An international team of astronomers has turbocharged the Hubble Space Telescope , enabling it to observe a brightly glowing disc of matter that is being sucked into its galaxy's central black hole. Such discs, known as quasar accretion disks, are typically about 100 billion kilometers across, and most lay billions of light-years away. So how was Hubble able to observe it? Through a technique called gravitational microlensing in which the light from a background object is bent by gravity around a foreground object. If scientists can line up a quasar almost exactly behind a much closer galaxy, they are able to see not one quasar but two or four magnified images of the same quasar—a gravitational lens. (In the image above the more distant quasar HE 1104-1805 is seen as the two larger images on either side of the smaller yet closer lens galaxy [WKK93] G.) The stars in that lens galaxy then act like ultra-high resolution telescopes (see the NASA video ). The level of detail involved is equivalent to being able to study individual grains of sand on the surface of the moon while standing on Earth. This enabled the astronomers to measure the diameter of the accretion disc and plot its various temperatures, providing a new experimental confirmation of how quasars work.
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