Based on his measurements of the velocity of the stars, Zwicky calculated that the amount of mass required for the cluster to have sufficient gravitational pull was 400 times greater than the contribution of the measured luminous mass—the matter that emits light.
I suppose I cannot really complain when reading a pop science book, but this is a really preposterously brief description of the measurements. I want a little more of the how than just "measurements based on the velocity of stars"?
How do we determine the velocity of stars in a distant galaxy? Do we mean velocity in the physics sense, with mass attached, or in a more colloquial sense of just speed? Are these measured together or separately?
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Brian
