Star

We find ourselves then in a galaxy which is just one amid per haps a million spiral nebulae. The nearest of the other systems is so remote that light takes nearly a million years to traverse the void which separates us from it. The number of stars in our own galaxy is at the lowest estimate 3,00o millions—a prodigious number, but we might, perhaps, recall that it amounts to less than two stars apiece for the human inhabitants of the earth. To one of the less distinguished stars of this host is attached the earth, which is our home. The evolutionary process which has swept across this system, condensing stars out of the primitive mist, has occupied a time which must probably be reckoned in millions of millions of years. From this orgy of inconceivable numbers we may, perhaps, glean an impression of man's place in the material universe. The space and the time which belong to him are as nothing in the far extent and the untold ages of inorganic nature. Is terrestrial man the one and final purpose of it all? Nothing in astronomy has appealed more to the imagination than the conception that each of the myriad points of light in the may be giving warmth and light to planets like our earth. It would seem a presumption to deny to them inhabitants of the same order of creation as ourselves. Nevertheless, we must not forget the prodigality of Nature. If indeed she has no grander aim than to bring forth her favoured child, Man, it would be just like her methods to scatter a million stars whereof but two or three might haply achieve her purpose. Strong reasons have been given by J. H. Jeans for regarding our solar system as a very unusual development. Although probably not unique, the formation of a system of planets is not the normal course of evolution of a star. It only happens when, at a critical stage of development, disrup tion is caused by the accidental approach of another star—a f or tune which, perhaps, not one star in a hundred millions would be likely to undergo. This theory is subject to much uncertainty; but it is at least a useful corrective to the view, often too facilely accepted, which assumes an almost infinite plurality of living worlds.

History of the Study of Stars.

In the older books on astron omy most of the space is devoted to the bodies of the solar system (sun, moon, planets, comets) ; the little that was known of the stars could be compressed into two or three chapters at the end. It is natural that our first acquired knowledge should relate chiefly to the bodies that are nearest to us. Much labour was de voted to observing the stars, but that was largely because they had to serve as graduation marks in the sky against which the move ments of the planets and comets could be recorded. Of late the centrifugal march of astronomical knowledge has been very re markable, and the study of the stars is now the largest field of astronomical research. The following may be selected as the chief landmarks in this progress: I. The first variable star (Mira Ceti) was discovered by Fabricius in 1596.

2. The first double star (Mizar) was discovered by Jean Bap tiste Riccioli in 165o.

3. Edmund Halley first detected the proper motions of stars (Arcturus, Aldebaran, Sirius) in 1718.

4. Sir William Herschel may be regarded as the pioneer of studies of the sidereal system. He discovered the solar motion (motion of the sun through the system of the stars) in 1783. By his famous counts of stars (star-gauges) he established the flat tened form of the stellar universe. In 1803 he was able to an nounce, from his measures of double stars made during the pre vious 25 years, the confirmation of the theory (urged by Christian Mayer in 1779) that the components revolve round one another.

5. In 1839 distances of the two stars 61 Cygni and a Centauri were found by measurement of parallax by F. W. Bessel and T.

Henderson, respectively. This gave the first definite idea of the scale of the stellar system.

6. In 1862 William Huggins applied the spectroscope to the stars and identified many familiar terrestrial elements as present in their atmospheres. The classification of stars according to their spectra was instituted by Angelo Secchi shortly of terwards.

7. Photography of the stars was developed between 1882 and 1887, the international photographic chart of the heavens and catalogue (not yet completed) being planned in the latter year.

8. Measurement of the velocities of stars in the line of sight by means of the spectroscope was developed very slowly in the latter part of the 19th century. The early measurements were entirely untrustworthy, and it is difficult to say when this power ful aid to stellar research properly started. Good results were obtained by H. C. Vogel and J. Scheiner in 1888-91. A great development of this branch of work was brought about by W. W. Campbell from 1897 onwards. The first spectroscopic binary star (Mizar) was found by E. C. Pickering in 1889.

9. J.

C. Kapteyn discovered, in 1904, that, at least in our local system, the stars are moving in two main streams. This gave great stimulation to statistical studies of the sidereal universe.

10. The distinction between giant and dwarf stars made mani fest by the researches of E. Hertzsprung and H. N. Russell began to be generally accepted about 1913.

II. The first direct determination of the angular diameter of a star, i.e., the apparent size of its disc, was made by F. G. Pease and J. A. Anderson in 1920, using an interferometer designed by A. A. Michelson.

12.

The physical theory of ionization was applied by M. N. Saha to the interpretation of spectral features of the stars in 192o. This led to a branch of research which has had most extensive developments.

The Twinkling of

Stars.—Before entering on a fuller account of this progress we must refer to a question which is often put.

Why do the stars twinkle? If we trace the beam of light, roughly -1.in. in diameter, which fills the eye-pupil back towards the star, it has passed through some miles of terrestrial atmosphere. Dif ferences of density due to slight inequalities of temperature will cause continual changes of refraction at one point or another on the path, giving rise to a general unsteadiness. More important still is the "interference of light." The irregularity of density may be such that the light at opposite edges of the narrow beam has traversed different effective thicknesses of air, so that the corresponding waves are delayed by different amounts. If the delay amounts to half a wave-length the waves will arrive crest on-trough, and, instead of summing up to give a bright star image, they will cancel one another. Thus the image pops in and out as the changes of atmospheric density succeed one another. The condition for cancelling is different for light of different colours (corresponding to different wave-lengths), consequently the star sparkles with different colours. To understand why this does not happen to the light of the planets we must recall that the disc of the planet, unlike the star-disc, subtends a consider able angle at the eye. The diameter of Saturn, for example, is normally about 20 seconds of arc, which means that at the height of a mile the rays which reach the eye fill an area 6 in. in diam eter. This gives plenty of scope for the variations of the individual beams in. in diameter) to average out, and consequently they combine to give a fairly steady brightness. _Interference or cancel ling of waves occurs between waves reaching different parts of the pupil, but not between waves starting from different parts of the disc of the planet or star; thus twinkling depends on the ratio between the angular sizes of the celestial object and of the eye pupil as viewed from an average height in the earth's atmosphere