The study of nebulae is essentially a photographic problem for cameras
of wide angle and reflectors of large focal ratio. The photographic
plate presents a definite and permanent record beside which visual
observations lose most of their significance. Perhaps the one field
left for the older method is the measurement of sharp nuclei deeply
enshrouded in nebulosity. New nebulae are now but rarely seen in the
sky, although an hour’s exposure made at random with a large reflector
has more than an even chance of adding several small faint objects to
the rapidly growing list of those already known. About 17,000 have
already been catalogued, and the estimates of those within reach of
existing instruments, based on the ratio of those previously known to
those new in various fields, lie around 150,000.
[1] A dissertation submitted to the Faculty of the Ogden Graduate
School of Science of the University of Chicago in candidacy for the
degree of Doctor of Philosophy.
Extremely little is known of the nature of nebulae, and no significant
classification has yet been suggested; not even a precise definition
has been formulated. The essential features are that they are situated
outside our solar system, that they present sensible surfaces, and
that they should be unresolved into separate stars. Even then an
exception must be granted for possible gaseous nebulae which appear
stellar in the telescope, but whose true nature is revealed by the
spectroscope. It may well be that they differ in kind and do not
form a unidirectional sequence of evolution. Some at least of the
great diffuse nebulosities, connected as they are with even naked-eye
stars, lie within our stellar system; while others, the great spirals,
with their enormous radial velocities and insensible proper motions,
apparently lie outside our system. The planetaries, gaseous but well
defined, are probably within our sidereal system, but at vast distances
from the earth.
In addition to these classes are the numberless small, faint nebulae,
vague markings on the photographic plate, whose very forms are
indistinct. 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. Fath made a
beginning by photographing each of the Kapteyn fields within reach of
the Mount Wilson 60-inch reflector with uniform exposures of one hour.
He discovered more than eight hundred new nebulae, and confirmed the
fact that the small nebulae avoid the Milky Way. This last is vital in
its bearing on the question of whether or not these objects belong to
our system. A survey with long exposures suggests itself, analogous to
that of Kapteyn, but based on the Milky Way rather than on the equator.
The writer attempted such a program with the Yerkes 24-inch reflector,
giving two-hour exposures. Little progress was made, but one fact stood
out, namely, that in the fields of galactic latitude -60° nebulae were
very scarce when compared to the numbers met with in galactic latitude
+60°.
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. He measured the positions
of more than four thousand new nebulae, and devised a classification
which, while admittedly formal, offers an excellent scheme for
temporary filing until a significant system shall be constructed.
The present paper has to do with certain clusters of small, faint
nebulae which the writer found during the years 1914 to 1916 while
photographing with the 24-inch reflector of the Yerkes Observatory.
From about 1000 uncatalogued objects, 512 in 7 well-defined clusters
were chosen for measurement. Known nebulae in the clusters numbered 76;
hence there were, in all, some 588 objects, or an average of 84 per
cluster. The fields are as given in Table I.
The problem of measuring and reducing accurate positions of objects
at a considerable distance from the center of plates taken with
a reflector of so large a focal ratio, 1:4, presented serious
difficulties. The area covered by each plate is a square of some 110´
to the side. With the full aperture the stellar images are sensibly
round only within 5′ of the optical center of the plate. From there
outward the coma becomes more and more prominent, distorting the images
first into an oval, and finally, near the edge of the plate, into the
shape of an arrow, while the point about which the images build up
becomes more and more eccentric. For images of various sizes this point
will be at various distances from the centers of figure, and at 40′
from the center will fall very nearly at the point of the arrow. This
introduces at once an overwhelming magnitude-error, masking whatever
distortion of the field may exist.