
PA: ?

Image gallery

Sketches  (1)

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Photos  (1)

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Remarks

The famous Hercules cluster was discovered with the naked eye by Edmond Halley in 1714. Houstin writes: "The following year he published a paper that was the first to describe a half dozen 'nebulous' objects in detail rather than mention them as asides to a star catalogue. Of the great globular Halley wrote: 'This is but a little Patch, but it shews itself to the naked Eye, when the Sky is serene and the Moon absent.' . . . This cluster is noted for its numerous star chains stretching outward in graceful arcs from its bright core. Recently, an obscure feature of M13 has become a visual quest for amateurs. During the middle of the last century, observers using Lord Rosse's large reflectors in Ireland noted three dark lanes radiating propeller-like from a common point southeast of the cluster's centre. These lanes were well enough known that the Rev. T. W. Webb mentioned their visibility with a 9-inch reflector in his classic late-19th century observing guide 'Celestial Objects for Common Telescopes'. Soon after, however, the lanes abruptly disappeared from observing books. I strongly suspect that photography was to blame, since the lanes do not show well in long-exposure photographs .. they are best seen with telescopes larger than 8 inches, and high magnification will definitely help."

Easily seen with binoculars, a telescopic view will show it about a third the diameter of the Full Moon. With a magnification of about 100x, individual stars within the cluster can be seen, the brightest of which are about 12th magnitude. In the middle of the 19th century, observers using Lord Rosse's great reflectors noted a propeller-shaped feature within the cluster, formed by three dark lanes. John Bortle, using a 12.5-inch reflector, saw the lanes in 1981. The lanes show best in 10" and larger scopes working at about 200x. The feature is offset to the southeast edge of the cluster.

Historical observations

William Herschel (c.1784)

In the Philosophical Transactions, 1814, William Herschel wrote "May 16, 1787. 20 feet telescope. The 13th of the Connoissance des Temps is a most beautiful cluster of stars. It is exceedingly compressed in the middle and very rich. The most compressed part of it is round and is about 2 or 2.5' in diameter, the scattered stars which belong to it extend to 8' or 9' in diameter, but are irregular." In the Philosophical Transactions, 1818, William Herschel wrote: "1799, 1805, It is very plainly to be seen by the eye. 1799, 7 feet finder. Very visible. 1783, 7 feet telescope. with 227 power plainly resolved into stars. 1799, 10 feet telescope. With an aperture of 4 inches the stars cannot be distinguished; with 9 inches, very beautiful. 1787, 1799, 20 feet telescope. The stars belonging to the cluster extend to 8 or 9' in diameter; the most compressed part about 2' or 2.5'; the latter is round, the former irregular. 1805, large 10 feet telescope. A brilliant cluster all resolved into stars."

Webb, T.W. (1893)

In the 5th edition of Webb's Celestial Objects for Common Telescopes it is described as "superb globular cluster .. finest of its class; just visible to naked eye. Halley discovered it in 1714; Messier was sure it contained no stars; but it is spangled with glittering points in a 5.5-foot achromatic, and becomes a superb object in large telescopes. John Herschels speaks of thousands of stars, 10 or 11 to 15 or 20th mag; his father had estimated them at 14,000. Smyth calls it an extensive and magnificent mass of stars, with the most compressed part densely compacted and wedged together under unknown laws of aggregation. John Herschel describes 'hairy-looking curvilinear branches,' well seen with 8-inch mirror; E. of Rosse, who noticed this spiral tendency, detected also three dark ;lanes' or rifts in its interior, beautifully seen by Buffham with 9-inch mirror. I have also perceived them. . . In Se.'s achromatic the outliers, inconspicuous in ordinary instruments, fill a field of 8'. . . Burnham sees a solitary close double star (0.9 arcseconds) near the centre, and notres that in all bright compressed clusters there seems to be a remarkable absence of close double stars. Klein has well remarked that our understanding strives in vain to answer the inquiry, What is the object of these thousands on thousands suns? The mere aspect of this stupendous aggregation is indeed enough to make the mind shrink with a sense of the insignificance of our little world. Yet the Christian will not forget that, as it has been nobly said, He took of the dust of this Earth, and with it He rules the universe! The neighbourhood is beautiful with a low power. A faint nebula [NGC 6207] about 40' N.f."

Barnard, E.E. (1906)

On the Vacant Regions of the Sky. Popular Astronomy, 14, 579-583.

[1906PA.....14..579B]

Brief note of dark nebulae visible in M13 (p 580)

Published comments

Doig, P. (1925)

Journal BAA, 35, p159

glbaulr cluster, denser parts nearly 12' diam., visible to naked eye .. There is no nebuilosity visible in these clusters, so that Raynard's conclusion (mentioned n Webb) is erroneous. The faint nebula 40' n.f. was seen by Barnard with 5-inch refractor."

Doig, P. (1925)

Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part II. M.N.R.A.S., 35(8), 280.

Melotte, P.J. (1915)

A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 7.0 mag globular cluster.

Vorontsov-Velyaminov, B. (1925/1926)

Vorontsov-Velyaminov, B. (1925) "Catalogue of integrated magnitudes of star clusters", Astron. Nach. 226.195. Comparing the brightness of the cluster with the extrafocal images of stars, he estimates the magnitude as 6.82.

Bailey, S.I. (1908)

"!! globular cluster, condensed; The Great Cluster in Hercules"

Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.

Harris, W.E. (1997)

RA 16 41 41.5 (2000) Dec +36 27 37 Integrated V magnitude 5.78 Central surface brightness, V magnitudes per square arcsecond 16.80 Integrated spectral type F6- Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.49 Core radius in arcmin .88. ["Catalog Of Parameters For Milky Way Globular Clusters", compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487) ]

Modern observations

[sci.astro.amateur] discussion

M13's dark 'Y'; discussed on sci.astro.amateur

From: darksky@shore.intercom.net

Subject: M13's Dark Y

Date: 31 Jul 1996 19:55:38 GMT

As M13 is well situated for observing at this time, I wonder if others have observed the "Y" shaped dark lanes that Walter Scott Houston(sp?) wrote about in his S&T observing column years back. I see it quite nicely in the 20 F5 but really do not remember seeing it in my old 10 inch SCT. I'm not sure I ever looked for it back then.

The "Y"'s fork in an inverted (not flipped) image eminates from south (up) with the left fork coming down from 1:30 and the right from around 2:30 and they meet just up and right from the core and the single base extends through the core. It's a large feature but dominates the southern portion of M13. I am posting from memory so I hope I was as acurate as possible. This is a low contrast feature but did not require averted vision to see. For a 'chance' configuration caused by a diminishing stellar population of brighter giants (if that is what it is caused by), it's very symmetrical! Any comments would be of interest so since everyone looks at this great ball of stars anyway, why not check out the dark "Y"?

/Russell Lederman

From: krumins@staff.uiuc.edu (krumins george)

Date: 31 Jul 1996 20:25:02 GMT

darksky@shore.intercom.net wrote:

: As M13 is well situated for observing at this time, I wonder if others

: have observed the "Y" shaped dark lanes that Walter Scott

: Houston(sp?) wrote about in his S&T observing column years

: back. I see it quite nicely in the 20 F5 but really do not remember

: seeing it in my old 10 inch SCT. I'm not sure I ever looked for it

: back then.

Saw it about ten years ago from rural Illinois using a modified 10-inch Odyessy. It was *really* cool, but it was also the middle of Winter, and it was *really* cold. As I recall the transparency was excellent that night.

Regards,

George Krumins

--

I have seen it several times in various apertures; my logbook is at home so I can't say which at present.

Jay Reynolds Freeman -- freeman@netcom.com -- I speak only for myself.

-----

I have also seen the "Y" in a variety of apertures (from 8 to 20"), and its a rather remarkable effect. But this is not an optical illusion - these are actually the result of dark nebula obscuring part of the globular. M5 also similar, though less pronounced dark "rifts"

Now I need to contact my observing partner (and Dark Nebula Fanatic) David Riddle to get the actual published references.

- Rich Jakiel

Intersting you should mention a *dark* "Y". When I had my Mom take a peek through the 14 at M13, she picked out a *bright* "Y", and I saw it then too, made up of stars of course. Now I'll go check for a dark "Y"....

--

Hi there. We used to call it "the propeller", although that also applied to the stars at the front of the "water jar" in Aquarius. I have seen it in an 8" Newtonian fairly easily. Clear skies to you.

David Knisely

--

I regulary see in with my 12.5" here in Florida. However, at TSP last may, I had a real hard time picking it out. Even in a 20". Guess the sky was so dark that dim stars I don't see her in FL were filling in the dark bands.

Steve

Steve & Susan Carroll -- LaBelle, FL Scarro ll2@usa.pipeline.com

------------------------------------------

From: Steve Coe (stevecoe@worldnet.att.net)

Date: Wed, 31 Jul 1996 22:43:15 -0700

Howdy;

I observed M-13 recently in my 13" f/5.6 and found the "propellor" or dark "Y" most promenent at 220X. I counted 41 stars in the NW quadrant of the cluster and saw most of the stars as silvery, but a few where light yellow or orange. I could hold the propellor steady at this power and saw the core "sitting" on the intersection of the three dark lanes. A small and thin dark lane cuts into the cluster on the opposite side from the propellor feature. I also noticed a chain of 9 stars that run E-W across the core that stood out as very noticable at this magnification. Lots of fun.

Clear Skies;

Steve Coe

I see it pretty regularly with my 10" dob. It is pretty easy once you know it is there.

Joe Larkin

Hi folks,

The Finnish amateurs use the suitable designation, "The Mercedes' star",; for the dark feature. It's obvious in my 17.5" at 100x. Lower; magnification does not reveal it. There are a few other conspicuous; dark lanes in the outskirts of the cluster, two at the opposite side of; the "Mercedes' star", cutting the edge. Some bright, hairlike lanes; consisting of stars, are seen curving gracefully on the edges. To me,; M13 is reminiscent of a spider.

Clear skies, Timo Karhula

Harrington, Phil (1986)

Harrington, P. (1986) An observer's guide to globular clusters. Sky&Telescope, Aug, 198.

finest globular north of the celestial equator. under dark skies it can be seen with the naked eye.. a 4inch will reveal some of its individual stars, and a 16inch causes the cluster to explode into a globe of innumerable stars. Lok carefully and you may see some star chains at the outer edge that have been noted by such observers as John Herschel and Lord Rosse.."

Ling, Alister

Letter by Alister Ling about the visibility of the Y: Astronomy magazine, 1994 October, p 12. Refers to a photo in the April 1994 Sky Almanac which shows these lanes.

Hartung, E.J. (1968) Astron.Obj.South.Tel

Hartung notes: "even at the comparatively low altitude of culmination in southern skies, it is wonderfully effective. The dense broad centre is very bright and sparkling and the outliers in extended rays cover a diameter of at least 6'; even 7.5cm will show some of these and the cluster is well resolved with 10.5cm."

Sweetman, Michael E. (1992)

Michael Sweetman (Tucson, Arizona, USA) observing with a 6-inch refractor, writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "At x48, an impressive, brilliant ball of light over a bright, nebular background, all blanketed with resolved stars. Reslution to the core. Appears to show stars in the outer areas on the S.p. side which gradually fades out, while on the N.f. side the stars fade more rapidly. Appears to be a bright arm of stars extending from the inner core to the S side, with a slight curve to the p. side. The nearby galaxy NGC 6207, shows as a small, faint smudge of light to the N.f. side of the field." He also reports a second observation: "Observed at x204, the cluster is not uniform in stellar distribution. Good views were obtained at x152, showing the shape of the outer envelope; averted vision shows numerous outer stars. Hints of the dark, inner lane can still be made, but is more readily observed at x204.."

Bortle, John (1976)

John Bortle (Webb Society Quarterly Journal, January 1976) using 10x50 binoculars, estimates the visual magnitude as 5.8.

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "5.7M; 23' diameter; bright, large and spectacular showpiece resolved nicely at 100x but higher-x with a wide field will blow you away! three subtle, narrow dark lanes intersect near, but off the center in a trillium-like fashion detracting slightly from this cluster's radial symmetry; SP GAL N6207 (12.3M; 2.7' x 1.2' extent) 20' to NNE; sweet and easy! tough, 15M SP GAL I.4617 is about halfway between M-13 and N6207, just off the W-most of a four-star, Lyra-like, parallelogramic asterism."

Moyer, James (1997)

Observer: James Moyer (e-mail: dr.@who.net) Instrument: 12.5-inch equatorial reflector Location: Buckingham, PA, USA Light pollution: moderate Transparency: excellent Seeing: good Time: Wed May 21 03:00:00 1997 UT Obs. no.: 139

This observation was made two days before full moon. At 55x M13 appeared very bright and large with many stars resolved, esp. towards the outskirts of the cluster but also continuing towards the center. The resolved stars gave the distinct impression of being superimposed over (in front of) the unresolved globular's central region. At 205x M13 filled the field of a 7mm Nagler eyepiece with many dozens of stars resolved, tiny pinpoints of light scattered across the entire field. The resolved stars were easily seen with direct vision at both magnifications. This is my very first deep sky observation made with a new NGT-12.5 after completing collimation and star testing.

Callender, John (IAAC)

Observer: John Callender (e-mail: jbc@west.net, web: http://www.west.net/~jbc/); Instrument: 8-inch Dobsonian reflector Location: Carpinteria, CA, USA; Light pollution: light Transparency: good Seeing: fair; Time: Sun Jun 29 09:45:00 1997 UT Obs. no.: 169

The climax of my Hercules session; I definitely saved the best for last! A big, beautiful, condensed swarm of stars at 49x, with a central glow behind it. As with M92, the central region seemed irregular at 244x, while 122x gave the prettiest view.

Observer: John Callender (e-mail: jbc@west.net, web: http://www.west.net/~jbc/); Instrument: 8-inch Dobsonian reflector Location: Carpinteria, CA, USA; Light pollution: light Transparency: fair Seeing: fair; Time: Mon Mar 17 12:45:00 1997 UT Obs. no.: 113

My first viewing of M13 with this telescope. At 49x gave the appearance of a big, circular star cluster with an unresolved, condensed glow centered behind it. Best view was at 122x, with some of the central glow being resolved into faint stars.

Brian Skiff

naked eye - mod f spot. adjacent fld *s also vis, so effect is of arc of three sim-br *s. BS, 29Apr1992, TSP.

7x35mm - mod-strong concen to sub*ar center. BS, 29Apr1992, TSP.

6cm - fairly br haze, no detail. FtL.

7cm - vbr cl less easily res than M5 @ 30x, where it is only gran. 75x: part res w/vfine-grained texture. halo reaches 2/3 way to closer of two m11 *s W. concen is strong-broad to mod-even. BS, 15Apr1993, Anderson Mesa.

15cm - easily found. quite granulated, many *s vis around edges. higher powers showed no more *s. HM/BS, 27June1970, FtL.

- 75x: outliers can be seen w/much gran twd center. on Srn side is a * much brtr than others, perhaps a cl var at max. HM/BS, 1Jul1970, FtL.

- fine br cl nicely res @ 80x, where spidery star-chains radiating from core are consp. two chains form embayment around apparent dk patch opening nrly due S; another sim but smlr/narrower pair of arcs open N. inner halo 7'-8' across, core ~1/3 of this; mod even concen in this region. outliers only about twice inner halo diam ---> 15' diam total; outliers do not reach either m7 * nr the cl. bkgrnd too rich to distinguish f outliers from fld. BS, 22May1988, Anderson Mesa.

25cm - outliers to 15' diam. br part 8' across. wreathes of *s everywhere. dk blotches on inside fine @ 250x.

- shockingly br, irreg round. at 280x outliers fill 14' fld. cottony underglow to layer of resolved *s in center. on S is curl of *s extending S & W from origin. protruding into SE side is narrow wedge of dkness, prominent. on NE is similar but less obvious wedge. outliers obviously elong in pa165. BS, 23May1982, Anderson Mesa.

30cm - fantastic @ 297x, lots of *s. core cottony, 5' across. dk spot on E 0'.5 across. arms N & S give bird-like appearance. more outliers on E & W. E side flattened.

Contemporary observations

Magda Streicher

1998 August

Globular Cluster, Hercules

Location: Campsite (23 16 South - 29 26 East).

Telescope: Meade 8" 18mm eyepiece, 36 arcmin f.o.v.

Date: August 1998.

Sky conditions: Not very good.

Description: Beautiful, very large (20 arc minutes) and bright extremely compressed wide core, with clear star ouliers randomly visible. Stars disperse like flares, seems more to the south, with faint stars visible just outside this globular to the southwest, although they could be part of the fringy edges,[R.V.1] spread thinly on the periphery. Using 26mm wide, a bright star just outside this globular to the east.

(no date)

Alldays (22.50S, 20.12E, 770m).

12-inch f/10 SCT (76x, 218x, 346x)

Beautiful, very large and bright extremely compressed wide core, with clear star outliers randomly visible. Stars disperse like flares, seems more to the south, with faint stars visible just outside this globular to the south-west, although they could be part of the fringy edges, spread thinly on the periphery. Two stars 8 magnitude just outside this globular to the north. The globular looks slightly cut off on the NW side. Outliers more to the west. IC 6417 a very small galaxy picked up 40 just north of M13 with care mind you and between NGC 6207 (346x). The dark markings is very evident in high power. Auke see the feature also known as the Mercedes Benz symbol, will take a careful look again

Tom Bryant

2008-05-28 23:00:00

Observing site: Little Tycho Observatory

Telescope: C-8

[16h 41m 42s, 36 28m 0s] This cluster was truly dazzling, as when observed with my well dark adapted eye, it looked extremely bright with hundreds of stars enveloped in a bright nebulosity of all the fainter ones.

2007-06-22 23:00:00

Observing site: Little Tycho Observatory

Telescope: C-8

[16h 41m 42s, 36 28m 0s] A glorious sight, despite the moonlight. Hundreds of stars, in a glow of thousands, like spilled sugar.

Carol Botha

2010 - 08 -08

Location: Kambro, Britstown

Time: 21:10

Telescope: 8" Dobsonian f5. Eyepiece 15mm. FOV- 45'

Sky conditions: Seeing 3/5

Actual dimensions: 16.6'x 16.6'(Cartes Du Ciel)

Object description:

Globular cluster in Hercules

Round object - brighter nucleus fading gradually to edges

Very faint stars resolved

9mm eyepiece resolves stars to the core and reveals strings of stars that look like wind toy

In the fov one very bright star to the E and one S. Between these stars are two dimmer stars. W just inside the fov are two dim and two very faint stars.

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