A radio telescope can measure not only the intensity of 21 centimeter radio emission but also the velocity of the gas that produces the emis- sion. If the gas emitting this 21 centimeter spectral line is moving, the emissionΓÇÖs frequency will Doppler shift. So if the gas is moving away from us at 200 kilometers per second, the spectral line will shift .067 percent, from 21 to 21.014 centimeters. This image shows this type of Doppler shift. The blue represents gas that is approaching us; the red, gas that is moving away. Since the galaxy is rotating, one side of the disk is moving toward and the other away from us. The image reflects this movement.
