beam of light is bent by a gravitational field, a process known as gravitational lensing. The effect was famously first observed by Eddington in the early 1900s, and was one of the three great “tests” of general relativity.
Gravitational lensing can also occur on a cosmological scale, when a bright quasar happens to lie far behind a closer galaxy. The galaxy largely prevents us from observing the quasar directly, but light from the quasar is deflected toward us by the mass of the closer galaxy, which gives us the illusion of multiple identical quasars clustered around the galaxy.
These alignments of quasar and galaxy are not common. Of the nearly 170,000 observed quasars, active galactic nuclei and blazars (the ones listed in a famous catalog), there are only about 120 examples of the effect. The most famous example of the effect is Q2237+030, known as the Einstein Cross. The video below shows both the observation of the Einstein Cross and a visualization explaining the effect.
While such an observation is amazing on its own. we’re also able to take it one step further. Occasionally a star from the galaxy will pass in front of one of the quasar images, causing some of the quasar’s light to be deflected away from us, just as our Sun can deflect starlight. This causes the quasar image to vary and distort, a process known as microlensing. Observations of these microlensing events allow us to determine the size and composition of the region near the black hole driving the quasar.
We wouldn’t be able to study the quasar’s black hole region if the galaxy weren’t in the way. Sometimes obstacles really are opportunities.