As you may recall from History Page 7, one of the predictions of the theory of general relativity and the first one to be proved was that a strong gravitational field should be able to bend the path of light traveling through it.
In our solar system, the only object massive enough to really bend light is, of course, the Sun. When the light from a distant star passes close to the Sun its path is bent around the Sun a small ways. For anyone who sees this light after it has been bent, it appears as if the distant star is at a slightly different place in the sky than where it really is! Of course, in practicality, we can only really observe this distortion during a solar eclipse because at all other times the Sun’s own light makes it difficult to see the other stars.
However, there are other objects out in the cosmos capable of bending light even more than the Sun can. Just think of what the gravity from an entire galaxy could do! When a massive galaxy falls into the light path of a quasar (a star-like object believed to be the most distant and most luminous object in the universe), the galaxy's huge mass is able to pull separate images of the quasar around it! These separate images consist of a real image (formed by light rays which travel directly to Earth from the quasar) and multiple virtual images (formed by light rays which are bent around a massive object then toward Earth). The virtual images are basically mirages and are the result of an effect called gravitational lensing.
