How do we know that AGN have sizes on the order of our Solar System?
A) Quasars and Seyfert galaxies are stellar-like sources.
B) Their brightness varies by factors of a few.
C) The emission lines in their spectra show gas rotating at speeds of thousands of km/s.
D) Their brightness varies on timescales of hours to a day.

Question

Quizplus · Accepted Answer

The rapid variability in brightness on timescales of hours to a day suggests that the emitting region is very compact, similar in size to our Solar System, because light cannot travel across larger regions in such short times.

How Do We Know That AGN Have Sizes on the Order