Einstein Rings

In the early 20th century, Albert finstein developed his general theory of relativity, which
implied that massive objects in the universe would bend light, He predicted that if one
object was directly behind another, it would create an "Einstein Ring", but he didn't think
we'd ever observe one.

However, there are now hundreds of known gravitational lenses, some of which are Einstein
Rings. Here is a famous one, called the "Cosmic Horseshoe", observed by the Hubble Space
Telescepe.

Some interesting facts:

• The lens galaxy in the foreground is reddish, while the background galaxy is blueish
• The radius of the ring on the sky is about the same as a one dollar coin viewed from a
  distance of 700 kilometres.