The above figures relate to the whole sky. Interesting results are found when we compare the density of distribution of the stars in different parts of the sky, particularly when we compare regions in the plane of the Milky Way with those near its poles. The galactic concentration (ratio of the number of stars in equal areas in the galactic plane and at the galactic pole) increases from 3.4 :1 for mag. 4 to 4.3 :1 for mag. ro, and to for mag. 18. These results refer to photographic magnitude, no sufficiently ex tensive counts for visual magnitude being available. The great excess of stars in the galactic regions is not wholly or even mainly due to the star-clouds of the Milky Way itself ; the star-density in creases steadily from the poles to the galactic plane, and the con centration is conspicuous before any part of the Milky Way is reached. This phenomenon was discovered by Sir William Her schel and the modern results are in good agreement with the star counts made by him and by his son, Sir John Herschel. It seems certain that the stars surveyed by us (the "local system") form an aggregation strongly flattened in shape like a bun or lens.
Only 20 stars are ranked as brighter than mag. 1.5. Of these 17 (including the variable Betelgeuse) are named in the above list. The other three are the southern stars, a Centauri (mag.
o.o6), Centauri (o.86), (3 Crucis (1.5o).
Stars not bright enough to have a constellation letter or num ber are designated by their reference number in some catalogue. Thus we have names like Bradley 1,940, Groombridge 2,23o, Boss 3,932, the last reference (Lewis Boss, Preliminary General Cata logue), being especially preferred. The most comprehensive source of reference is the Bonn Durchmusterung made by Argelander. This work, completed in 1862, lists the places of 324,00o stars between the north pole and S. An extension to 23° S. by Schonfeld adds 133,000 stars. Star charts to be used with the catalogue are published, and there is no difficulty in identifying in the sky a star referred to, for example, as B.D.-f-r3°, 2,966. The Cordoba Durchmusterung (C.D.M.) and Cape Photographic Durchmusterung (C.P.D.) fulfil a similar function for the rest of the southern sky. For stars fainter than the limits of these sur veys an author must describe the identity as best he can.
Double stars are generally designated by the name of the dis coverer, with the ordinal number of his discovery, or the number in Burnham's General Catalogue may be given. Variable stars are assigned (in order of discovery in the constellation) Roman capital letters from R to Z, or a pair of these letters, e.g., X Cygni, RT Aurigae.
called the colour-index. (See PHOTOMETRY : Celestial.) A classification which runs nearly parallel with classification by colour is afforded by examination of the spectrum of the light. In the Draper classification, now universally adopted, a continuous sequence of types is recognized denoted by the letters 0, B, A, F, G, K, M. Intermediate types are indicated by decimal divi sion, e.g., A2 is a type two-tenths of the way between Ao and Fo. A cursory examination of the spectrum is sufficient to determine the type, and 225,000 stars have been classified in this way by Miss A. J. Cannon (Harvard Annals, vols. 91-99). The sequence 0 to M corresponds to progression in colour from blue to red, and progression in temperature from the hottest to the coolest stars. The sun is of type Go. The percentages of stars catalogued in these classes are: B, 2; A, 29; F, 9; G, 21; K, 33; M, 6. Type 0 is rare, not more than 20 stars brighter than mag. 6.25 being known. The letters N, R, S denote other rare types whose relation to the recognized sequence is still uncertain. The prop erties of the stars (distribution, mass, luminosity, motion, varia bility, etc.) are all closely correlated to the spectral type; so that in nearly all investigations it is necessary to study the different types separately. The terms "early type" and "late type" are still commonly used in accordance with the order in this series, i.e., for the bluer and redder stars respectively. The terms originally had reference to a theory of the progress of stellar evolution which is now entirely discarded.
Some progress has been made with the direct measurement of the heat received from the stars, and the corresponding bolomet ric magnitudes (related to heat intensity in the same way that visual magnitudes are related to light intensity) have been deter mined. The star-image is focussed on a small disc in which a thermoelectric junction is embedded, and many hundreds of stars are found to give sufficient heat to be measured in this way. The main trouble in these investigations is that the radiant heat over a certain band of wave-lengths is largely absorbed by the water vapour in our atmosphere, and rather large corrections must be applied on this account. The bolometric magnitude may also be inferred from the visual magnitude by applying a correction de pending on the spectral type of the star. This correction, called the heat-index, is analogous to the colour-index, and varies in correspondence with it. Naturally, the red stars are "brighter" bolometrically than visually, because their radiation is of low luminous efficiency. The greatest luminous efficiency is for stars of types F and G, perhaps because the human eye has been developed so as to make the most of sunlight. In still hotter stars there is a falling off of luminous efficiency, the radiation being too far in the violet and ultra-violet to suit the eye.