. 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. ies in the way of accepting this theory. In the first place,the planetaries are comparatively rare objects; out of so manyhundred thousand stars in all stages of development it is verystrange, in fact inexplicable, that there should be fewer than onehundred and fifty at this particular early stage. Then, too, theirspace velocities are very much higher than that of the averagestar. Why should the planetaries stand so decidedly a
. 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. ies in the way of accepting this theory. In the first place,the planetaries are comparatively rare objects; out of so manyhundred thousand stars in all stages of development it is verystrange, in fact inexplicable, that there should be fewer than onehundred and fifty at this particular early stage. Then, too, theirspace velocities are very much higher than that of the averagestar. Why should the planetaries stand so decidedly apart inthis respect, and how can this gap be bridged over ? Though buta theory as yet, perhaps the most acceptable hypothesis, becauseof their high speeds and small numbers, is that the planetarynebulae are to be regarded as a somewhat sporadic case instellar evolution, arising through some collision or cataclysm,and not to be regarded as cases typical of the general run ofstellar development. When we pass on to the third subdivision, the great class - They are very closely allied in spectrum with a comparatively rare class ofstars known as the Wolf-Rayet Fig. 1—.-^t left, region in the Milky Way showing ten to twenty thousand stars,one planetary (N. G. C. 6563), and no spirals. Fig. 2—At right, region near N. G. C. 2507, some distance from the Milky Way,showing few stars and tifty-thrce small nebulae, indicated by rings. Thearea of eacli lialf is somewliat larger than tliat covered bv the full Moon.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectastronomy, bookyear19
