Agnes E Wells' Ph.D. thesis
Agnes Ermina Wells was awarded a Ph.D. from the University of Michigan in 1924 after submitting her theses A study of the relative proper motions and radial velocities of stars in the Pleiades group. We give below some extracts from the thesis.
A study of the relative proper motions and radial velocities of stars in the Pleiades group, by Agnes E Wells.
A dissertation submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the University of Michigan.
A study of the positions and proper motions of one hundred and fifty stars of the Pleiades group measured on photographic plates taken at the Goodsell Observatory was undertaken in order to determine the positions of many of the faint stars up to the fifteenth magnitude near Merope and between Merope and Alcyone; and to compare the results for stars up to the ninth magnitude with those for forty six stars, whose positions had been well determined at Königsberg, Ya1e, Columbia, Paris and Oxford. The work was begun at Goodsell Observatory, Carleton College, Northfield, Minnesota, in 1915. Two photographic plates were taken in December 1915 and were measured and reduced during the spring of 1916. A third plate taken in 1899 was measured and reduced during that same period. In the spring of 1917 a fourth plate, taken in January 1916, was measured at Goodsell Observatory. During the fall of 1923 this plate was reduced at the Detroit Observatory of the University of Michigan, Ann Arbor.
Scientifically the Pleiades group is of great interest. About fifty years ago, Alcyone, the brightest of this group of brilliantly white stars, was considered by Maeder to be the centre of the universe. Young, in his Manual of Astronomy (1902) says, "It is certainly within bounds to deny that there is any conclusive evidence of such a motion, and it is still less probable that the star Alcyone is its centre, if the motion exists." This theory is again brought forward by André, in 1900, in his work on Stellar Astronomy, who shows "that it agrees with several statistical facts relating to the proper motion of the stars."
Robert Trumpler in a paper on The Physical Members of the Pleiades group (Lick Observatory Bulletin, 1921), discusses the reasons why some of the stars of this group are to be considered physical members and why others are background stars. He says, "The probability of a star of this region of the sky being a member is determined by three factors:
(1) For a star near the centre of the group it is greater than for a star situated at a considerable distance from the centre.
(2) The less the proper motion of a star differs from the mean proper motion of the group, the more likely it is to be a member. A similar statement might be made for the radial velocities, but these have been observed only for a few of the brightest stars and a high degree of accuracy could hardly be claimed for them.
(3) The degree of accordance with the relation which has been found between the spectral type or colour of the members and their brightness."
The determinations of correct positions and proper motions, in order to understand the homogeneity of the group, were many, before the work in radial velocities was developed. The positions determined by Bessel, Schluter and von Busch at Königsberg, Pritchard at Oxford, Elkin and Smith at Yale, all heliometric determinations, those by Wolf at Paris by micrometer measures, and by Jacoby at Columbia, by photographic determinations, have been accepted as most authentic.
The result of the study of the positions and radial velocities lead to many interesting suggestions of the possibility of motion in the group. Pritchard suggested that there might be a motion of rotation of the group which would be indicated by a study of the motions in the line of sight. Jung in his dissertation says that this undoubtedly cannot be because it would mean positive radial velocities on one side and negative on the other side, and that his results did not show that distribution. Hartmann discusses the problem in an article "Die Bewegung der elf hellsten Plyadensterne." He concludes that there is no motion of rotation.
The results of this investigation indicate a decided predominance of positive radial velocities in the N.W. quadrant and a predominance of negative velocities in the S.E. quadrant. The average for the N.W. quadrant is approximately 6 km per sec., and for the S.E. quadrant is approximately -2 km per sec. The proper motions of these stars do not indicate anything singular except perhaps Bessel 26s, which may not belong to the group.
In conclusion I wish to express my appreciation to those who have aided me in this investigation: Prof H C Wilson of Carleton College, and Prof W J Hussey of the University of Michigan, for putting the equipment of their observatories at my disposal, and for their helpful direction; to Prof R H Curtiss for expert direction and thoughtful supervision; to all of the observers who obtained spectrograms of the Pleiades; to Dean B McLaughlin for making and photographing the star chart; and to Prof Rossiter for help during observations.
Ann Arbor, Michigan
14 January 1924.
Last Updated September 2023