From Proceedings of the National Academy of Sciences of U. S. A.
Such a discous nebula has been investigated by the astronomer F. G. Pease of the Carnegie Observatory on Mount Wilson. By means of the spectroscope he has studied the motion of Nebula No. 4594 in the New General Catalogue (see [Fig. 3]). This body is believed to be a spiral nebula like those in [Figs. 4] and [5] but viewed from the side so that the spirals appear as a band. As the picture shows, this band is coursed through by a thick dark line, owing, it is believed, to a cold non-luminous dust-cloud outside of the spiral. The bright band is broadest in the centre. The curvature of the dark middle line in the shape of an arc, whose apex points downward, combined with the fact that the major portion of light falls above this arc, indicates that we do not view the nebula exactly on edge but from the upper (north) side of a plane through the arms of the nebula. The nucleus speeds away from us at the dizzying rate of 1180 km. (730 miles) per second. The east edge, i. e. the left on the picture, departs with the still higher velocity of 1630 km. (1100 miles) per second, while the west (right) edge retreats at the rate of only 800 km. (495 miles) per second. According to Pease, the nebula rotates as a solid disk so that the difference between the velocity of any point and that of the centre increases in the same proportion as the distance of the point in question from the centre. It is probable that we have been prevented from observing the outer parts, corresponding to the spiral arms proper, by the ring of dust which encircles the nebula. The visible portion occupies an arc of 2¼ minutes on each side of the centre. Its spectrum corresponds to that of star-group F-5 among the yellow stars in the Harvard classification. Therefore, it is not the light of the coherent gas-cloud which preponderates but rather that of the stars consolidated within the cloud, and corresponding to the stars in the Milky Way. This star-light is so bright that it entirely suppresses the radiation from the gas-cloud itself.
Such parts of the gas-aggregation as are most removed from the place of collision continue in their course through space little affected by the attraction of the central mass because of the great distance involved. The portions nearer the point of impact receive orbits curved by this same attraction and the curvature becomes the sharper the nearer the axis of rotation. One result of the mutual gravitation between the central mass and the particles in the outer sections of the nebula is also that the velocity in the spiral arms becomes smaller the farther the section in question is located from the centre, just as comets in the solar system move slower the farther they are removed from the Sun. But in all portions outside the central region the attraction is too weak to give circular orbits to the gaseous matter. All substance in these localities, therefore, departs ever more from the centre. As the spiral arms stretch out into straight lines such matter finally leaves the central disk altogether. It is possible that only the disk itself remains in the nebula computed by Pease.
Another astronomer on Mount Wilson, A. Van Maanen, has investigated a nebula, No. 101 in Messier’s catalogue ([Fig. 4]). This nebula lies nearly at a right angle to the line of vision which consequently almost coincides with the axis of rotation. The motion of the different parts of this nebula has been calculated with the help of photographs taken in the years 1899, 1908, and 1914, whereby its changed position with reference to surrounding fixed stars has been recorded. Out of 87 points in the spirals only 9 moved in the direction of the hands on a clock while the other 78 moved in the opposite sense. The mean angular velocity is 0.022 seconds of arc per year, corresponding to 85,000 years for one complete revolution at 5 minutes’ distance from the centre. The absolute velocity 2 minutes of arc from the centre is 1.5 times as great as at 7.5 minutes’ distance.
[Fig. 4] is reproduced from Van Maanen’s original. It shows clearly the general regularity in the motion of the component parts as well as the numerous exceptions to the rule. Such exceptions may be caused by perturbations due to invading masses, which impart their own motion to the entangling matter. These foreign bodies have probably condensed surrounding vapours and this created the bright knots which stud the nebular spirals. The upward motion amounts on the average to 0.007 seconds per year. While the points of condensation describe half a revolution around the centre they depart therefrom to about twice their original distance. More than a million years is therefore likely to elapse before the outer portions of the nebula are so far removed from the nucleus that the spiral form of the nebula is no more apparent.
Fig. 4. Internal motions in Messier 101. The arrows indicate the direction and magnitude of the mean annual motions. Their scale (0”1) is indicated on the plate. The scale of the nebula is 1mm = 10”5. The comparison stars are enclosed in circles.
From Proceedings of the National Academy of Sciences of U. S. A.
Fig. 5. Spiral nebula No. 51 in Messier’s Catalogue; situated in Canes Venatici, and photographed February 7 and 8, 1910, from Mount Wilson Observatory in California. Scale: 1 millimeter = 5 sec. arc.