The stars differ enormously in their actual luminosity, that is, in the actual amount of light emitted by each. If they were all equally lumi nous we could form an idea of their relative distance by their brightness; the farther a star was away the fainter it would shine. But as a matter of fact, some of the brightest stars in the heavens are so distant that their parallax proves to be immeasurably small. This is not the case with Sirius, the brightest of all, which has a parallax of 0" .38, showing it to be situ ated at a distance from which light would travel in about nine years. But the next bright est, Canopus (not visible farther north than our Southern States), is so distant that no parallax can be shown by the most refined measures. The conclusion is that it must be 10,000 times as luminous as our sun; possibly 100,000 times. This is the case with several stars in or near the constellation Orion. At the other extreme, we find that among the nearer stars are several not visible to the naked eye. It is true that the nearest of all, Alpha Centauri, is nearly of the first magnitude; but the next in order, 61 Cygni, is only of the sixth magnitude; two others are quite invisible to the naked eye. This shows that there are stars much less luminous than the sun, and that the greatest stars are probably millions of times more luminous than the smallest ones.
Ball, Sir R., (Story of the Heavens' (new ed., New York 1905) ; id., (Popular Guide to the Heavens' (London 1905) ; Burnham, S. W.. (General Catalogue of Double Stars> (2 vols., Carnegie Institution, Washington 1906) ; Campbell, W. W., (Stellar Motions> (New Haven 1913) ; Chambers, G. F., 'Handbook of Descriptive and Practical As tronomy' (Vol. III, 4th ed., Oxford 1909) ; Clerke, Agnes M.,