IN STARRY SKIES

Evening Post, Volume CVII, Issue 55, 8 March 1929, Page 14

IN STARRY SKIES

•THE GREAT NEBULA IN

ANDROMEDA

(By "Omega C'entauri.■")

The Great Nebula in Andromeda, as ft appears to us, is tho largest of all jfto Spirals. Its greatest diameter is ht least' 135 minutes of arc, more than .four times that of the moon. Tho nojbula may, however, bo exceeded by many others in absoluto size, for it is one of tho'jioarest to us. Since photography revealed its marvellous form it las been justly regarded, as ono of the jchief glories of the heavens. <

It can bb detected with the naked jeye, but even, telescopes of considerable pize fail to.show the beautiful detail given in the photographs.

The best time to look for it hero is an October.or November, when during the evening the great square of Pegasus shines out clearly in the Northern eky. The bottom right hand corner of ;the square does not really belong to the plying Horse, for it is just over the iboi'derv of. the constellation, and is

■;£A.lpha,. the chief genij of Andromeda. ,'^Fhe next, three brightest stars of this '(constellation form a lino curving down'jwards from Alpha • -towards the right. ffhese, ml order, are Delta, Beta, and jGramina, and the spaces between the four aroabout soven, eight, and eleven jdegrees; respectively, the side, of the jgreafc. square being about twelve degrees. ;To find the Nebula, start from Beta, arid follow a line of stars that branches from the other almost at right angles, -downwards towards the left. Four degrees- along this line is '■ the [double star Mv, and three degrees^further the star Nu. The Great Nebula is less than ,two degrees to the left of Nu. It never rises more than about eight Idcgrces above our h.orizou, so unless the Bky is very clear it is best to look for it with field-glasses. It is only the central portion that you • will ace, as the epiral amis are very faint. In the best photographs the ■■light of the . brilliant Jiu'eleus has'been restrained to prevent lOver-exposuro.

The -photograph here shown was (taken with the 24in Yerkes refle'etor en 18th September, 1901, by Professor JG-. \V. Bitchey. Air exposure oi four ihours was given on a Cramer Crown plate.. :A.:;,photograpli taken at Mount [Wilson,';in 1925, with an exposure of tonly.twphqurs,.through tholOOin telescope, resolves portions of the, nebulos-;. ity into stars. It :s interesting to notice that two small nebulae are shown in the Yerkes' photograph. Thcsolook as if they bear to the Great Nebula

somewhat the same relation as the Magellan Clouds do to, the Galaxy. If they are really connected with the Great; Nebula- evon these smaller ones must ho of immense sizo. Doig in Ms "Stellar Astronomy" estimates that one has a diameter of 2700, ana the other a diameter of 1000, light years. These -estimates- are based on the distance of the Great Nebula, deduced from tile Copheid Variables in it, of 930,000 light'years. If this distauco is correct light must take about 50,000 years to cross from ono sido of tho nebula to tho" other. Tlio light given out by the nebula is estimated to equal that.of 630 million suns, whilst Hubble estimates its mass to bo that, of 3500 million suns. . ' . Spiral nebulae air appear to bo flat disc-like structures,.this form being probably tho result of extremely rapid rotation. In the Andromeda nebula the speed is 45 miles a second within two and a half minutes of the centre, and in another spiral a rotation nearly fivetimes as rapid has been observed. The discs are presented to our view at allangles. The Whirlpool, we saw, is almost broadside on. The Andromeda nebula is seen obliquely,'and others are exactly edge on. The Great Nebula was familiar to the Persians in the tenth century, but it appears to havo.been unknown, in Europe until it was observed by Simon

Marius in 1614, It is ; No. 31 in Messier Js catalogue '-of. objects likely to be mistaken for comets. In spito of his care it still leads to disappointment amongst comet' seekers. Huggins' was the : ..\'first to show that its spectrum is continuous with absorption line^, a spectrum rather like that of a G-type star. The lines in the spectrum are displaced, showing a volocity, of approach of about 20p miles-a second. In this respect the nebula is exceptional, for whilst nearly every spiral shows a high radial velocity, the great majority of them' are receding, from us. Many of their velocities are exceedingly high, one being over 1100 miles a second, and the average about 400 .miles a second. The Great Nebula in Andromeda, is remarkable for the number of Novae discovered in it. Between: 1909 and 1926 sixty-seven were observed, mostly in the inner regions of unresolved nebulosity. They were all voTy faint, most? ]y of the seventeenth magnitude photographically. ■ But on 19th August, 1885, one 'appeared, apparently near the centre of the .nucleiis, which reached the.' 7th magnitude....v'lf it was really within the nebula it must have been the most brilliant,object, nver seen in tho heavens.'. It ;was ;teii magnitudes brighter than the average of the others, that is it was ton thousand times as bright. It must, therefore, have shono a hundred million times as brightly as the sun. Five years later it had.sunk to the sixteenth: magnitude., .■."".'