STARS.

Hauraki Plains Gazette, Volume XXXIV, Issue 4602, 17 September 1923, Page 4

STARS.

HOW MANY ARE THEBE? (By Dr. Wm. J, S. Lockyer, of the Norman Lockyer Observatory.) How many stars are there in the sky ? To make only a first approximation to the answer to the above question it is necessary first to find out what is the faintest in the sky that can be recorded by a telescope with an attached photographic plate. Photography must be employed because visual observations are inadequate, owing to the fact that the photographic plate is capable of recording stars which not even the largest telescope in the world can show us. The second step is to photograph the whole sky from pole to pole and then counit up the stars that are on all the plates. The result of such a count will only then give us a rough idea of the number of stars in the sky, because, in the first place, the more powerful the telescope the greater the number of stars that can be recorded, and we have not yet arrived at the limit of size of the telescope. Again, the speed of the photographic plate is probably quite slow now compared with what itj eventually will be. Lastly—and this is of extreme importance—there is a great amount of dark matter in space, scattered here and there both in large and small quantities, which lies between us and the distant stars, cutting them off from oui vision. Everyone knows that stars are not all of the same brightness. Astronomers use the term “magnitude” for brightness and adopt the system of changing from one magnitude to the next greater magnitude by multiplying by two and a half; thus a star of the first magnitude is two and a half times brighter than a star of the second magnitude, a second-magnitude stop two and a half times that of the third, and so on. Now, with the unaided eye an observer' with a keen vision can see stars a little fainter than the sixth magnitude. To him, therefore, there will be about 7000 stars visible, but only half this number will be above the horizon at any time. Every gain in magnitude means a great increase in the number of sitjars recorded. Photographs taken with such a powerful telescope as the 60in reflector of the Mount Wilson Observatory, with an exposure of five hours, show stars to the twentieth magnitude. The biggest instrument in the world, the lOOin leflector. at the same observatory, phptographs stars to the twenty-first magnitude, and probably fainter. A survey of the whole heavens with this latter instrument would tell us best of all how many stars there are. Such a survey has not been made, so one has to be content with a more modest one—very complete, however —made with a lOin lens by a British amateur astronomer and extending to stars of the seventeenth magnitude. The counts of these plates (show that up to the tenth magnitude thete are 271,800 stars ; up to the fifteenth magnitude 15,470’,*000 stars are recorded; while 54,900,000 stars are .shown up to the seventeenth magnitude. By an ingenious mathematical process these sit,ar numbers have been extended beyond the seventeenth magnitude, giving 760,1000,000' stars up to that magnitude. Reasoning from these results, it is probable that the total number of stars is not less 't,han 1,000,000,000 (one thousand million), and cannot greatly exceed twice this amount. The figure 1,500,000,000 (fifteen hundred millions) may therefore be taken as an approximate value for the number of stars in the sky.