Scientists using the Hubble Space Telescope have used the magnifying capabilities of a faraway galaxy to take a picture of an even more distant dusty disc surrounding an active black hole.

The observations of the quasar's accretion disc – the glowing disc of matter that has been heated up as it is sucked into the central black hole – have revealed unique details of its size and temperature, observations that were only made possible using the gravitational lensing technique. This method relies on stars in an intervening galaxy located between the space telescope and the distant black hole magnifying light streaming from the quasar.

For one quasar, known as HE 1104-1805, the stars in the intervening galaxy passed through the path of the light from the quasar such that they amplified the signals from different parts of the accretion disc, enabling astronomers to construct a colour profile across the disc.

The colours could then be translated into temperatures, since hot matter closest to the black hole emits at bluer wavelengths than material further out. These temperature readings could then be converted into distance measurements, allowing the astronomers to arrive at an estimate of the diameter of the disc of between four and eleven light-days across, or 100 to 300 billion kilometres.

"These kinds of observations are quite difficult," says lead scientist Jose Muñoz. "But our team is leading projects to detect as much as possible this effect in order to constrain much better the physical properties of the quasar accretion discs. This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone."