Island Universes Research Articles

Classics of Science: The Island Universes

Classics of Science: The Island Universes

ON THE MOTIONS OF SPIRALS

In a note on Messier 331 I made use of Dr. van Maanen's results for the translational motion of the nebula, but not of his results interpreted as internal motions for the same object.2 Dr. Harlow Shapley has called my attention to this fact and expressed the opinion that van Maanen's measures practically eliminate the island universe theory. It is obvious that the parallax for Messier 33 estimated by van Maanen from the internal motions in this object is not in harmony with the parallax value suggested by me as based upon certain spectroscopic results.1 I was aware of these facts, and the only reason for not dealing with this discrepancy in the note in question was my desire to discuss in a later paper the present state of the problem of the distances of spirals and in this connection use van Maanen's data. Although agreeing with van Maanen's statement that definite conclusions as to the real meanings of these motions must await further measurements, I will make here some preliminary comments on the exceedingly interesting measures of motions in spirals, recently published by van Maanen. For two spirals, N. G. C. 4594 and N. G. C. 224, Pease has found a component of the radial velocity, evidently due to a rotation of the nebulae in the plane of the spiral arms. The rotation in the first of these objects was discovered by Slipher, and his measurements are confirmed by Pease's investigation. The motion is a purely rotational one, increasing with the distance from the center. The internal motions in four spirals measured by van Maanen from photographic plates separated by intervals of time ranging from 5 to 15 years appear, if interpreted as real motions, to be of another kind, since after subtracting the translational motion the points close to the centers have a rotational velocity practically equal to that of the outlying regions. For Messier 51 and 81 there seems to be a slight increase in the rotational component with distance from center, but in Messier 101 that component decreases a little. As there is no reason to sup-

NOVAE IN THE SPIRAL NEBULAE AND THE ISLAND UNIVERSE THEORY

NOVAE IN THE SPIRAL NEBULAE AND THE ISLAND UNIVERSE THEORY

Our Astronomical Column

THE STRUCTURE OF THE UNIVERSE.—If the distance of each star were known in addition to its position in the sky, our knowledge of the present structure of the universe would be complete. In order to determine the change in the structure, it would be only necessary to know the motions of the stars. We know the positions of a great number of the stars and their motions across the line of sight; we know also the velocities in the line of sight of a few, and the distances of a still smaller number. The data for the solution of the problem are therefore very meagre. Nevertheless, there are indirect methods of attacking the problem which may tend to lead one to an approximate solution, and it is these methods which form the subject of the very instructive article which appears in the July number of Science Progress by Mr. H. Spencer Jones, of the Royal Observatory, Greenwich. The idea, as he states, that the centre of our system is occupied by an immense sun, many thousands of times larger and more glorious than our own sun, and that round about it are millions of lesser suns of various sizes, together forming the nucleus of an immense spiral nebula, of which the spiral arms coiling around the nucleus appear to us as the Milky Way, and that this to us immense system is but one, and perhaps a comparatively small, island universe amongst thousands or millions of other island universes in space, is an idea which by its magnificence appeals to the mind of man. What forms the substance of the article is the basis of truth upon which this conception is founded, and the straightforward and clear way in which the author has marshalled his evidence makes the article of particular interest.