Edwin Hubble

“ If this is true, it is natural to suppose them physically connected, as is the case in star-clusters. It is not possible to form a conception of this state of affairs until some idea of their distance is acquired. Suppose them to be extra-sidereal and perhaps we see clusters of galaxies; suppose them within our system, their nature becomes a mystery.
Fig. 2.—Distribution of size of nebulae in Wolf’s Perseus
and three Yerkes fields
The question of nebular distances is of first importance, for it is in terms of this quantity that the various dimensions may be expressed. ”

“ They may give gaseous spectra, or continuous; they may be planetaries or spirals, or they may belong to a different class entirely. They may even be clusters and not nebulae at all. These questions await their answers for instruments more powerful than those we now possess.
Our present hope is to study them statistically, but until motions, either radial or transverse, have been detected we must content ourselves with the problem of their distribution. The first step is to make a systematic survey with powerful telescopes. ”

“ As the small nebulae and the spirals inhabit the same regions of the sky, it is probable that the order of distance of the two classes is the same. Classes e, f, may even turn out to be spirals themselves. ”

“ The velocity of escape is 280 km/sec.
At the lower limit of average distance of spirals the typical nebulae would be fifteen light-years in diameter, forty-five million times as massive as our sun, and 3 × 1013 as dense as our atmosphere. On the other hand, if the typical spiral nebula is placed at a suitable distance, its dimensions assume the same order of magnitude as those of our own stellar system. ”

“ If they are within our sidereal system, then, as they are most numerous in the direction of its minor axis, the dimensions of our system must be much greater than is commonly supposed.
The observations point to very large values for the rotational components of velocity, although the necessarily small scale of the instruments employed in their study renders the measuring difficult. Pease has determined the velocity of rotation for N.G.C. 4594 with some degree of accuracy. At 120′′ from the nucleus it amounts to 300 km and varies linearly with the distance outward. ”

“ The tendency of small nebulae to gather in clusters has been known for some time. Stratonoff’s map of the distribution of faint nebulae in the Northern Hemisphere shows it very plainly. Max Wolf’s more detailed study of the ecliptic regions with the 16-inch Bruce camera and the 30-inch reflector demonstrates that within these larger regions of the sky where nebulae tend to congregate there are points of accumulation about which the clustering is more marked. ”

“ Save for very exceptionally clear and steady skies, the longer exposures add much labor for negligibly greater results. The diameters of the faintest [Pg 6] stars near the center of the field, with the full aperture, fine sky, and an hour’s exposure, which are of about magnitude 17.5, are about 2.0′′. Longer exposures are apparently subject to change of focus, differential refraction, and other disturbances which tend to increase the size of the images unduly, and hence to spread the total light over a larger area. ”

“ The principal objection lies in their apparently systematic distribution with respect to the Milky Way. The matter is usually stated in the form that “spirals avoid the Milky Way.” There are less than 300 nebulae known to be spiral in form. The greatest of them all is just on the border. It is suggested that the spirals seem to follow the distribution of the small faint nebulae. ”

“ There is a double star in the position published in the Strassburg Annals.
[7] See Plate IV, enlarged from negative R 3352, taken with 120m exposure on February 26, 1916. The numbers were marked on only those nebulae which promised to be readily visible on the engraving, and which were separated enough to give room for inscribing the number. The B.D. stars are designated by letters, for which the key is as follows. ”

“ The curves (Fig. 1) take the same form, save that for most the maximum frequency is for diameters between 20′′ and 25′′. One of his lists was made from plates taken with the 30-inch reflector at Königstuhl. ”