NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, B.A., D.Sc, F.R.A.S. 



THE NUMBER OF VISIBLE STARS.— The enquiry as 

 to the number of stars in the entire heavens that can be seen 

 with our best telescopes, or photographed by them, is a very 

 interesting one. Various estimates have been given at 

 different times, some going as high as a thousand millions, 

 but it now appears that this number is greatly in excess of 

 the truth. Naked-eye observers are often surprised on 

 hearing that they can seldom see more than two thousand 

 stars at a time. In the same way, when one has seen some 

 of the rich fields of the sky in a large telescope, with the 

 whole region thickly studded with stellar points, one is apt to 

 make extravagant estimates of the number in the whole sky. 

 Mr. R. H. Tucker has a paper in Publications of the 

 Astronomical Society of the Pacific for August, in which 

 he gives, by the collation of various authorities, the number 

 which can be seen visually (i.e., down to the 17th magnitude) 

 as forty million, or a thousand to the square degree. This is 

 about the same as the number of inhabitants of Great 

 Britain. He gives the number that can be photographed 

 (i.e., down to the 20th magnitude) as a hundred millions. It 

 is well known that the relative increase in number for each 

 additional magnitude falls off for the fainter stars, indicating 

 either an absorption of light in space or our actual approach 

 to the limits of our system. In any case, the number given 

 must be less than the real number of stars in our system, for 

 there are many stars of small intrinsic luminosity which are 

 faint even when near us, and would be invisible in the more 

 remote parts of the system. Of course, no account can be 

 taken of dark, burnt-out stars, as we have no means of 

 forming an estimate of the number of these. If we suppose 

 the star density to remain constant throughout our system 

 (which is probably not true) and take the number of stars 

 within sixteen light-years as eighteen, then a hundred million 

 stars would fill a sphere of radius three thousand light-years, 

 which is the same value as Newcomb's for the distance of the 

 outer parts of the Galaxy. But the value rests on such 

 doubtful assumptions that it is only a rough guess. 



The same publication contains an article by H. D. Curtis 

 on the unit to be used for stellar distances. He advocates 

 the continuance of the use of the light-year, instead of the 

 astron or parsec, on the grounds that it is more easily grasped 

 by the general reader, and also that it is known to one part 

 in ten thousand, or ten times as accurately as the solar 

 parallax. As regards the first point, I do not think that anyone 

 proposes to drop the light-year for popular purposes ; the 

 second does not seem to me to carry much weight, in view of 

 the fact that star-distances can only be found in terms of the 

 sun's distance, and also that no stellar parallax is trustworthy 

 to more than two significant figures. The parsec is advocated 

 because it enables us to pass almost instantaneously from 

 parallax to distance, thus saving a large amount of mental 

 arithmetic. 



There is one point in his paper which gives some food 

 for thought : he speaks of the light-year as being six trillion 

 miles. This is an instance of the American system that calls 

 a thousand millions a billion, a thousand billions a trillion, and 

 so on. The English system makes a million millions a billion, 

 a million billions a trillion, and so on. It has the logical 

 advantage that the bi-, tri-, and so on, express the powers to 

 which a million is raised, whereas the prefixes are meaningless 

 on the American system ; moreover, there is the important 

 advantage that fewer new names are introduced in the 

 expression of large numbers. The adoption of a uniform 

 system of arithmetical notation throughout the world seems a 

 more important matter than the question of the relative 

 advantages of the light-year or the parsec. 



GIANT AND DWARF STARS.— Professor H. N. Russell 

 gave an interesting address on this subject at the June 

 meeting of the Royal Astronomical Society, and a summary 

 of it appears in the Observatory for August. He takes all 

 the stars for which parallaxes worthy of any confidence have 

 been obtained, and classifies them according to intrinsic 

 brightness and spectral type. He concludes that all the 

 intrinsically very faint stars are red, of spectral type K or M, 

 while all the stars of types A, B are much brighter than the sun. 

 There are, however, a number of red stars that are intrinsically 

 very bright, such as Arcturus, Aldebaran, Antares. The stars 

 of types K, M are either much brighter than the sun or much 

 fainter. Of the dwarf stars of class M not one is visible to the 

 naked eye, although one is the second nearest star in the 

 heavens. 



Discussing the masses, he gives reasons for supposing that 

 these differ among themselves much less than the intrinsic 

 brightness, and that the dwarf stars are those of very low 

 surface brightness. 



He conjectures that the giant red stars are at an early stage 

 of low temperature, low surface brightness, low density, great 

 surface, and consequently great total light. As each contracted 

 it would grow hotter and whiter, but smaller, so that its total 

 light would not change much. When hottest it would have a 

 spectrum of type A or B ; only massive stars would attain the 

 highest temperature, and stars of types A, B are found to be 

 massive in those cases where their mass can be determined. 

 After passing the maximum temperature it would grow smaller, 

 redder, and duller, i.e., it would pass to the class of dwarf stars. 

 According to this view the difference between giants and 

 dwarfs is not one of total mass, but of age and degree of con- 

 densation. It is, of course, rather curious that stars in 

 such different conditions should give the same type of 

 spectrum, i.e., that type M should occur both at the 

 beginning and the end of a star's career; but the evidence 

 seems strong that these M stars do form two different classes. 



Professor Russell hopes to publish his researches more fully 

 next year, and we may look forward to their appearance 

 as likely to throw new light on the life-history of the stars. 



A NEW COMET. — The second comet of the year was 

 found by Dr. J. Metcalf, on September 2nd, being his third 

 discovery of the kind. It was of the tenth magnitude, in 

 Right Ascension 6 h 50 m , North Declination 57°. Daily 

 motion minus l m 16 s , North 34'. If an ephemeris for October 

 should be ready in time, it will be given in another column, 

 with " Face of the Sky " for November. 



Later. — The comet was nearest to the Sun on July 20th, but 

 approached the Earth in September so as to grow slightly 

 lighter ; but it will not be conspicuous. Another faint comet 

 was found by Neujmin on September 3rd. 



BOTANY. 



By Professor F. Cavers, D.Sc, F.L.S. 



BOTANY AT THE BRITISH ASSOCIATION.— The 

 doings of Section K (Botany) at the Birmingham meeting 

 presented various features of interest. For the first time 

 in the history of the British Association, a lady was chosen as 

 President of a Section, and in her address Miss Ethel Sargant 

 referred in felicitous terms to this innovation as an honour 

 done to herself as a botanist, but also as an act of generosity 

 — greater because done in the face of custom and prejudice. 



RECENT RESEARCH ON EMBRYOLOGY OF 

 ANGIOSPERMS— Miss Sargant, after a tribute to the work 

 and influence of the late Lord Avebury, as a representative of 

 a small but distinguished class of naturalists, to whom she 



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