Object Name: Messier 92
Alternative Designations: M92, NGC 6341
Object Type: Class IV Globular Cluster
Right Ascension: 17 : 17.1 (h:m)
Declination: +43 : 08 (deg:m)
Distance: 26.7 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 14.0 (arc min)
Locating Messier 92: Once you’ve identified the “keystone” asterism in the constellation of Hercules, finding Messier 92 is a snap. Simply draw a mental line between Pi and Eta, (the widest part of the keystone and the northernmost two stars) and create a triangle in your mind. Now, focus the finderscope at the apex of the of the triangle and look for a small, smudgy spot just to the north. You’re on it! Bright and easy, M92 can be spotted in small binoculars and becomes recognizable as a globular cluster in larger models. At close to magnitude 6, it’s near unaided eye visibility from a dark sky location and makes an excellent object for urban telescopes – even on partially moonlit nights!
What You Are Looking At: Wonderfully resolvable in large aperture, 26,000 light years distant M92 seems to literally explode with stars… and no wonder! This sixteen billion year old globular has perhaps 330,000 of them packed into a 109 light year radius. While only about 16 of these stars are variables, there are also some eclipsing binaries in there as well as some unusual ultra-violet objects. “The cores of several Galactic Globular Clusters (GGCs) has been observed with the Hubble Space Telescope (HST) WFPC2 through visual, blue and mid- and far-UV filters in two programs devoted to study the evolved sequences in the Color Magnitude Diagrams (CMDs) of GGCs. In the UV CMDs of two clusters (M13 and M92) we have discovered a population of faint objects with a strong UV excess which lie significantly outside the main loci defined by normal stars.” says F.R. Ferraro (et al). “Some of these UV objects are nearly coincident to positions of low luminosity X-ray sources. We suggest that these stars might be a, perhaps new, subclass of cataclysmic variable.”
So what causes one globular cluster to have different properties than the rest? Apparently it isn’t age, because according to research, all globular cluster may have formed roughly at the same time. “A differential fit of the NGC 2419 CMD to that of the similarly metal-poor `standard’ cluster M92 shows that they have virtually identical principal sequences and thus the same age to within 1 Gyr. Previously published studies of many other low-metallicity globular clusters throughout the Milky Way halo show that they possess this same age to within the ¸ 1 Gyr precision of measurement.” says William Harris (et al). “The addition of the remote-halo object NGC 2419 to this list leads us to conclude that the earliest star (or globular cluster) formation began at essentially the same time everywhere in the Galactic halo throughout a region now almost 200 kpc in diameter.”
So, is age relative? Or can it be determined precisely? According to N. Paust; “While isochrone fitting is the routine method used to determine cluster ages, it is not the only possible method. Indeed recent observations of M92 (NGC 6341), M3 (NGC 5272), M13 (NGC 6205), and M14 (NGC 6402) have shown that the shape of the red giant branch (RGB) luminosity function (LF) can be used to simultaneously determine the distance modulus and age of clusters with gigayear precision.”
History: This awesome globular cluster was originally discovered by Johann Elert Bode on December 27, 1777 who wrote: “A nebula. More or less round with pale glow. On this occasion, I also want to announce that on December 27, 1777 I have discovered a new nebula in Hercules, not known to me, southwest below the star s in his foot, which shows up in a mostly round figure with a pale glimmer of light. Its longitude is about 11 deg [Sgr] [251 d] and its latitude 66d north.Together with two small [faint] stars, which don’t occur at Flamstead, it appears in the reversing telescope as shown in fig k (in the following volume).”
Charles Messier would be the next to encounter it on March 18, 1781. In his notes he writes: “Nebula, fine, distinct, and very bright, between the knee and the left leg of Hercules, it can be seen very well in a telescope of one foot [FL]. It contains no star; the center is clear and brilliant, surrounded by nebulosity and [it] resembles the nucleus of a large Comet: its brightness, its size, approach much that of the nebula which is in the girdle of Hercules. See No. 13 of this Catalog: its position has been determined, by direct comparison with the star Sigma Herculis, fourth magnitude: the nebula and the star are on the same parallel. (diam. 5′)”
Sir William Herschel would be the first to resolve it into stars, but it was Admiral Smyth who gave M92 the true attention it deserved: “A globular clusterof minute stars, preceding the right leg of Hercules. This object is large, bright, and resolvable, with a very luminous centre; and, under the best vision, has irregular streamy edges. It is immediately preceded by a 12th-magnitude star, distinct from the outliers, and there are several other stars in the field, of which the brightest is of the 7th magnitude in the nf [north following, NE], with a Delta AR = 28s. Messier, who enrolled it in 1781, remarks that “it is easily seen with a telescope of one foot [FL];” and it really demands very little optical aid to render it visible. Messier’s own instrument did not, it seems, resolve it, for he compares the shining centre, with its attendants, to the nucleus of a comet surrounded by nebulous matter; but of course, it rose into a brilliant cluster, of 7′ or 8′ in diameter, before the reflectors of Sir W. Herschel in 1783. The mean place was obtained by carefully differentiating the cluster with Eta Herculis, from which it bears north-by-east, 1deg 1/2 distant; bearing to the north of Alpha Herculis, and west of Wega.”
Top M92 image credit, Palomar Observatory courtesy of Caltech, M92 Hubble Image, M92 2MASS Image, M92 HR Diagram (AANDA), and M92 image courtesy of Hillary Mathis, REU program/NOAO/AURA/NSF.