What does a black hole look like? Scientists have, for the first time, directly observed the disc that surrounds this area of tremendous gravitational pull, from which no light can escape.
Black holes power quasars, which are thought to be the extremely bright centers of certain galaxies surrounding supermassive black holes. Scientists believe that large amounts of energy are released when matter falls into a black hole, producing the light of the quasar. Around a black hole is an accretion disk, made of dust and gas.
The thing about quasars, though, is that they're so far from Earth that even at a typical size of 100 billion kilometers across, they appear so small to us that there will probably never be a telescope powerful enough to see their structures directly. That in turn means that most of our knowledge of quasars' inner structure is theoretical – not based on direct observations.
In order to learn more about the nature of these still-largely-mysterious objects, then, astronomers had to devise a new way to study quasars. That's exactly what they've done with the Hubble Space Telescope: They used the stars in a galaxy as a scanning microscope. The gravitational lensing of these stars amplifies the light from different parts of the quasar itself, resulting in detailed color information for a line through the accretion disc.
By combining several images produced using this technique, astronomers were able to see subtle color differences over the observation times, which translates to a full color profile across the black hole's accretion disc. The color profile in turn allowed the team to determine that the disc is between 100 to 300 billion kilometers across – a wide range, to be sure, but still incredibly accurate given the incredible distance between the relatively small quasar and Earth.