. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. y they were and had measuredtheir apparent brightness. Because of difficulties described inan earlier section, the distances of only a few objects have beendirectly measured, and these alone tell us little of the realbrightness of the stars. Fortunately, as has been shown by Adams and Kohlschiitter,it is possible to ^nd the absolute magnitude directly from the * This is equivalent to saying that the radius of the Earths orbit s
. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. y they were and had measuredtheir apparent brightness. Because of difficulties described inan earlier section, the distances of only a few objects have beendirectly measured, and these alone tell us little of the realbrightness of the stars. Fortunately, as has been shown by Adams and Kohlschiitter,it is possible to ^nd the absolute magnitude directly from the * This is equivalent to saying that the radius of the Earths orbit seen fromthe standard distance would subtend an angle of A length of one foot at adistance of 400 miles subtends the same angle. The value of the standard dis-tance commonly used, in light-years, is 33. The Brightness of the Stars 231 spectrum, at least for all but the bluest spectral types. Therelative intensities of certain pairs of lines, even in spectra ofthe same class, vary with the intrinsic brightness of the star;and by observing these critical lines, the absolute magnitude isquickly and accurately determined. +4 +6 +8 +10 +12 F0-F9 G0-G9 K0-K3 K4-K9. Fig. 19. Vertical Distances Represent Numbers of Stars HavingTHE Absolute Magnitudes Given at the Top of the Diagram,dififerent spectral classes being shown separately. The grouping ofthe stars as giants and dwarfs is clearly indicated, the former havingan absolute magnitude of about +1, while the magnitude of the dwarfsincreases as the M stars are approached. The curves are from theinvestigation by Adams and Joy, Mt. Wilson Contr., No. 142; Astro-physical Journal, 46, 335, 1917. The classification and study of such absolute magnitudesas are now available lead to a very remarkable result. Thebluest stars, on the average, are about one hundred times moreluminous than the Sun. Their mean absolute magnitude is notfar from zero, and the individuals differ but little from themean. A similar result holds for the A stars, but
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectastronomy, bookyear19
