Cosmic Lens Reveals the Anatomy of a Quasar express
A distant quasar appears as four points of light arranged in a cross. This is how the gravity of a galaxy lying along our line of sight distorts the view: it acts like imperfect glass, multiplying the image. Each point is the same object, but its light travels different paths. So when the quasar flares, the four images respond at different times and to different degrees.
Astronomers gathered 25 years of observations from telescopes in both hemispheres. They devised a way to more accurately subtract the light from the lensing galaxy itself, so it wouldn't interfere. The result was very clean light curves for each of the four images. These curves show that the flickering is caused not only by the lens itself, but also by individual stars within it: they act as tiny additional lenses.
From this flickering, they extracted the key quantity: the size of the quasar's glowing region. In blue light, it spans about 10 light-days across — roughly the distance from the Sun to the Oort cloud. In red light, the region is noticeably larger. The longer the wavelength, the broader the source. The growth is nearly linear: the red source is about twice as large as the blue one.
This result is direct evidence that a quasar is not a point. It is a disk of incandescent gas surrounding a supermassive black hole. The inner, hotter parts of the disk shine in blue and ultraviolet, while the outer, cooler parts shine in red. Hence the size increases with wavelength. Now models describing quasar emission face a stringent test: they must reproduce exactly this size–wavelength relation.
The light from this quasar traveled about 8 billion years to reach us — more than half the age of the Universe.
In the movie Interstellar, the characters witness a similar phenomenon: the black hole Gargantua warps the light from its accretion disk, creating multiple images.