M92 Globular Cluster in Hercules and “Trouvelot’s Hook”

The Observer’s Challenge complete report:  Click on the following link:  JULY 2016 OBSERVERS CHALLENGE – M-092

The following write-up and sketch by Jaakko Saloranta of Finland, one of the most talented and gifted visual observer’s in the world today.  Roger Ivester 

Messier 92

The baby brother of Messier 13. Brightest star in the cluster is magnitude 12.1 so it is fairly easy object to resolve. However, it is a difficult naked eye object: barely visible with optimal averted vision at an altitude of 57 degrees. Easily visible with a 8×30 binoculars as a non-stellar smudge.

Forms a triangle with two 10th magnitude stars. Partial resolution is achieved – only a handful of stars visible – with a 3 inch refractor @ 133x (23′). With a 4.5 inch SkyQuest XT @ 152x (20′) M92 appears as fairly well resolved, with a few dozen stars visible. Bright core, might appear slightly elliptical but is probably just an illusion caused by unresolved stars NW of the nucleus.

Messier 92 contains a little known small feature nicknamed “Trouvelot’s Hook” (named after 19th century French astronomer Étienne Léopold Trouvelot) . It is a hook-shaped chain of stars with dark bays at both sides. The feature is visible in two separate sketches made by Trouvelot. The first one is from 1874 and the second from 1877. Both sketches of M92 are made from Harvard College Observatory.

Having seen Roger Ivester’s notes on M92, it is obvious that Roger has noted – at least a part of – “Trouvelot’s hook”. He described it as “a faint chain of four stars follows the flattened edge”. This is part of the very same chain sketched by Trouvelot! I personally could not make out the dark lane sketched by Roger. I only saw a couple of bright stars just E of the cluster’s core.

Pencil Sketch By Jaakko Saloranta of Finland:


The following sketches by French Astronomer, Trouvel0t. The first one was made in 1874 and the second 1877, both from the Harvard College Observatory.


Below:  Sketch by Trouvelot in 1877



The following information and pencil sketch by the writer.  RI 

M92 – NGC 6341 – Globular Cluster in Hercules:  Date: May 27, 2016

Telescope: 10-inch f/4.5 Equatorial Reflector

Eyepiece: 11 mm – Also Other Observations With The Employ Of a 2.0x Barlow
Magnifications: 104x and 208x intermittently

Bright with an intense round core and a granular texture at 104x. When increasing the magnification to 208x, excellent resolution of stars in the outer regions and in the halo, with many outliers.

The overall shape has a subtle N-S elongation. The NNE-WSW edge is flat, which is one of the more noticeable and recognizable features of the cluster. When using averted vision a faint chain of four stars follows the flattened edge. These stars were not visible on two of the three nights of observations, and appeared intermittent or not constant.

The following is a pencil sketch using a 5 x 8 blank notecard, with the colors inverted using a scanner.  Please note the faint star chain on the eastern side of the cluster, on both my sketch and Jim Dire’s image.  It was extremely difficult for me, requiring averted vision, and could only see intermittently on the third night of observations.

Roger Ivester 

Scanned Image 162110000

M92:  The following image and text by James Dire of Hawaii


James Dire, Ph.D.

M92 is the middle in size and brightness of three globular cluster located in the constellation Hercules. The brightest and largest is M13 while the smallest in our sky is NGC6229. All three globular clusters can be spied with an 8-inch or larger telescope.

Globular clusters are highly compact groupings of tens of thousands to millions of stars. There are approximately 150 of these clusters forming a spherical halo around our Milky Way galaxy. Globular clusters are also known to exist in other galaxies. The Andromeda galaxy probably has 2-3 times as many as the Milky Way.

M13 is the brightest globular cluster visible in the northern hemisphere and the third brightest visible from Earth. At magnitude 5.8, has a diameter of 25 arc minutes, nearly as large as the Moon. In comparison, at magnitude 6.4 M92 is roughly half as bright. It spans 15 arc minutes. Although both appear in Charles Messier’s famous catalog, he did not discover either of them. Edmond Halley discovered M13 in the year 1715 and Johann Bode discovered M92 in 1777.

M92 is found six and one-third degrees north of Pi Herculis, the northeastern-most star in the Keystone. Like M13, M92 can easily be spied in 50mm binoculars or finder scopes. Many stars can be resolved in both clusters using telescopes. Larger apertures will reveal more individual stars. I recommend eyepieces that yield 100x, or higher if the seeing is steady.

At magnitude 9.4, NGC6229 is quite a bit more challenging to find than M92. NGC6229 is located just north of the center of Hercules’ club, or 11 degrees north of M13. The easiest way to find it is to center the 5th magnitude star 42 Herculis in the eyepiece and hop two degrees to the southeast. William Herschel discovered NGC6229 in 1787. He was also the first to resolve stars in M92 six years later.

My image of M92 was taken with a 10-inch f/6 Newtonian with a Paracorr II coma corrector yielding a 1753mm focal length. The scope was atop a Paramount ME German equatorial mount and the image was taken with an SBIG ST-2000XCM CCD camera. The exposure was 20 minutes. The brightest star in the field of view, left (east) of the cluster, is HD156821 shining at magnitude 9.76. The faint star to the left of this is a 16.1 magnitude star. The yellow orange star on the northeast side of the cluster is HD156873, magnitude 9.98. The third brightest star in the field, to the lower right of the cluster shines at magnitude 10.9. None of these stars are members of M92.   James Dire 

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