Probably the most impressive of all nebulæ is the great one in Orion ([Fig. 146]), whose position is shown on the star map between Rigel and ζ Orionis. Look for it with an opera glass or even with the unaided eye. This is sometimes called an amorphous—i. e., shapeless—nebula, because it presents no definite form which the eye can grasp and little trace of structure or organization. It is "without form and void" at least in its central portions, although on its edges curved filaments may be traced streaming away from the brighter parts of the central region. This nebula, as shown in [Fig. 146], covers an area about equal to that of the full moon, without counting as any part of this the companion nebula shown at one side, but photographs made with suitable exposures show that faint outlying parts of the nebula extend in curved lines over the larger part of the constellation Orion. Indeed, over a large part of the entire sky the background is faintly covered with nebulous light whose brighter portions, if each were counted as a separate nebula, would carry the total number of such objects well into the hundreds of thousands.
The Pleiades ([Plate IV]) present a case of a resolvable star cluster projected against such a nebulous background whose varying intensity should be noted in the figure. A part of this nebulous matter is shown in wisps extending from one star to the next, after the fashion of a bridge, and leaving little doubt that the nebula is actually a part of the cluster and not merely a background for it.
[Fig. 147] shows a series of so-called double nebulæ perhaps comparable with double stars, although the most recent photographic work seems to indicate that they are really faint spiral nebulæ in which only the brightest parts are shown by the telescope.
According to Keeler, the spiral is the prevailing type of nebulæ, and while [Fig. 144] presents the most perfect example of such a nebula, the student should not fail to note that the Andromeda nebula ([Fig. 140]) shows distinct traces of a spiral structure, only here we do not see its true shape, the nebula being turned nearly edgewise toward us so that its presumably circular outline is foreshortened into a narrow ellipse.