STAR SYSTEMS

Evening Post, Volume CXIII, Issue 54, 5 March 1927, Page 3

STAR SYSTEMS

THE SPIRAL NEBULAE

AN ISLAND UNIVERSE

mi. HUBBLE'S DISCOVERIES

The great 100-inch reflecting telescope of the Mount Wilson Observatory, California, has more than justified its existence. By means of this gigantic instrument the diameters of the largest giant stars were successfully measured some years 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 members of a mysterious class of objects (writes the astronomical correspondent of the "Manchester Guardian"). Ever since the cloudylooking spots on the sky known as nebulae were first closely investigated by Herschel 150 years ago, they have been the subject of much speculation and controversy. At first Heiscliel regarded the nebulae as, like the globular clusters, great outlying systems of stars, and independent of the stellar system to which the sun and stars belong. The great astronomer himself abandoned this view, and latterly came to regard the bright nebula in Orion and other kindred objects, as well as the small, round "planetary" nebulae, as mast masses of primeval worldstuff, out of which, he believed, the suns and worlds of the future would be evolved. That Herschel was rigi't, inasmuch as these great diffuse nebulae are non-stel-lar, was proved beyond a doubt when Huggins first applied the spectroscope to the nebulae in 18G4. But these great diffuse nebulosities do not exhaust the nebular family. Herschel 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 oi 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. AN OLD CONTROVERSY. Por many years controversy has 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-Hue 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 diffusi gaseous nebulae are concentrated to Vie plane of the Milky Way, while the spirals are about as far from the Milky Way—the referenceplane 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 systems at 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-consti-tution.

Two years ago Dr. Hubble, using the great 100-inch telescope on Mount Wilson, succeeded in partially resolving two of tho brightest spirals—the nebulae in Andromeda aud Triangulum respectively—into stars. . lie further succeeded by means of the variable stars in these nebulae in fixing their distances, which caiiK out as just under a. million light-years or six million billion miles. Dr. Hubble has now published in pamphlet form the results of his study of the spiral in Triangulum, known as Messier 33, from its -lumber in Messier's famous catalogue. This nebula is just visible to the unaided eye not far fro its companion spiral, Messier 31, better known as thr great nebula iv Andromeda. Dr. Hubble has resolved the outermost portions of the nebula into "swarms of actual stars." The central region is unresolved, aud the "stars appear to merge smoothly into the unresolved background." Indeed, Dr. Hubble holds "it is doubtful 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.

LATEST DISCOVERIES. Dr. Hubble tells us in his |>apcr that ho 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. Tlunebula, (lien, is composed of stars and nebulae very similar to those in the stellar system. Another point of resemblance has also been brought out by Dr. Hubble. He succeeded in counting the number of stars :lown to the nineteenth photographic magnitudel. The stars of the brightest mag nitudc appear to be distributed in n manner similar to those in the region of our stellar system round the sun Dr. Hubble has further confirmed the distance of nearly a million light years which he found from his study of the variable stars. The distance" beinc known, the angular diameter «uf tho spiral can be converted into linear di-i 13,000 light-years. The spiral would thus appeal to be a much smaller system than the main stellar system, the inn nnSn CV OiV vllich apponra l" be abou' 4U0,000 light-years.

Five outlying systems are mow known -the two spirals in Triangulum and Andromeda, a faint cloud-iike obiecf known as N.G.C. 6822, „„* rhe two Nubeculac or Magellanic Clouds visible only j,, the Souther,, Hemisphere Unlike the globular clusters, which -,re symmetrically arranged relative to the stellar system, these five appear to be quite independent and, therefore, to merit the designation of "island uni-