1 4 D O C U M E N T 9 M A R C H 1 9 1 9
AKS (NNPM, MA 4725 (16)). [11 226.1]. The verso is addressed “Herrn Dr. E. Freundlich Rathaus-
str. 2 Neubabelsberg.,” and postmarked “Berlin [W] 30 1.3.19 7–8N[achmittag].”
In January 1919, Freundlich requested financial assistance from the board of the Kaiser Wilhelm
Institute of Physics (KWIP), as well as payment of his overdue 1918 research allowance. Einstein was
asked to approve the request on 1 March (see entries of 20 and 28 January, 2 February, and 1 March
Freundlich (1885–1964) was appointed an Assistent at the KWIP for a period of three years, be-
ginning 1 January 1918, charged to make preparations for the observation of light deflection by the
Sun during the total eclipse of 1919 and to find evidence for the redshift in the spectra of stars other
than the Sun (see his plan of research submitted to Einstein on 4 December 1917 [Vol. 8, Doc. 402]).
As no special building was set aside for Einstein’s institute, Freundlich carried out his research in the
Astrophysical Observatory in Potsdam (see Gustav Müller to Einstein, 9 January 1918 [Vol. 8,
Walther Nernst (1864–1941) was Professor of Physical Chemistry at the University of Berlin
and a member both of the board of directors and the board of trustees of the KWIP. Einstein submitted
a short report on 16 June 1919 (see entry for this date in Calendar).
Kohlschütter 1919 is a report on Eddington 1917a, 1917c, and 1918c which showed, by applying
the equation of radiative equilibrium to the interior of a star, how radiation pressure could counterbal-
ance the internal pressure created by a star’s own weight. The theory made predictions of the mass,
density, and surface temperature of stars.
Eddington 1917c adopted a “molecular weight” of 2 for the solar gas (replacing an assumed
average value equal to the atomic weight of iron, 56). He observed that, whatever the gaseous sub-
stance of the star may be, the atomic weight of elements is approximately twice as large as the number
of their electrons and that, with total ionization in the extreme temperatures of the solar interior, the
gas must consist of free electrons and positive ions. Einstein notes that this molecular weight may not
apply at lower temperatures, where ionization would be less effective, leading to a higher “molecular
Stars of type A or B may be either blue giants or white dwarfs, while stars of type M and K may
be either red giants or red dwarfs. Distinguishing between giant and dwarf stars of the same spectral
type had been addressed in Adams and Kohlschütter 1914, but was not yet a wholly solved problem.
Einstein’s knowledge of astronomy had become much more current since his letter to Freundlich of
March 1915 (Vol. 8, Doc. 59), in which, in preparation for presenting a paper of Freundlich’s to the
Academy, he asked for brief characterizations of the spectral classes of stars.
In the sentence, the second “letztere” should be “erstere.”
In Eddington’s theory, blue giants of spectral type A and B are typically of smaller radius with
densities far higher than red giants of spectral type M and K, and should therefore exhibit a much
greater gravitational redshift (Kohlschütter 1919, p. 90, gives a factor of 2500 increase in density
between type M and type A stars). Freundlich’s research focused on stars in the Orion nebula, which
is rich in blue giants (see Doc. 14).
9. To Karl Scheel
[Berlin,] Montag [17 March
Die Kritik des Kollegen Jakob ist
und es ist gut, dass die ziemlich
konfusen, dabei auch anspruchsvollen Ausführungen Petzolds nicht unwiderspro-
Der Angriff auf Cohns Modell ist ganz und gar unberechtigt. Ein
Modell ist keine Kopie; es ist eine Sache, die naturgemäss nur einen Teil der Eigen-