Number and Magnitude of the Stars

NUMBER AND MAGNITUDE OF THE STARS On a clear dark night the stars seem to be innumerable in multi tude, but in reality the number in the whole sky visible to the unaided eye is about 6,000. A small telescope greatly multiplies the number, and the largest instruments (aided by long photo graphic exposures) would reveal some hundreds of millions. The brightness is expressed by "magnitudes," the progressively fainter stars being of progressively greater magnitude. A star is defined to be one magnitude higher than another if its light is fainter in the ratio 2.512; this ratio is adopted so that a difference of five magnitudes may correspond to a light-ratio of i :zoo. The faint est stars visible to the naked eye on clear nights are about the 6th magnitude. The brightest star, Sirius, has magnitude —1.6 i.e., 7.6 magnitudes brighter, or just over i,000 times more lumi nous, than the faintest naked-eye stars. Classified on the same scale, the sun's magnitude is —26.7. It is possible with the largest instruments to reach stars of about the 21st magnitude. It gives some idea of the light-grasp which has been attained if we notice that the interval from the sun to Sirius (25 magnitudes) is com parable with the interval from Sirius to the faintest stars ob served (23 magnitudes).

If the stars were distributed uniformly through infinite space and their light suffered no absorption, each magnitude would comprise 3.98 times as many stars as the preceding magnitude (the number 3.98 is the power of the light-ratio 2.512). This theoretical star-ratio is not realized in the actual distribution. It may be taken as a rough rule that an increase of one magni tude multiplies the number of stars about threefold for the brighter stars, but the factor falls off rapidly for the faintest stars. This is an indication that the density of distribution of the stars in space falls off at great distances—that we are in the midst of a limited cluster or star-cloud. The following table gives the results of F. H. Seares and P. J. van Rhijn (1925) for the number of stars in the sky brighter than the indicated visual magnitude : Probably this is one of many star-clouds lying in the galactic plane, the whole being coiled into a spiral nebula. From other in

dications it appears that we are very far from the centre of the spiral, which is found to lie in the direction of the constellation Sagittarius. The sun cannot be said to occupy a privileged posi tion among the stars; it is nearly central in the star-cloud to which it belongs, but this star cloud is an outlier of the main ga lactic system—which in turn is only one among thousands of its kind.

Nomenclature of the Stars.

The brighter stars are desig nated by a Greek letter followed by the name of the constella tion, according to a system introduced by J. Bayer in his star maps in 1603. The letters are assigned roughly in order of bright ness, but for stars of nearly equal brightness the order is that of position in the constellation usually beginning at the head of the object figured. Alternatively, a system of numbers (followed by the constellation name) introduced by Flamsteed can be em ployed; this covers many stars which have no Bayer letter. Flam steed's numbers were assigned in order of right ascension. For example, 27 Geminorum = E Geminorum ; 3 2 Leonis = aLeonis = Regulus. A number of ancient names of conspicuous stars sur vive; many of these are of Arabic origin. The following list gives those which are familiar to the modern astronomer, together with their Bayer equivalents and their visual magnitudes: Up to a certain point each magnitude contributes more star light than the preceding, the increased number of stars in the mag nitude-interval more than counterbalancing their individual faint ness. The maximum is reached between the r ith and 12th magni tudes, the r 2 million stars in this interval giving a combined light equivalent to 84 stars of magnitude r.o. Beyond this the contri bution falls off. All the stars together give as much light as r,roo stars of magnitude r.o—a result which has been checked closely by direct measurement of the general light of the sky.