IN STARRY SKIES

Evening Post, Volume CVII, Issue 67, 22 March 1929, Page 16

IN STARRY SKIES

A SPIRAL NEBULA SEEPJ EDGEWISE

(By "Omega Centauri.")

Coma Berenices is a small and rather an insignificant constellation wedged in amongst others .that are larger, more brilliant, and much better known, but it contains so many objects of interest that these quite make up what it lacks in size and brightness. The Herdsman Bootes, with his brilliant Arcturus, is beside it on the east, the Lion, with Eegulus, on the west, the Great Bear and the Hunting Dogs are to the north and the Virgin to the south. None of the stars of Berenice's Hair are as bright as the fourth magnitude, but they are so numerous that they- make a- fine sight through field glasses. But the chief interest of this region is revealed only by the telescope, which shows that spiral nebulae are astonishingly abundant round the northern polo of the galaxy, which lies between Beta and Gamma in this constellation.

This photograph was taken by Professor G. W. Eitchey on March 6th and 7th, 1910, with the 60in reflector

at Mount Wilson. An exposure of 5 hours was given on a seed 23 plate. The nebula does not seem to have a popular name although it is one of the best known of the spirals, this very photograph having been reproduced in many books" on astronomy. The Nebula is number 24 in Herschel's fifth catalogue, and number ■ ;4565 in the N.G.C. The nebula is within three degrees of the star Gamma, the third brightest in the constellation, and about two degrees from the Galactic pole. To find its position in the sky, imagine a line from Regulus to Arcturus divided into 5 equal parts. The nebula is about 4 degrees below the third point of division. The nebula is thus rather nearer to Arcturus than to Eogulus. During March it is not on the meridian until after midnight, but by the beginning of June it will bo due north about 8 p.m. The nebula, though an extremely interesting object, is rather faint and disappointing when seen through a small telescope. It is fortunate that such an excellent photograph has been taken of it. It looks very different from such nebulae as the Great Nebula in Anclroii'.eda and its wonderful rivnl in the- Tnanglc. But the difference is chieily one of aspect. The nebula in the Triangle- is broadside on to us. The nebula in Andromeda we see obliquely, but H. 24 is almost directly edge-on to us. It is not peculiar in this respect, but it is the largest northern nebula so seen.

The field shown in the photograph is about 25 by 20 minutes of arc. Ono of the most interesting features of this nebula is the dark marking which crosses the nucleus and almost cuts it in two. This puzzled the early observers very greatly. Even Sir John Horschol seems to have been uncertain whether it was a single or a double nebula. Admiral Smyth in his "Cycle of Celestial Objects" describes it as a long white nebula in the centre of the tresses. "It is," ho says, "a curious long and streaky object somewhat of a weaver's shuttle shape. Prom the description I received viva voce from Hcrschel, my attention was intently fixed upon this nebula; and after long and patient gazing, a parallel patch on the following limb was rather inferred than made out by a peculiar glow on that part. The parallel appendage to this nebula is a most extraordinary phenomenon, and is very beautifully figured in the Phil. Trans. 1833 by H., who considers the two as constituting a flat annulus seen at a great obliquity, but having very unequal breadths and densities in its two opposite semicircles. Or, asks he, must we admit the appendage to be a separate and distinct nebula, dependent, by some unknown physical relation, on its brighter neighbour?" The photograph leaves no such doubt. It shows what is clearly a single structure with a glowing nucleus partly hidden by a vast ring of obscuring matter. Many other nebulae seen edge-on show the same phenomenon. It seems, indeed,

to be a characteristic of spiral nebulae- to have around them darker and cooler matter, which, when seen edgewise, hides or obscures some of the brightest parts within. Curtis sees in this an explanation of the strange distribution of the spiral nebulae in the sky. None are seen near the galactic plane. The numbers increase as we go away from it, and there is considerable concentration of such systems near the galactic polos, especially around the northern one. Curtis suggested long ago that spirals may be distributed more uniformly, and that those in the direction of the galactic plane may be hidden by obstructing matter, which is almost certain to exist if the galaxy is, as most now believe, an immense spiral nebula.

To realise in full the wonderful nature of the object seen in this picture we must remember that the light by which its photograph was taken had probably been about ten million years on its journey towards us, that the mass of the nebula is probably equal to thnt of some hundred million suns, that it is but one of 103 known nebulae in a space uot ten ck-grecs across in. tlii:; small constellation. In crossing ever, from 0110 sida of: this nebula to thu other, light is supposed to take about ten thousand'years, and the nebula itself 'has been found by Slipher of the Lowe! Observatory to be moving away from us at the astonishing rate of u'2s miles a second.