How do we know that AGNs 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 ranging from hours to a day.
E) We can measure their angular size, which gives us the physical size when we know the distance.

Question

Quizplus · Accepted Answer

The rapid variability in brightness on timescales of hours to a day suggests that the emitting region is relatively small, comparable to the size of our Solar System, because light cannot travel across a larger region in such a short time.

How Do We Know That AGNs Have Sizes on the Order