Soon after discovery, it was observed that quasars had a starlike appearance which was blurred by Earth's atmospheric "seeing". Despite the blurring, it was realized that the intense energy-emitting regions of quasars must be small because they
A)have high radial velocities.
B)are very luminous.
C)are surrounded by quasar fuzz.
D)radiate huge amounts of energy.
E)fluctuate rapidly on time scales as short as a few hours.

Question

Quizplus · Accepted Answer

The rapid fluctuations in brightness observed in quasars indicate that the regions emitting this energy must be relatively small. This is because, according to the light-travel time argument, the size of the emitting region is constrained by the time it takes for light to travel across it. If a quasar's brightness changes significantly over a period of a few hours, the region responsible for these changes cannot be larger than a few light-hours across, implying a compact size.

Soon After Discovery, It Was Observed That Quasars Had a Starlike