Relativistic Headlight Effect
The observed luminosity of an object depends on the relative velocity of the observer and the source. When the observer moves toward the source at nearly the speed of light, the luminosity of the source appears to increase; moving away from the source, the luminosity appears to decrease. This Demonstration illustrates rays of light emitted by a source, with the rays becoming more or less concentrated along the line of relative motion as the velocity is increased. The red arrow inside the source indicates the relative velocity.
For more information, see the Relativistic beaming entry on Wikipedia.