The southern hemisphere is rich in nebulae smaller but of the same kind as these three magnificent objects, the Keyhole, the Looped, and the Orion Nebulae—that is, large irregular masses of gas, often spangled with stars—and each has some special beauty of its own; but for most of them large telescopes are needed to grasp the faint details. There is a nest of them in the northern part of Sagittarius: a cloudy streak visible to the naked eye, a little north of the star γ Sagittarii, represents three nebulae and clusters close together—M 8, M 20, and M 21. The first is a wonderful combination of a bright scattered star-cluster and a gaseous nebula, with dark rifts dividing the cloudy structure. The second is the celebrated Trifid Nebula, less bright and large, but with even more striking black lanes which split the principal part into three almost separate portions. Many faint stars are scattered over it, but as they are scarcely more numerous than in the surrounding regions, most of them probably are not connected with the nebula. M 21 is a star-cluster. Near these, where Sagittarius borders on Aquila,[11] is a small but very remarkable nebula, known from its shape as the Horseshoe or the Omega Nebula (M 17). It has a curious mottled appearance, with bright knots here and there. [11] On Scutum in maps where this constellation is not included in Aquila. And a little further west, near together, are two wonderful nebulae which surround the two stars Rho Ophiuchi and Nu Scorpii. Professor Barnard, who has studied and taken exquisite photographs of many nebulae, considers the first of these the finest in the sky, because of its dark, winding lanes and the veiling of the stars in places by partly transparent nebulous matter. XV OTHER TYPES OF NEBULAE The large irregular nebulae described in the last chapter are all more or less mingled with stars, at least in appearance, and it has been suggested that they are star-clusters in process of formation, with larger and brighter masses of filmy nebulosity all about them than at later stages, for long-exposure photographs reveal some exceedingly faint nebulosities surrounding Kappa Crucis and the Pleiades and other fully-developed star-clusters. But this can only be a guess until we know more about the nature of nebulae. In some regions of the sky we find vast spaces thinly veiled by nebulosity so faint and transparent that it seems to have reached the very limit at which matter can exist and be recognised as such. Thus in the constellation of Orion nearly all the bright stars are connected together by the vast convolutions of an exceedingly faint diffused nebula in spiral form, the innermost curve of which ends in the Great Nebula of the Sword, and the whole region within is filled with faint light.