THE SPIRAL NEBULAE
Last and most important of all are the spiral nebulae. The finest example
is in the constellation Canes Venatici, and its spiral configuration was
first noted by Lord Rosse, an epoch-making discovery. The convolutions
of its spiral are filled with numerous starlike condensations,
themselves engulfed in nebulosity. Photography possesses a vast
advantage over the eye in revealing the marvelous character of this
object, an inconceivably vast celestial whirlpool. Naturally the central
regions of the whorl would revolve most swiftly, but no comparison of
drawings and photographs, separated by intervals of many years, has yet
revealed even a trace of any such motion.
The number of large spiral nebulae is not very great; the largest of all
is the great nebula of Andromeda, whose length stretches over an arc of
seven times the breadth of the moon, and its width about half as great.
This nebula is a naked-eye object near Eta Andromedae, and it is often
mistaken for a comet. Optically it was always a puzzle, but photographs
by Roberts of England first revealed the true spiral, with ringlike
formations partially distinct, and knots of condensing nebulosity as of
companion stars in the making. While its spectrum shows the nongaseous
constitution of this nebula, no telescope has yet resolved it into
component stars.
Systematic search for spiral nebulae by Keeler, and later continued by
Perrine, at the Lick Observatory, with the 36-inch Crossley reflector,
disclosed the existence of vast numbers of these objects, in fact many
hundreds of thousands by estimation; so that, next to the stars, the
spiral nebulae are by far the most abundant of all objects in the sky.
They present every phase according to the angle of their plane with the
line of sight, and the convolutions of the open ones are very perfectly
marked. Many are filled with stars in all degrees of condensation, and
the appearance is strongly as if stars are here caught in every step of
the process of making.
The vast multitude of the spiral nebulae indicates clearly their
importance in the theory of the cosmogony, or science of the development
of the material universe. Curtis of the Lick Observatory has lately
extended the estimated number of these objects to 700,000. He has also
photographed with the Crossley reflector many nebulae with lanes or dark
streaks crossing them longitudinally through or near the center. These
remarkable streaks appear as if due to opaque matter between us and the
luminous matter of the nebula beyond. Perhaps a dark ring of absorptive
or occulting matter encircles the nebula in nearly the same plane with
the luminous whorls. Duncan has employed the 60-inch Mount Wilson
reflector in photographing bright nebulae and star clusters in the very
interesting regions of Sagittarius. One of these shows unmistakable dark
rifts or lanes in all parts of the nebula, resembling the dark regions
of the neighboring Milky Way.
Pease of Mount Wilson has recently employed the 60-inch and the 100-inch
reflectors of the Mount Wilson Observatory to good advantage in
photographing several hundred of the fainter nebulae. Many of these are
spirals, and others present very intricate and irregular forms. A search
was made for additional spirals among the smaller nebulae along the
Galaxy, but without success. Several of the supposedly variable nebulae
are found to be unchanging. Many nights in each month when the moon is
absent are devoted to a systematic survey of the smaller nebulae and
their spectra by photography. The visible spiral figure of all these
objects is a double-branched curve, its two arms joining on the nucleus
in opposing points, and coiling round in the same geometrical direction.
The spiral nebulae, as to their distribution, are remote from the Galaxy,
and the north Galactic polar region contains a greater aggregation than
the south. The distances of the spiral nebulae are exceedingly great.
They lie far beyond the planetary and irregular gaseous nebulae, like
that of Orion, which are closely related to the stars forming part of
our own system. Possibly the spiral nebulae are exterior or separate
"island universes." If so, they must be inconceivably vast in size, and
would develop, not into solar systems, but into stellar clusters. The
enormous radial velocities of the spiral nebulae, averaging 300 to 400
kilometers per second, or twenty-fold that of the stars, tend to sustain
the view that they may be "island universes," each comparable in extent
with the universe of stars to which our sun belongs.
Recent spectroscopic observations of the nebulae applying the principle
of Doppler have revealed high velocities of rotation. Slipher of the
Lowell Observatory made the first discovery of this sort and Van Maanen
of Mount Wilson has detected in the great Ursa Major spiral, No. 101 in
Messier's catalogue, a speed of rotation at five minutes of arc from
the center that would correspond to a complete period in 85,000 years.
As was to be expected, the nebula does not rotate as a rigid body, but
the nearer the center the greater the angular velocity, and Van Maanen
finds evidence of motion along the arms and away from the center.
These great velocities appear to belong to the spiral nebulae as a class,
and not to other nebulae. Thirteen nebulae investigated by Keeler are as a
whole almost at rest relatively to our system, as are the large
irregular objects in Orion, and the Trifid nebula. This would seem to
indicate that the spiral nebulae form systems outside our own and
independent of it.
Quite different from the spirals in their distribution through space are
the planetary nebulae. The spirals follow the early general law of nebulae
arrangement, that is, they are concentrated toward the poles of the
Galaxy; but the planetary nebulae, on the other hand, are very few near
the poles and show a marked frequency toward the Galactic plane.
Campbell and Moore have found spectroscopic evidence of internal
rotatory motion in a large proportion of the planetary nebulae.
The distribution of the nebulae throughout space, like that of the stars,
is still under critical investigation, but the location of vast numbers
of the more compact nebulae on the celestial sphere is very
extraordinary. The Milky Way appears to be the determining plane in both
cases; the nearer we approach it the more numerous the stars become,
whereas this is the general region of fewest nebulae and they increase in
number outward in both directions from the Galaxy, and toward both poles
of the Galactic circle. Obviously this relation, or contra-relation of
stars and nebulae on such a vast scale is not accidental, and it also
must be duly accounted for in the true theory of the cosmogony. The
nebulae which are found principally in and near the Milky Way are the
large irregular nebulae, and vast nebulous backgrounds, like those
photographed by Barnard in Scorpio, Taurus and elsewhere, as well as the
Keyhole, Omega, and Trifid nebulae. Allied to these backgrounds are
doubtless some of the dark Galactic spaces, radiating little or no
intrinsic light, and absorbing the light of the fainter stars beyond
them. A peculiar veiled or tinted appearance has been remarked in some
cases visually, and examination of the photographs strongly confirms the
existence of absorbing nebulosity.
The spiral nebulae are so abundant, and so much attention is now being
given to them, both by observers and mathematicians, that their precise
relation to the stellar systems must soon be known; that is, whether
they are comparatively small objects belonging to the stellar system, or
independent systems on the borders of the stellar system, or as seems
more likely, vast and exceedingly remote galaxies comparable with that
of the Milky Way itself. Our knowledge of the motions of the spirals,
both radial and angular, is increasing rapidly, and must soon permit
accurate general conclusions to be drawn.