IN STARRY SKIES

Evening Post, Volume CX, Issue 50, 27 August 1925, Page 15

IN STARRY SKIES

WHAT ARE THE SPIRAL

NEBULAE?

POLAR CAPS OF GALAXY OR DISTANT,UNIVERSES

(By "Omega Centauri.")

Our mental picture of a spiral nebula is conditioned very largely by the estimate which we form of its distance, and the determination of that estimate is inextricably bound up with the question whether or not the White Nebulae form an intergral part of our galactic system. Unless they do- so, they must be regarded as island universes, comparable in size and importance with the Galaxy itself. K. A. Proctor was a strong advocate of the former point of view. Although he believed in the infinite extent of the universe, and tne infinite number of cosmic systems contained within it, he did not accept the idea that nebulae are external galaxies. As the; spaces be*tween the stars are enormous in comparison -with the dimensions of the stars themselves, so, he contended, were the spaces separating system from system in comparison with the sizes of individual systems. Since it is impossible to see the outermost limits of the Galaxy, it must be useless, he argued, to look for anything beyond. He thought it highly probable that island universes exist, but did,not recognise the spirals as being of this nature.

Professor A. W. Bickerton is another who argues strongly that the spiral nebulae form an essential part of the galactic system, but his reasons for upholding this view differ entirely from those of Proctor. They are* set out in detail in "The Eomance of the Heavens./' In this Bickerton pictures the Milky Way, as having its origin in the encounter of two great cosmic systems, in the course of which a'large portion of the material of each system was swung out into a great spiral arm. These spiral arms form the vast encircling ring of the Galaxy. In the- pprtiona of the ■original systems which interpenetrated one another innumerable collisions must have occurred, and these stellar impacts must have filled a vast region of space with gas at an enormously high temperature. This gas would tend to escape in all directions, but most of that near the plane of the galaxy would be picked up by the stars. In directions at right angles to this plane, there would be little to prevent escape, and so vast, masses of gas would bo expelled, and would ultimately form nebulous caps in the region of the yoles of the Galaxy. On their outward journey these gas clouds would encounter one another, and often a rapidjy moving cloud would overtake and collide with ianother moving more slowly. It is to such encounters that Bickerton attributes the charactertistio spiral forms of the non-galactic nebulae.

Eecently Professor Lindemann has proposed a somewhat similar theory, but he considers that the nebuloses material was expelled from the Galaxy chiefly by radiation pressure, and that the resulting nebulae are shining by the reflected light of the stars of the Milky Way. In "General Astronomy," 1923, H. Spencer Jones sums up his account of the nebulae in favour of the spirals belonging to the galactic system, but he adds: -"Although the balance of evidence at present seems opposed to the Island-Universe theory "the question cannot be regarded as yet definitely closed." One of the strongest advocates of the opposite view is Professor H. D. Curtis. In a delightful lecture, published in 1917, he discusses all types of nebulae. In summing up all available evidence bearing on the problem of the nature of the spirals he shows that if we regard them as members of our galactic system we have serious difficulty in explaining their distribution, their distances, their tremendous speeds, and their constitution as revealed by the spectroscope. Regarded in that way they do not fit in with any scheme of stellar evolution. If on the other hand we regard them as separate galaxies he maintains that most of the difficulties vanish. Distances ranging from ten to a hundred million lightyears accord with the negative results of parallan determinations. The spectra are what would be expected from con. geries of stars. Their peculiar nongalactic distribution is explained by the fact that non-luminous obstructing matter connected with the Galaxy cuts off from our view spirals and all other distant systems that lie at all near to the plane of the Milky Way. The appearance of Novae within these nebulae is natural if they are galaxies, but is hard to explain if they are clouds of dust or gas belonging to our system. Their great velocities, agreeing with those of the magellanic clouds, seem more fitting for Island Universes than they do for parts of a single system. In the Lick Observatory Publications, volume XIII., 1918, Professor Curtis says:—"lt is my belief that all the many thousands of nebulae, not definitely to be classed as diffuse or planetary, arf> true spiraJs, and' that the very minute spiral nebulae appear as textureless discs or ovals solely because of their small size. Were the Great Nebula of Andromeda situated 500 times as far away as at present it would appear as a structureless oval about 0.2 minutes long, with very bright centre and not to be distinguished from the thousands of very small, round or ovjl nebulae found wherever the spirals are iormed. "I see no reason; at present existing, for changing the estimate made in this paper, that at least 700,000, and very probably a million, small spirals are within reach of the large reflecting telescopes." In the lecture already referred to, -which was published as one of the A^dolfo Stahl lectures, he says: "The future may possibly bring to light new facts which will enable us to give some other explanation, but our present evidence, so far as it goes, leads to the belief that the spirals are composed of great clouds of stars, so infinitely distant that we cannot make out the individual stars, much as our own Milky Waly, which is seen in the telescope to be made up of millions of closely packed stars, to the unaidad eye appears as a faint nebulous luminous band across the sky."

Dr. W. W. Campbell expressed very similar views. He considered that the faint nebulae are almost all spirals. They are found to have radial velocities up to 600 or 700 miles per second, and it is probable that the average of their real velocities in space is of the order of 500 miles a second. And yet, in spite of this, changes in apparent position in any of these nebulae are exceedingly difficult to detect, even in long periods of time. It seems, therefore, that these systems, whatever they may be, nre exceedingly distant and enormously large. Some probably contain enough material to make tens of thousands, or perhaps millions, of stars comparable in j mass with our sun. The fact that their spectra aro such as might be expected i if they ■were known to consist of liinlli; i tudcs of stars, lends support to this view.. This was tlio stage readied in tlio -it; i tempt!) to noWo the prubJnm of theii juaturj) jei&bti pr nine yejre ftgQ, ■3ttt.*i

pendulum of opinion seemed to be swinging definitely towards the Island "Universe side. But during the 'ast few years new methods of judging distance have been developed, and many astronomical discoveries have been made. The problem has been re-dis-cussed in the light of* the new knowledge. We must leave until next week the consideration of the latest views on the problem.