Abstract
Eta Aquilæ and the Cepheid Problem.—Vol. 4, No. 8 of the Publications of Michigan University Observatory contains a study of the spectrum of Eta Aquilse by Dr. D. W. Lee. The star has been known as a variable since 1784, its period being about 7.2 days. It was soon recognised that the eclipse explanation did not fit this case and many other theories were tried in turn, tho ono usually accepted at present being Prof. Shapley's pulsation theory. The present paper supports this theory, but with the modification that the pulsations are to a large extent in the star's atmosphere. The spectro-heliograph has enabled us to study the behaviour of gases at different heights in tho sun's atmosphere, and the experience gained there may bo extended to stellar spectra. D. Lee notes that if the pulsation arises from a central impulse, there would bo a lag in the phases of the outer layers as compared with the inner ones; his observations confirm this, and indicate the presence of a compressional wave which is traced through four of the lower layers of the atmosphere. A companion paper by W. Carl Rufus in No. 7 of the same publication contains a diagram of the mean velocity curve for all levels. This shows a marked pause in the middle of the ascending portion of tho velocity curve. The curves from hydrogen and strontium lines have a secondary maximum at this point. The light-curve has a similar pause, which is supposed to indicate a stage of comparative rest in tho atmosphere.
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