THE HEAVENS

Hawera Star, Volume XLVI, 1 April 1927, Page 8

THE HEAVENS

SECRET OF SPIRAL NEBULAE. WHAT LAST TWO YEARS HAVE REVEALED. The great Kkf-inch reflecting telescope or the Mount, Wilson Observatory, California, has more than justified its existence. By means of this gigantic instrument the diameters or the largest giant stars were successfully measured some yeafs ago, and at long last the enigmatical spiral nebulae appear to have yielded up the secret of their nature and structure.

The spirals have been, indeed, the most mysterious memoers 0.l a mysterious class of objects (writes the astronomical coriespondent or the “Manchester Guardian'’). Ever since tire eloudy-lcoking spots on the shy known as nebulae were nrst ciosely investigated by iierschel 150 years ago, tiiey have been the subject of muen speculation and controversy. At first rterschel regarded the neoulae as, like the globular clusters, great outlying systern a of stars, and independent of tne stedar system to which tne sun and stars belong. The great astronomer himselr abandoned tliis view, and latterly came to regard the brignt- nebula in Orion and other kindred objects, as well as the small, round “planetary" nebulae, as vast masses of primeval worldstufl, out of which, he believed, the suns anci worlds of the future would be evolved. That Hensehel was right, inasmuch as tnese great diffuse nebulae are nonstellar, was proved beyond a doubt when Huggins first applied the spectroscope to the nebulae in 1864. But these great diffuse nebulosities do not exhaust the nebular family. Herscliel, was unaware of the existence of the large class of “spiral" nebulae. Many of the members of this class were known to him, but his telescopes were not powerful enough to show the spiral form; and so he. registered them as cloudy spots of light. Lord Rosse’s great telescope in 1845 first revealed the true form of fourteen of these, and this discovery, which was received with considerable scepticism by a number of astronomers, was placed beyond a doubt in the latter part of the nineteenth century, when several astronomers undertook great photographic surveys of the sky. Many thousands of these spiral nebulae have ’ been photographed and catalogued. For many years controversy lias raged concerning the nature of the spirals. That they are totally different in kind from the great diffuse nebulosities typified by the nebulae in Orion, Cygnus, and Sagittarius admits of no doubt. These great clouds give bright-line spectra, which proves them to be clouds in fact as well as in appearance; the spirals have continuous or stellar spectra. The great nebulae are almost at rest in space, while the spirals are moving with very rapid velocities. Further, the * diffuse nebulosities typified by the*, nebulae in Orion. Oygnus, and Sagittarius admits of no doubt. These great clouds give bright-line spectra, which proves them to be clouds in fact as well as in appearance; the spirals have continuous or stellar spectra. The great nebulae are almost at rest in space, while the spirals are moving with very rapid velocities. Further, the diffuse gaseous nebulae are concentrated to the plane of the Milky Way, while the spirals are about as far from the Milky Wav—the reference-plane of the stellar system —as they can be. Accordingly many, astronomers during the last quarter of a century have looked with favour on the view that these spirals are independent stellar system as very great distances. Others, again, held as strongly to the theory that the spirals are true nebulae, and this seemed to be confirmed by some measurements of photographs at Mount Wilson, which appeared to indicate an appreciable movement of the condensations or “knots” in the arms of some of the spirals. This suggested comparative proximity and non-stellar constitution .

Two years, ago Dr. Hubble, using the great 100-incli telescope on Mount Wilson, succeeded in partially resolving two of the brightest spirais—the nebulae in Andromeda and Triangulum respectively—into stars. He further succeeded by means of the variable stars in these nebulae in fixing their distances, which came out as just under a million light-years, or six million billion miles. Dr. Hubble lias now published in pampnlet form the results of liis study of the spiral in Triangulum, known as Messier 33, from its number in Messier’s famous catalogue. This nebula is just visible to the unaided eye from its companion spiral, Messier 31, better known as the gre’at nebula in Andromeda. Dr. Hubble lias resolved the outermost portions of the nebula into “swarms of actual stars." The central region is unresolved, and “the stars appear to merge smoothly into the unresolved background.” Indeed, Dr. Hubble holds “it is doubtiul how far the process of resolution can be carried.” But the evidence is decisive that the nebula is a great cluster of stars, in many ways akin to the main stellar system, and apparently disconnected with it.

Dr. Hubble tells us- in liis paper that he has found forty-two valuable stars in addition to three previously known, and that two temporary stars have been detected. Several patches of diffuse nebulosity were detected in the outer arms of the spiral. The brighter of these patches have been separately studied by means of the spectroscope and show bright lines. As in the main stellar system, these gaseous nebulae appear to be connected with the hottest stars, which are blue in colour. The nebula, then, is composed of stars anti nebulae very similar to those in the stellar system. Another point of resemblance has been brought out by Dr. Hubble. He succeeded in counting the number of stars down to the nineteenth photographic magnitude. Dr. Hubble lias further coniirmcd the distance of nearly a million light years which he found from liis study of the variable stars. The distance being known, the angular diameter of the spiral can, be converted into linear diameter, and this comes out at about 13,000 light-years. The spiral would thus appear to be a much smaller system than the main stellar system, the diameter of which, appears to be about 300.000 light-years. Five outlying systems are now known —the two spirals in Triangulum and Andromeda, a faint cloud-like object known as N.G.C. 6822, and the two Nubeculae or Magellanic Clouds, visible only in the Southern Hemisphere. Unlike the globular clusters, which are symmetrically arranged relative to the stellar system, these five appear to be quite independent, and, therefore, to merit the designation of “island universes.’’