As early as 1933, the astronomer Fritz Zwicky discovered that the galaxies in the Coma Cluster should escape if the cluster is only composed of light-emitting matter. This NASA image demonstrates the problem: there is insufficient gravitational force from the hot gas (emitting x-rays, red false color) and the galaxies (optical wavelengths, green false color) in the Coma Cluster to hold it together. Other matter must be present that cannot be seen –– dark matter.

Vera Rubin, in 1965, made observations that demonstrated conclusively to most astronomers that there is dark matter. She observed the orbital speeds of stars that were at the very edge of spiral galaxies, and discovered that the speed of a star remains almost constant as it orbits a galaxy, contrary to what is expected from Newton’s Universal Law of Gravitation and Newton’s Second Law as demonstrated below. Because of the presence of dark matter, the yellow star can travel at a larger speed than the red star without careening away from the galaxy, imagined to be at the center of the rotation. The blue star's large speed demonstrates that a star at the edge of a galaxy does not move as if it is a fixed part of the galaxy. Relative star speeds are shown accurately, but absolute speeds are greatly exaggerated. (The CDF player may take a couple of minutes to load.)

See also gravitational lensing by dark matter.

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Stars Orbit Galaxy Demo

South Carolina State University, 01/24/2015
This material is based upon work supported by the National Science Foundation under Grant Number AST-0750814. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.