Kapteyn Discovers Two Star Streams in the Galaxy (Great Events from History II: Science and Technology Series)
Article abstract: Kapteyn discovered that the proper motions of stars were not randomly distributed but tended in two opposite directions, implying the rotation of the galaxy.
Summary of Event
Following several years of study of the structure of the Milky Way, Jacobus Cornelis Kapteyn concluded, on the basis of exhaustive star counts in sampled portions of the sky, that the proper motions (perpendicular to the line of sight) of the stars were not randomly distributed. To his own satisfaction, Kapteyn confirmed previous perceptions of the galaxy as a flattened ellipsoid (lens-shaped). Yet, because gas and dust toward the center of the galaxy distorted his counting in that direction, he incorrectly placed the solar system relatively near the center of the galaxy.
Having constructed what he believed was a correct view of the galaxy, Kapteyn then measured the proper motions of many stars for the purpose of verifying that these were randomly distributed, as theory indicated. Instead, he discovered systematic departures from randomness, with the stars showing movement in opposite directions in different parts of the sky. The two streams of movement of the stars implied that the galaxy was not static with the sun near the center of random individual stellar...
(The entire section is 2259 words.)
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