The number of the stars is not less amazing than their distance. It is true that the number visible to the unaided eye is not by any means so great as might be imagined on a casual survey. On a clear night the eye receives the impression that the multitude of stars is so great as to be utterly beyond counting; but this is not the case. The naked-eye, or 'lucid,' stars have frequently been counted, and it has been found that the number visible to a good average eye in both hemispheres together is about 6,000. This would give for each hemisphere 3,000, and making allowance for those lost to sight in the denser air near the horizon, or invisible by reason of restricted horizon, it is probable that the number of stars visible at any one time to any single observer in either hemisphere does not exceed 2,500. In fact Pickering estimates the total number visible, down to and including the sixth magnitude, to be only 2,509 for the Northern Hemisphere, and on that basis it may safely be assumed that 2,000 would be the extreme limit for the average eye.
Region of the Milky Way in Sagittarius. Photographed by Professor E. E. Barnard.
But this somewhat disappointing result is more than atoned for when the telescope is called in and the true richness of the heavenly host begins to appear. Let us take for illustration a familiar group of stars—the Pleiades. The number of stars visible to an ordinary eye in this little group is six; keen-sighted people see eleven, or even fourteen. A small telescope converts the Pleiades into a brilliant array of luminous points to be counted not by units but by scores, while the plates taken with a modern photographic telescope of 13 inches aperture show 2,326 stars. The Pleiades, of course, are a somewhat notable group; but those who have seen any of the beautiful photographs of the heavens, now so common, will know that in many parts of the sky even this great increase in number is considerably exceeded; and that for every star the eye sees in such regions a moderate telescope will show 1,000, and a great instrument perhaps 10,000. It is extremely probable that the number of stars visible with the largest telescopes at present in use would not be overstated at 100,000,000 (Plate [XXVII.]).
It is evident, on the most casual glance at the sky, that in the words of Scripture, 'One star differeth from another star in glory.' There are stars of every degree of brilliancy, from the sparkling white lustre of Sirius or Vega, down to the dim glimmer of those stars which are just on the edge of visibility, and are blotted out by the faintest wisp of haze. Accordingly, the stars have been divided into 'magnitudes' in terms of scales which, though arbitrary, are yet found to be of general convenience. Stars of the first six magnitudes come under the title of 'lucid' stars; below the sixth we come to the telescopic stars, none of which are visible to the naked eye, and which range down to the very last degree of faintness. Of stars of the first magnitude there are recognised about twenty, more or less. By far the brightest star visible to us in the Northern Hemisphere, though it is really below the Equator, is Sirius, whose brightness exceeds by no fewer than fourteen and a half times that of Regulus, the twentieth star on the list. The next brightest stars, Canopus and Alpha Centauri, are also Southern stars, and are not visible to us in middle latitudes. The three brightest of our truly Northern stars, Vega, Capella, and Arcturus, come immediately after Alpha Centauri, and opinions are much divided as to their relative brightness, their diversity in colour and in situation rendering a comparison somewhat difficult. The other conspicuous stars of the first magnitude visible in our latitudes are, in order of brightness, Rigel, Procyon, Altair, Betelgeux, Aldebaran, Pollux, Spica Virginis, Antares, Fomalhaut, Arided (Alpha Cygni), and Regulus, the well-known double star Castor following not far behind Regulus. The second magnitude embraces, according to Argelander, 65 stars; the third, 190; fourth, 425; fifth, 1,100; sixth, 3,200; while for the ninth magnitude the number leaps up to 142,000. It is thus seen that the number of stars increases with enormous rapidity as the smaller magnitudes come into question, and, according to Newcomb, there is no evidence of any falling off in the ratio of increase up to the tenth magnitude. In the smaller magnitudes, however, the ratio of increase does not maintain itself. The number of the stars, though very great, is not infinite.
A further fact which quickly becomes apparent to the naked eye is that the stars are not all of the same colour. Sirius, for example, is of a brilliant white, with a steely glitter; Betelgeux, comparatively near to it in the sky, is of a beautiful topaz tint, perhaps on the whole the most exquisite single star in the sky, so far as regards colour; Aldebaran is orange-yellow, while Vega is white with a bluish cast, as is also Rigel. These diversities become much more apparent when the telescope is employed. At the same time the observer may be warned against expecting too much in the way of colour, for, as a matter of fact, the colours of the stars, while perfectly manifest, are yet of great delicacy, and it is difficult to describe them in ordinary terms without some suspicion of exaggeration. Stars of a reddish tone, which ranges from the merest shade of orange-yellow up to a fairly deep orange, are not uncommon; several first-magnitude stars, as already noted, have distinct orange tones. For anything approaching to real blues and greens, we must go to the smaller stars, and the finest examples of blue or green stars are found in the smaller members of some of the double systems. Thus in the case of the double Beta Cygni (Albireo), one of the most beautiful and easy telescopic objects in the northern sky, the larger star is orange-yellow, and the smaller blue; in that of Gamma Andromedæ the larger is yellow, and the smaller bluish-green; while Gamma Leonis has a large yellow star, and a small greenish-yellow one in connection. The student who desires to pursue the subject of star colours should possess himself of the catalogue published in the Memoirs of the British Astronomical Association, which gives the colours of the lucid stars determined from the mean of a very large number of observations made by different observers.
In this connection it may be noticed that there is some suspicion that the colours of certain stars have changed within historic times, or at least that they have not the same colour now which they are said to have had in former days. The evidence is not in any instance strong enough to warrant the assertion that actual change has taken place; but it is perfectly natural to suppose that it does, and indeed must gradually progress. As the stars are intensely hot bodies, there must have been periods when their heat was gradually rising to its maximum, and there must be periods when they will gradually cool off to extinction, and these stages must be represented by changes in the colour of the particular star in question. In all probability, then, the colour of a star gives some indication of the stage to which it has advanced in its life-history; and as a matter of fact, this proves to be so, the colour of a star being found to be generally a fair indication of what its constitution, as revealed by the spectroscope, will be.
Another feature of the stars which cannot fail to be noticed is the fact that they are not evenly distributed over the heavens, but are grouped into a variety of configurations or constellations. In the very dawn of human history these configurations woke the imaginations of the earliest star-gazers, and fanciful shapes and titles were attached to the star-groups, which have been handed down to the present time, and are still in use. It must be confessed that in some cases it takes a very lively imagination to find any resemblance between the constellation and the figure which has been associated with it. The anatomy of Pegasus, for example, would scarcely commend itself to a horse-breeder, while the student will look in vain for any resemblance to a human figure, heroic or unheroic, in the straggling group of stars which bears the name of Hercules. At the same time a few of the constellations do more or less resemble the objects from which their titles are derived. Thus the figure of a man may without any great difficulty be traced among the brilliant stars which form the beautiful constellation Orion; while Delphinus presents at least an approximation to a fish-like form, and Corona Borealis gives the half of a diadem of sparkling jewels.
A knowledge of the constellations, and, if possible, of the curious old myths and legends attaching to them, should form part of the equipment of every educated person; yet very few people can tell one group from another, much less say what constellations are visible at a given hour at any particular season of the year. People who are content merely to gape at the heavens in 'a wonderful clear night of stars' little know how much interest they are losing. When the constellations and the chief stars are learned and kept in memory, the face of the sky becomes instinct with interest, and each successive season brings with it the return of some familiar group which is hailed as one hails an old friend. Nor is the task of becoming familiar with the constellations one of any difficulty. Indeed, there are few pleasanter tasks than to trace out the figures of the old heroes and heroines of mythology by the help of a simple star-map, and once learned, they need never be forgotten. In this branch of the subject there are many easily accessible helps. For a simple guide, Peck's 'Constellations and how to Find Them' is both cheap and useful, while Newcomb's 'Astronomy for Everybody' and Maunder's 'Astronomy without a Telescope' also give careful and simple directions. Maunder's volume is particularly useful for a beginner, combining, as it does, most careful instructions as to the tracing of the constellations with a set of clear and simple star-charts, and a most interesting discussion of the origin of these ancient star-groups. A list of the northern constellations with a few of the most notable objects of interest in each will be found in Appendix II.