Universitätsbibliothek Heidelberg, Heid. Hs. 3695 EC4,4 Campbell, William Wallace; Wolf, Max [Adr.]: Briefe von William Wallace Campbell an Max Wolf: Brief von William Wallace Campbell an Max Wolf (Mount Hamilton, 22.7.1903)

Mt. Hamilton, Cal., July 22,1903.

Professor Dr. Max Wolf, Director,

Astrophysical Observatory, Heidelberg.

Dear Sir:-

The number of variable stars now known is about 1200. They
are among the most interesting bodies in the heavens. Their variations
in brightness have been very carefully studied in a 1 rge number of cases
by able astronomers. In general, such observations do not require expens-
ive or new telescopes.

The successful study of variable star spectra, on the contrary,

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of certain spectral types, that others

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the study of variable-star spectra is an

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requires powerful and expensive equipment, and the subject is one of
great difficulty. Almost no systematic work on them has been done. If we

of variable-star spectra. Beginning

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ield? Is it desirable that increased ro-

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i, ;3 Lyrae, Eta Aquilae, Zeta Geminorum, and
!he same types, our knowledge is confined