1931

Humason, Milton L., "Apparent Velocity-Shifts in the Spectra of Faint Nebulae," ApJ, 74, 35-42 (1931) - NADS
    With one exception, possibly the velocity of an isolated object seen in projection on a remote cluster, the new data fully confirm the velocity-distance³ previously formulated and extend the observational range to a distance of about thirty-two million parsecs.
    ³It is not at all certain that the large red-shifts observed in the spectra are to be interpreted as a Doppler effect, but for convenience they are expressed in terms of velocity and referred to as apparent velocities.
[p. 35]
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Lemaître, Georges, "Expansion of the Universe, A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulæ," MNRAS, 91, 483 (1931) - NADS
    According to the theory of relativity, a homogeneous universe may exist such that all positions in space are completely equivalent; there is no center of gravity. ...
    Two solutions have been proposed. That of de Sitter ignores the existence of matter and supposes its density to be equal to zero. It leads to special difficulties of interpretation ... but it is of extreme interest as explaining quite naturally the observed receding velocities of extra-galactic nebulæ, as a simple consequence of the properties of the gravitational field without having to suppose that we are at a point of the universe distinguished by special properties.
    The other solution is that of Einstein. It pays attention to the evident fact that the density of matter is not zero, and it leads to a relation between this density and the radius of the universe. This relation forecasted the existence of masses enormously greater than any known at the time. These have since been discovered, the distances and dimensions of extra-galactic nebulæ having become known. From Einstein's formulæ and recent observational data, the radius of the universe is found to be some hundred times greater than the most distant objects which can be photographed by our telescopes. . . .

1933

Lemaître, Georges Edouward, "Discussion on the Evolution of the Universe," [Get ref.]
    [States the theory of the Big Bang.]

1935

[The following two papers are products of the imposed settlement between Hubble and van Maanen on the issue of astrometric evidence of internal motions in spiral nebulae. See: Hubble's Demolition of van Maanen? on this website's Cosmology's Missing Mass Problems page. Scroll down about 2/3rds of the way from the top of that page.]
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Hubble, E., "Angular Rotations of Spiral Nebulae," ApJ, 81, 334-335 (1935) - NADS
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Van Maanen, A., "Internal Motions in Spiral Nebulae," ApJ, 81, 336-337 (1935) - NADS

1936

Hubble, Edwin, The Realm of the Nebulae, Yale University Press, New Haven (1936), Page 141.
    Messier 33 -- ...The nucleus resembles in appearance a giant globular cluster, although no evidence of resolution is found. It is semistellar, M = -8, spectral type F5, color excess-appreciable, radial velocity, -320 km/sec.*, as derived from moderately large-scale spectra.
    * [A recent best value for M33's systemic heliocentric radial velocity is -180 km/sec. Hubble's disparity may need explaining. See the Gordon (1971) entry below.]
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Humason, Milton L., "Is the Universe Expanding?," ASPL, 2, 161-164 (1936) NADS
    The interpretation of these recessional velocities beyond the observable facts is still controversial. So far as is known at the present time, the only cause which can produce the observed displacements of the lines in the spectrum of a nebula is motion toward or away from us. On the assumption that the displacements represent motion, the observations tell us that almost without exception the extra-galactic nebulae are moving away from us. The most reasonable explanation of this fact at present is that the universe is expanding. [p. 164]
    . . .   If the displacements are not interpreted as motion, we find in the redshifts a hitherto unrecognized and highly important phenomenon whose implications are unknown. [p.164]

1937

Zwicky., F., "On the Masses of Nebulae and of Clusters of Nebulae," ApJ, 86, 217 (1937) - ipac-caltech

1938

Lindblad, Bertil, "On the State of Motion in the Stellar System and the Probable Relation of the Galaxy to the Sequence of Types of Spiral Nebulae," POBV Conference, 1 September (1938), Page 15 - NADS [No abstract or article.]
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Babcock, Horace W., "Spectrographic Observations of the Rotation of the Andromeda Nebula," PASP, 50, 174-175 (1938) [Abstract] - NADS
    The central core of the nebula, to a radius of some 4', appears to rotate with constant angular velocity, in agreement with the results of Pease. A linear velocity of rotation of 90 km/sec in the plane of the spiral is measured at this distance. From this point the velocity falls off, reaching zero at a distance of 10' from the center. Beyond 10', the velocity increases again, in the same direction, and from 22' to 30' maintains a value of about 150 km/sec. [p. 175]

1940

Lindblad, Bertil, "On the Interpretation of Spiral Structure in the Nebulae," ApJ, 92, 1 (1940) - NADS
    Previous work had indicated that the full development of spiral structure is likely to give an approximate logarithmic shape of the arms, in which the particles of the arm describe roughly circular orbits of uniform angular velocity around the center. It is concluded here that such a formation is possible. . . .[p.1]
    The verification of the theoretical results by observational data is discussed in some detail. H. W. Babcock's results concerning the rotation of the Andromeda nebula are in good agreement with the main theoretical result concerning the possibility of a uniform rotation of the spiral structure.

1942

Mayall, N.U. and Aller, L.H., "The Rotation of the Spiral Nebula Messier 33," ApJ, 95, 5 (1942) - NADS
    ...the velocity of the system is found to be -167 ±5 km/sec...   the main body of the spiral, some 18' in radius, appears to rotate [based on spectroscopic measurements and assumption of simple circular motion in the plane] almost like a solid body (rotational velocity increases fairly uniformly with distance), while the outer parts, represented by a zone having least and greatest radii of 18' and 30' respectively, appear to rotate like a planetary system (rotational velocity decreases with distance). The transition between these two types of motion occurs in the general vicinity of 16' (1000 parsecs) from the center, at which the rotational velocity attains a maximum of approximately 120 km/sec. ... In most of these [earlier] investigations [with the exception of Babcock's study of the Andromeda nebula] the rotational velocities were measured only in the immediate vicinity of the nucleus.

1946

Joy, Alfred H., "Adriaan van Maanen, 1884-1946," PA, 54, 107-110 (1946) - NADS - [No abstract or article.]
    [According to Joy, van Maanen died of a heart attack on January 26, 1946. His article is a tribute to van Maanen's scientific career. The following quote from the article deals with van Maanen's long term study of internal motions of spiral nebulae.]
    "From 1914 until 1923, in addition to his parallax program, he [van Maanen] spent a great amount of time and effort in an attempt to measure the internal motions in spiral nebulae by comparing plates taken at different epochs from 5 to 15 years apart. Although the material was not homogeneous, some of the plates being taken at the Cassegrain focus, some at the Newtonian, and some even with other instruments, the results were strangely accordant in showing a rotation period of the order of 100,000 years or, perhaps a motion outward along the arms of the spirals. Slow as this motion is, it is much too rapid to be admitted with our present knowledge[*] of the distances of these extragalactic bodies. The fact seems to be that the time intervals between the plates were too short and the observational material inadequate to cope with the difficulties of the problem. For the present, at least, we shall have to rely on spectroscopic results for our picture of the motions in the spiral nebulae." [p. 109]
  *[This would be knowledge based on redshifts being used as measures of cosmological distance. Arp's discordant redshifts, regardless of their ultimate cause, may eventually lead to a re-evaluation of van Maanen's findings.]
    [See: Arp, Halton, Seeing Red: Redshifts, Cosmology and Academic Science; Apeiron, Montreal (1998), and Arp, Halton, Research with Fred.]

1948

Milne, E. A., "Star-streaming and the Stability of Spiral Orbits in Spiral Nebulae   I - Motion round a point-nucleus," MNRAS, 108, 309 (1948) - NADS

1969

Gordon, Kurtiss J., "History of our Understanding of a Spiral Galaxy: Messier 33," QJRAS, 10, 293-307 (1969) -
http://nedwww.ipac.caltech.edu/level5/March02/Gordon/Gordon_contents.html

1970

Smith, M.G., Weedman, D.W., "Internal Motions in Galactic and Extragalactic H II Regions," ApJ, 161, 33 (1970) - NADS - Emission profiles have been observed for the total Hα emission from nine H II regions in the galaxies M101 and M33. ... The most probable velocities of internal motions are found to range from 19 to 34 km sec^-1. [Added 07 Feb 2007.] [Chandar (2002) found young cluster velocities of 87 ± 11 km sec^-1 in M33.]

1971

Berendzen, Richard,; Hoskin, Michael, "Hubble's Announcement of Cepheids in Spiral Nebulae," ASPL, 10, 425 (1971) - NADS
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Gordon, Kurtiss, J., "A 21-CENTIMETER Study of the Spiral Galaxy Messier 33," ApJ, 169, 235-270 (1971) - NADS
    The "best value" for the systemic radial velocity of M33 is S = -180 km/sec (heliocentric). The rotation peaks at Vr = S ± ~85 km/sec at 30' from the galactic center. ...Velocities in the wings deviate by 40-50 km/sec from those predicted by the rotation curve. [Measures of the systemic velocity for M33, since 1942, have been within eight percent of Gordon's "best value" of -180 km/sec. It is of interest to note that in 1916 Pease's measurement was -70 km/sec and in 1936 Hubble's stated value was -320 km/sec. Hubble did not specify who made the measurements.]

See: Systemic Radial Velocity Measurements of Messier 33. [This website]