George Paddock – early work on cosmological redshift

In addition to the work on extragalactic objects, George Paddock did some work on planets, stars, and galactic nebulae. He also worked on the radial velocity equations of binary stars. Here, I will concentrate on his extragalactic work (which contains only couple of papers).

Paddock (1916) discussed spiral galaxies in their relation to the galactic stellar system (this was well before it was established that spiral galaxies are not part of our own galaxy). He made an observation from the radial velocities of different objects:

The average radial velocities except the spirals range in increasing magnitude from zero to fifty kilometers per second. But a considerable jump is noticed from the fifty kilometers to 400 kilometers for the average of the spirals.

Based on this he presented a question:

Are the spirals dissociated from the star system?

Paddock then mentioned some Slipher’s arguments of the radial velocities of spiral galaxies. Paddock also discussed solar motion and its possible effect to the radial velocities of spiral galaxies. He noted that the spirals having measured radial velocities by that time were distributed in two groups and the Magellanic clouds were a third group, but he also said:

These objects, however, can hardly be considered to form a unitary system of associated objects, for it must be noticed that the average velocity of each of the three groups of objects is decisively positive, which means that they are receding not only from the observer or star system but from another.

What he describes here is an expanding motion. He continued:

Accordingly a solution for the motion of the observer thru space should doubtless contain a constant term to represent the expanding or systematic component whether there be actual expansion or a term in the spectroscopic line displacements not due to velocities. This brings up the question whether these large displacements are to be interpreted as due entirely to velocities.

13 years before Hubble’s redshift-luminosity relation, Paddock was already pondering similar questions. He brought up NGC 1068 with its fuzzy and broad spectroscopic lines as a possible example showing that all of the redshift might not be due to velocity (note that later there has been lot of discussion on the possible discordant redshifts in NGC 1068 system). He suggested that there might be a constant term resembling the K-term of stellar radial velocities and went on to quantify the term from the solar motion derived from the radial velocities of galaxies. He got a rather large value for the K-term (about 250-350 km/s) but he concluded that it is likely be due to small sample size and that he expected it to diminish with larger sample.

Campbell & Paddock (1918) discussed their spectroscopy on NGC 4151. They first mentioned that according to a photograph by Curtis, they thought that NGC 4151 was a planetary nebula. They then descibed their spectroscopy. They mentioned not finding the expected spectrum of a planetary nebula, and determined the radial velocity of 940 +/- 40 km/s for NGC 4151. They also noted that a new photograph by Curtis clearly showed a spiral structure, and that the character of the spectrum resembled the spectrum of NGC 1068.


Paddock, 1916, PASP, 28, 109, “The Relation of the System of Stars to the Spiral Nebulæ”

Campbell & Paddock, 1918, PASP, 30, 68, “The Spectrum and Radial Velocity of the Spiral Nebula N. G. C. 4151”


(University of California: in memoriam) George Frederic Paddock: Lick Observatory

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