Spiral Galaxies


Spiral galaxies, like or own Milky Way, rotate like a huge pinwheel.  To measure the speed of rotation, Rubin and Ford measured the distances of bright clouds from the center of the galaxy on images such as the one at right.  They then used a very sensitive spectrograph designed by Ford to observe the Doppler shift of the cloud to determine how fast it was moving towards or away from Earth.  For each galaxy they measured the speed of several clouds at various distances from the center.
The astronomers expected that like the planets in our solar system, the stars near the center of the galaxy would rotate faster than the stars near the outer edge.  But in fact the bright clouds at the edges of the spiral arms moved as fast as the clouds closer to the center.  The only way to explain these surprising results was to imagine that most of the mass in the galaxy was in some form of invisible matter throughout and beyond the visible parts of the galaxy.