1921

Van Maanen, A., "Internal Motion in Four Spiral Nebulae," PASP, 33, 200 (1921) - NADS
    For the rotational and radial components the results are collected in Table I. The rotational components would correspond to the following periods: For Messier 101, 85,000 years; for Messier 33, 160,000 years; for Messier 51, 45,000 years; for Messier 81, 58,000 years.
___

Van Maanen, A., "Investigations on Proper Motion - Fourth Paper: The Internal Motion in the Spiral Nebula Messier 51," ApJ, 54, 237 (1921) - NADS
    A comparison of a plate taken by Mr. Duncan on April 8, 1921, with one taken by Mr. Ritchey on February 7-8, 1910, both at the 25-foot focus of the 60-inch reflector, enabled the proper motion of the nebula and the relative motion of its parts to be determined. Measurements of 80 points of the nebula compared with those of 20 stars give for the nebula an annual proper motion of +0".006 in right ascension and +0".001 in declination. The internal proper motion is not a pure rotation since the mean radial component is outward and is 42 percent of the mean tangential component which is 0".019 ENWS; rather it is a spiral motion out along the arms at the rate of 0".021 per year together with a slight outward radial motion of 0".003.
    Two sixteenth magnitude stars with large proper motions, about 0".15 per year, were found near Messier 51. They are called f and r.
    [Plate II (in hardcopy original) shows internal motions of M51.]
    [With respect to his selected reference stars, van Maanen's measures of bulk internal motions for M51 are approximately three times greater than his measure of its systemic proper motion..]
___

Van Maanen, A., "Investigations on Proper Motion - Fifth Paper: The Internal Motion in the Spiral Nebula Messier 81," ApJ, 54, 347-356 (1921) - NADS
    [Plate IV (in hardcopy original) shows internal motions of M81.]

Lundmark, Knut, "The Spiral Nebula Messier 33," PASP, 33, 324-327 (1921) - NADS
    On small scale photographs the spiral appears as a bona find nebulous object but photographed with large instruments especially the outer parts of the spiral arms seem to be resolved into numerous star-like objects. Many of these secondary nuclei look exactly like stars but a number of them have a soft appearance which has led Ritchey to call them nebulous stars.   . . .
    There is in the spiral structure a great number of dark lanes and it is to be noted that these do not always define the space between the spiral arms but many times go across them.   . . . [p. 324]
    [To see an example of a dark lane cutting across spiral arms, see Robert Gendler's CCD photograph
    M81, Spiral Galaxy in Ursa Major.]
    [To this writer, the cross-cutting dark lane phenomenon is suggestive of a wake-like action caused by the rapid passage of some massive object (or a compact system of objects) skimming across the face of a spiral. The object transient time would have to be much shorter than the rotation period of the spiral, otherwise the trace would be curved by the differential motions in the spiral arms. If this is the case, it absolutely requires that the spirals have the relatively small spatial dimensions and distances from us, consistent with van Maanen's findings. RSF 25 Oct 2003.]
    From an unpublished investigation on the proper motion of 100 spiral nebulae, derived from micrometric and photographic measures, we give the following results for N.G.C. 604 [a giant nebulous star forming region in M33] measured as one mass to illustrate the accuracy of the measures. [The units are in decimal fractions of arcseconds per year.] [p. 326]

1922

Van Maanen, A., "Investigations on Proper Motion - Seventh Paper: Internal Motion in the Spiral Nebula NGC 2403," ApJ, 56, 200 (1922) - NADS
___

Van Maanen, A., "Investigations on Proper Motion - Eighth Paper: Internal Motion in the Spiral Nebula M94=NGC 4736," ApJ, 56, 208 (1922) - NADS

1923

Jeans, James Hopwood., The Nebular Hypothesis and Modern Cosmogony: Being the Halley Lecture delivered on 23d May 1922, Oxford, The Clarendon Press, London, New York [etc] H. Milford (1923). - NADS [No article or abstract available.]
___

Jeans, J. H., "Internal Motions in Spiral Nebulae," MNRAS, 84, 60-76 (1923) - NADS
    No interpretation, and very little discussion, has so far appeared on Mr. Van Maanen's highly interesting measurements of the internal motions of spiral nebulae. ... The theoretical part of the present paper has been held back for over a year, the first part of which was spent in an unsuccessful effort to find some less revolutionary interpretation of the observed motions than that here tentatively put forward, the second part being spent in an effort, again unsuccessful, to connect up the suggested interpretation with the general scheme of the theory of relativity....[p. 60] [More to come.]
___

Van Maanen, A., "Investigations on Proper Motion - Ninth Paper: Internal Motion in the Spiral Nebula Messier 63, NGC 5055," ApJ, 57, 49 (1923) - NADS
___

Van Maanen, A., "Investigations on Proper Motion - Tenth Paper: Internal Motion in the Spiral Nebula Messier 33, N.G.C. 598," ApJ, 57, 264-278 (1923) - NADS

    Comparison of two photographs taken in 1910 and 1922 by Ritchey and Humason, respectively, gives, with respect to twenty-four comparison stars, the annual proper motion of = +0".003, = -0".004, and the motions of 399 nebular points freed from this motion. The internal motions are shown on Plate XIX. They can be interpreted as a rotation or as a motion outward along the arms of the spiral, preferably the latter. Taken as a rotation, the motions indicate periods from 60,000 to 240,000 years. [p. 264]

    There seems to be a considerable increase of motion with distance from the center. Error analysis of measured displacements within seven spirals indicates that actual internal motions exist and are in agreement with Jeans' cosmogony. Parallaxes of larger spiral nebulae suggest diameters ranging from several light years to several hundred light years. Larger spirals are enormous compared to our solar system, but are small compared to the Milky Way. [p. 264.]
    For internal motions expressed as rotation (+ = N-E-S-W) and radial (+ = outward) components, the component means were found to be rotation = +0.020 ± 0.001 arcsec/year, radial = +0.003 ± 0.001 arcsec/year. [p. 273.]

1925

Jeans J. H., "Note on the Distances and Structure of the Spiral Nebulae," MNRAS, 85, 531-534 (1925) - NADS
    Results recently published by Hubble and Shapley seem to establish the inaccuracy of estimates I made some time ago of the distances and other quantities associated with the spiral nebulae. Hubble estimates the distance of M 31 (the Andromeda nebula) as 950,000 light-years, as against my estimate of 5000 light-years. Even apart from this, however, the time has come when my calculations may reasonably be revised in the light of new nebular knowledge.   . . .
    My original calculations made use of v. Maanen's determinations of the angular velocities of the nebulae. Recently Eddington has drawn attention to the close correlation between the luminosities and masses of actual stars. If the same correlation is assumed to hold for the stellar condensations in a nebula, we can dispense with v. Maanen's measurements altogether and (in theory at least) determine the distance of the nebula from the observed stellar magnitudes of its condensations. The method is as follows. [pp. 531-532]
    [Method is described]
    The method, although simple in theory, may encounter in practice a difficulty which may render it almost valueless. [Describes the potential problem.
    [In a footnote Jeans says, "The controversy does not appear to be one between Professor Shapley and myself, so much as one between the estimates of Van Maanen and Hubble as to nebular distances."] [p.532]
    [Using Pease's formula for line-of-sight velocity as a function of angular distance from the center of M31, and Hubble's estimated distance to the same object, Jeans arrives at a rotation period for M31 of 18 million years.] [pp. 533-534]
    It is difficult to imagine that those nebulae which exhibit a lenticular centre and filamentous arms with pronounced condensations can have a similar constitution [to that of our galactic stellar system and perhaps to the nucleus of the Andromeda nebula]. It seems more likely that the Andromeda nebula may be in a far later stage of development than the typical spiral; it may exemplify a state intermediate between the typical spiral and the galactic system. [p. 534]
___

Van Maanen, A., "Investigations on Proper Motion - Eleventh Paper: The Proper Motion of Messier 13 and its Internal Motion," ApJ, 61, 130 (1925) - NADS