# To Erik Larsen. 8 August 1938

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Albert Einstein (1879-1955)

To Erik Larsen. 8 August 1938

Albert Einstein (1879-1955). Typed letter signed ('A. Einstein') to Erik Larsen, Nassau Point, Peconic, Long Island, 8 August 1938.

In German. 1¼ pages, 278 x 2176mm. With two retained typed copies (one signed, one partially manuscript) of letters by Larsen.

Einstein's correspondent, Erik Larsen, was evidently a young Danish astronomer visiting the United States.

To Erik Larsen. 8 August 1938

Albert Einstein (1879-1955). Typed letter signed ('A. Einstein') to Erik Larsen, Nassau Point, Peconic, Long Island, 8 August 1938.

In German. 1¼ pages, 278 x 2176mm. With two retained typed copies (one signed, one partially manuscript) of letters by Larsen.

*Provenance*: collection of Carl-Gustav Danver (1908-1994, astronomer, attached to Lund University).**On spiral nebulae and spherical star clusters.**Einstein is struggling to understand a proposition Larsen has made about the structure of spiral nebulae: 'I do not understand what hypothesis about the temporal process of the phenomena in spiral nebulae you are starting from, in order to discover something about the structure of these nebulae at a specific time'. Einstein suggests he consider for comparison the case of spherical star clusters, where one can assign an arbitrary velocity and mass distribution at a specific time (although he notes wrily that 'it generally turns out that the star cluster neither remains spherical, nor that the density and velocity distribution remain stationary'). However, 'assuming general temporal constancy of these distributions, you will obtain specific results if you consider as forces only the mean gravitational field of the cluster as a whole in one point. The results become more definite if you assume that the interactions of the individual pairs of stars would not change this statistical distribution'. Einstein concludes by challenging Larsen as to what he can substitute for these hypotheses in the case of spiral nebulae: 'Your problem only makes sense for me if you first clearly decide this question'.Einstein's correspondent, Erik Larsen, was evidently a young Danish astronomer visiting the United States.

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