IN STARRY SKIES

Evening Post, Volume CIV, Issue 26, 30 July 1927, Page 27

IN STARRY SKIES

STELLAR DISTRIBUTION

OF STARS

SIZE AND SHAPE OF GALAXY

(By "Omega Centauri.") Ever since Hersehel began to sound •he depths of space, astronomers have jeen studying the distribution of the stars. Do we live within a limited issemblage of them, or are we in an .nfimte universe whose stars are spread .vith some approach to uniformity? This .s £he next subject treated in Professor Dingle's ''Modern Astrophysics." [Jnless light is absorbed in its journey trough space it is clear that stars :annot be distributed uniformly ;hrough an infinite volume, for, considinng a spherical space around us, the lumber of stars would increase as the ' :übe, whilst' the light would diminisn I is the square of the radius. Thus I ;he total amount of light received at my place from such surroundings would je infinite. Professor Dingle points out ;hat, if there were an infinite -uniform listribution of stars, the gravitational ines of force reaching the earth would ilso be infinite in number. This, he iays, they are not, but he does not >ay how we can tell. The attraction of iuch a universe on a particle withn it would then be equal in every lirection and the earth or any other jody would bo unaffected by it. But m absolute uniformity iii distribulon seems hardly worth considering, tor the stars that we see are certainly lot so arranged. The Milky Way ap)ears to be of fundamental importance. Infortunately throughout this chapter Professor Dingle uses the word universe where galaxy or cosmic system vould be more appropriate. The probem he considers is really whether the Jalaxy extends indefinitely into space ir whether, with the greatest teleicopos, evidence can be secured which iroves it to be limited in extent. If the stars are grouped into sys,ems such as the Galaxy, and these sysems are separated from, one another ty distances very great compared with heir own dimensions, and ii again hese systems form others of a higher irder similarly widely spaced, the whole argument based on the infinite intenity of light absolutely fails. The total adiation received at any place in such i, universe would be. represented by an nfinite decreasing geometrical series, vhich might well have a finite sum. We shall consider, therefore, that our irst problem is to determine the shape md size of the Galaxy, leaving all quesions regarding the infinity of the universe itself for consideration later. If i we examine any portion of the Milky Way with a series of telescopes if increasing size, or if we take a series if photographs of it with increasing engths of exposure, we shall see or ecord continually more and more itars. This fact might be taken as svidence that we can find no limit to ■he extent of the Galaxy. But we must nquire whether we see as many more itars aa we should expect with each inirease of power. A larger telescope jrings into view not only many stars ;hat were too distant to be seen with imaller instruments, but also numbers >f comparatively near stars that wore nissed beforo because of their intrinsic 'aintness. But if stars of all magni;udes were evenly distributed through space, the average luminosity of stars n any one region would be tlie same is in any other. But the average apparent luminosity in different regions yould vary inversely as the square of ;he distance. Now, any Btar is 2.512 times as bright as one of the next ligher magnitude. So by increasing ;he square of the radius of the region 3xamined in this ratio, we bring into dew stars whose average magnitude in a sphero is proportional to the cube af the radius. The number of stats >f any apparent magnitude should ;herefore be the square root of the cube jf 2.512 times the number of the pre:eding. This ratio is about 3.98, or very nearly four. If stars were uniformly 3pread, there should therefore be nearly four times as many in the first two magnitudes as in the first magnitude alone, nearly four times as many in the first three magnitudes as in the first and second and so on. Now there are 11 stars brighter than first magnitude , 39 brighter than the second Hid 133 brighter than the third. These numbers give ratios 3.5 and 3.4, not far short of the theoretical ones. But the ratio gets steadily smaller as we pass to higher magnitudes. Tho number of stars brighter than the 9th magnitude is about three times the number brighten than the eighth, but tho number brighter than the twentieth magnitude is only about 1.6 times the number brighter than the nineteenth. There is, therefore, every indication that we are approaching a limit. The Galaxy, although of vast extent, ia not boundless. But what can be said of its shape? The band of light we call the Milky Way extends right round the heavens. The lucid stars appear to the naked eye to be more numerous in its neighbourhood, and photographs show this galactic concentration niore distinctly still. The central plane of the Milky Way is almost a great circle in the sky. The solar system, however, seems to be .situated a little to the north of this plane, so that the centre lino of the Galaxy forms what is called a small circle just south of the great circle already mentioned. Tho central plane appears to be a plnne of symmetry in tho system. The average number of stars per Bquaro degree diminishes steadily as the galactic latitude, or distance from the - central plane of the Galaxy, increases. This tendency is especially strongly marked in the case of the fainter stars. The ratio of the number of stars per sqnaro degree in Galactic latitude 5 degrees, divided by the number in the same area in galactic latitude 80 degrees, is called the galactic concentration. For stars of apparent magnitude 8.5 this is 2.6, for stars of apparent magnitude 17.5 it is 21.9. Tho stars begin to thin out more rapidly towards the galactic poles than near the central piano of the Galaxy. ' This appears to moan that the system is very much flattened. In directions near the galactic poles our giant telescopes can reach regions in which stars are negligibly few. In the direction of the central plane the system extends beyond the most distant stars whose prcscneo is revealed by the highest telescopic powers. The galactic concentration, however, is not the same for stars of all spectral types. It is greatest for early types, and becomes less and loss as we pass along the Harvard sequence. This will have to be taken into account when we consider the life history of the system. The Harvard classification does not divide the stars into groups containing approximately equal numbers. Stars of types F and G are far more numerous than those of any other typos. Shapley has calculated the relative numbers of stars iv the different spectral types in the direction of the central plane of the Galaxy. His table gives the number of stars in a region of one million cubic parsees. Out of 8716 stars n less than 7600 are classed as F5 to GO dwarfs, whilst the types BO to B5 contain only 4.4, and the types K5 to Me, giants only, 22. This seems to indicate that a star takes an exceedingly long time to pass through the P to G stage of its life history. In most discussions of the form and extent of the Galaxy it is assumed that the absorption of liglit in space is negligible. Now we know for certain

that some loss of light does take place. There are periodical eclipses of bright i stars by dark ones, and opaque bodies I that are quite unknown to us must be very numerous compared with those that make their presence known in this way. Dark nebulae also are numerous and of vast extent. These, however, are avoided in choosing regions in which-to make the stellar counts. But there may be finely divided matter in the form of cosmic dust, which would produce a far more noticeable effect. Macmillan has calculated that were stars uniformly distributed to infinity it would only require one particle of dust, one thousandth of an inch in diameter, in every 560 cubic miles of Bpace, J to produce the observed reduction in the | star ratios ' for different magnitudes. | This seems a very small quantity, but it would imply that in the region of the sun seven times as much matter is in- the form of dust as is concentrated in the stars. But fine dust would scatter'light rather than absorb it, and shorter wave lengths would suffer more than longer. This would have tho effect of making distant stars appear redder thau I nearer ones of the same type. Shapley has used the great globular star cluster in Hercules in -testing for this effect. l He could find no appreciable differences in the colour indices of this cluster from those of stars of the same type which are known to be comparatively near to us. This appears to show that the absorption co-efficient is extremely small. Kapteyn pointed out that, this co-ef-ficient must be known with great accuracy to enable us to put our results on a firm foundation. It should, he said, be measurable to within a thousandth of a magnitude. The general opinion now is that cosmic scattering of light is negligible in amount, and that the Galaxy really has the shape and limitations indicated by the star coun-ts that have been made. It must be remembered that it is only such absorbing matter as exists within the boundaries of the Galaxy that affects this determination. If our cosmic system is a spiral nebula far removed from all others, the vast regions separating system from system may be comparatively devoid of scattered material. Within the Galaxy itself, howfiver, dark matter is probably far more abundant than has hitherto been re-f-ognised. The existence of eclipsing variubles not only proves the reality of dark stars but givos some hint of their abundance. The Harvard sequence leads us to imagine myriads of red dwarf stars gradualling sinking into invisibility. Where are all those that have done so in the past? The dark nebulae so successfully photographed by Barnard and others show the presence of non-luminous matter spread through vast regions of space. Practically every spiral nebula also, which is seeii edgewise, is surrounded by dark matter which cuts off a land of its light. Analogy suggests that oar Galaxy must be similarly surrounded. The fact that spiral nebulae are not observed' in the direction of the Milky Way supports this inference, since their feeble light would be unable to penetrate the dark veil. Our solar system appears to lio ec-. ceatrically within a vast flattened system of stars and gaseous nebulae. This system extends perhaps 300,000 light years from side to side, and contains at least some four thousand million stars. All these are in rapid motion, and each has its own special characteristics. Our sun is a typical dwarf of type G. Much information with regard to the stars can be learnt from an intensive study of the sun. Planetary - nebulae, gaseous nebulae, and dark nebulae, are also members of the Galactic system. The status of the Globular star clusters requires further consideration, as does also that. Jof the spirals. # As the sun teaches us to realise the nature of a star, the Galaxy may help us to appreciate the grandeur of the distant spiral nebulae.

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