Small and Bright

It is an awe-inspiring thing to contemplate an object so powerful and so distant. The most distant quasars are some of the most distant objects we can see.

Quasars are distant and bright, but also small. How do we know they are small? Many quasars flicker, varying in brightness, rapidly—on scales of days. We know that light must be able to travel across the size of an object for us to see it vary in brightness. Why? Because for a region to appear to brighten, you must detect photons from the

far side as well as from the near side of the object that is brightening. If an object is one light-year across, the "brighter" photons from the far side of the source will not reach you until a year after the photons on the near side. So this source could only flicker on scales of a year.

Any object that flickers on scales of mere days must be small—light days across, to be exact. This characteristic flickering reveals that the source of energy is perhaps the size of our solar system—presumably a gaseous accretion disk spiraling toward a supermassive black hole.

If a supermassive black hole is the source of a quasar's power, then about 10 suns per year falling into the black hole could produce its enormous luminosity.

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