Another object of the globular class, but less resolvable, is that known as 92 Messier, which lies between the stars Eta and Iota in Hercules, nearer the latter. Sir William Herschel’s telescopes showed it as seven or eight minutes of arc in diameter. It is considerably brighter at the centre. The larger components are easily visible in moderate-sized telescopes, but even Lord Rosse’s giant instrument failed to resolve the central blaze. There is no doubt, however, that it consists wholly of small stars, as the unerring eye of the spectroscope shows a stellar spectrum, similar to that of the neighboring 13 Messier.
Fig. 17.—The Region of Boötes and Hercules
Another fine example of the globular class is 5 Messier, which lies closely north, preceding the fifth magnitude star, 5 Serpentis. It is considerably compressed at the centre. Sir William Herschel counted 200 stars, but failed to resolve the central nebulosity. Messier, its discoverer, found it visible with a telescope only one foot long.
Another fine object is 3 Messier, in Boötes. Admiral Smyth describes it as “a brilliant and beautiful globular aggregation of not less than 1,000 small stars.” It is beyond the power of small telescopes, but it was resolved by Buffham, even in the centre, with a 9-inch reflector.
Numerous fine examples of the globular class are found in the Southern Hemisphere, which indeed seems to be richer in these marvelous objects than the northern sky. Of these the most interesting are those known as Omega Centauri and 47 Toucani. Omega Centauri, from its great apparent size—about two-thirds of the moon’s diameter—and its visibility to the naked eye, may perhaps be considered as the most remarkable object of its kind in the heavens. It shines as a hazy star of the fourth magnitude, and I have often so seen it in the Punjab sky. Its large size and globular form are clearly visible in a binocular field-glass, but, of course, its component stars are far beyond the reach of such an instrument. Sir John Herschel, observing it with his large telescope at the Cape of Good Hope, found it a “truly astonishing object. All clearly resolved into stars of two magnitudes, viz., thirteen and fifteen, the larger lying in lines and ridges over the smaller;... the larger form rings like lace-work on it.” If we take the average magnitude of the components at thirteen and a half, the apparent brightness of the cluster would imply that it contains about 15,000 stars.
The Great Nebula in the Constellation Cygnus