Distribution of Nebulae over the Sky
Now let us consider the surveys. The nebulae are great beacons scattered through space. We know something about their nature, and, in particular, we know their intrinsic luminosities. Therefore, we can study their distribution - investigate the characteristics of the observable region as a whole. There are two immediate problems - the distribution over the face of the sky, and the distribution in depth. In both cases the method of investigation depends upon the simple proposition that, in a statistical sense, apparent faintness indicates distance.
The distribution over the sky is examined by comparing the numbers of nebulae per unit area which are brighter than a particular limit of apparent faintness. In principle the method is simple and direct, but faint limits must be selected in order to include large numbers of nebulae. Statistical averages are significant only when they represent large populations.
The results of such surveys indicate that the apparent distribution over. the sky is not uniform. Nebulae are not found along he heart of the Milky Way, and they are scarce along the borders. Beyond this zone of complete or partial avoidance the numbers of nebulae per unit area increase directly with galactic latitude right up to the poles of the Milky Way.
The departures from uniformity in the apparent distribution follow a familiar pattern, namely, that due to local obscuration, to the absorption of light within the galactic system. From our position within the system we look through the swarm of stars into the universe beyond. But the system is not completely transparent. Great clouds of dust and gas are scattered among the stars, and they are especially prevalent along the central plane which defines the Milky Way. These clouds, piling up one behind another, completely obscure the very distant stars, and, of course, the more remote nebulae as well. Cloud absorption fully accounts for the zone of nebular avoidance which follows the Milky Way.
In addition to the scattered clouds, the main body of the galactic system seems to be embedded in a very tenuous medium which appears to be fairly uniform. Whatever its actual structure may be, the medium behaves roughly as though it were an extended uniform layer, centred on the galactic plane. From our position near the sun the shortest paths through the `uniform layer' are perpendicular to the galactic plane, towards the galactic poles. In these two directions the absorption by the medium is least - about 25 per cent. - and the nebulae are most numerous. As the line of sight departs from either pole and approaches the Milky Way, the path through the uniform layer lengthens, the absorption increases, and the numbers of nebulae decrease. This latitude effect is similar to the fading of the sun as it drops from the zenith to the horizon and its light travels along a constantly increasing path through the earth's atmosphere.
The apparent distribution over the sky must evidently be corrected for local obscuration before the true distribution is revealed. Absorption by the tenuous uniform layer is readily corrected; the latitude effect follows the familiar cosecant law. Along the belt of the Milky Way, however, the absorption is practically complete, and, consequently, the proper corrections are unknown. The true distribution of nebulae within the zone of avoidance cannot be determined from the observations.
Outside the zone of avoidance, the large-scale, true distribution is thoroughly uniform. The two galactic hemispheres are closely alike, and there are no systematic variations in either latitude or longitude. The uniformity in the explored areas of the sky runs up so smoothly to the edges of the unexplored regions that we have no hesitation in assuming that the uniformity extends over those latter regions as well.
Minor irregularities do exist. Nebulae are found singly, in pairs, triplets, and groups of various sizes up to the occasional great clusters. Our own nebula, the galactic system, is the chief component of a triple system, in which the two Magellanic Clouds are the satellites. This triple system, along with a few neighbouring nebulae, forms a typical, small group, isolated in the general field. In fact, the nebulae exhibit a pronounced tendency towards clustering. However, the tendency seems to operate on a modest scale; for instance, no cluster is known with as many as a thousand members. When large areas of the sky are compared, the irregularities average out, and the large-scale distribution appears to be thoroughly uniform. Our sample of the universe is isotropic, very much the same in all directions.