How can astronomers determine the size of an emission region in a very distant and unresolvable source?

Question

Quizplus · Accepted Answer

By measuring brightness variability, astronomers can estimate the size of the emission region based on the time it takes for the variation in brightness to occur. Since information cannot travel faster than the speed of light, the size of the emission region must be smaller than the time it takes for the brightness variability to occur. This method is known as the variability timescale method, and it has been used to estimate the size of emission regions in quasars and other active galactic nuclei. Option A is incorrect because the object's mass and density cannot be reliably measured from a distance. Option B is incorrect because radio interferometry requires the presence of multiple radio telescopes, and it may not be feasible to observe a distant and unresolvable source with enough precision. Option D is incorrect because the redshift of a spectrum is primarily determined by the object's velocity and distance, not its size.

How Can Astronomers Determine the Size of an Emission Region