Credit & Copyright: Geraint Lewis and Michael Irwin, William Hershel TelescopeExplanation: The famous "Einstein Cross" is a case where a single object is seen four times. Here a very distant QSO happened to be placed right behind a massive galaxy. The gravitational effect of the galaxy on the distant QSO was similar to the lens effect of a drinking glass on a distant street light - it created multiple images. But stars in the foreground galaxy have been found to act as gravitational lenses here too! These stars make the images change brightness relative to each other. These brightness changes are visible on these two photographs of the Einstein Cross, taken about 3 years apart.
For more information about this picture see the home page of the IAU Symposium 173 Astrophysical Applications of Gravitational Lensing in Melbourne, Australia 9-14 July, 1995.
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Based on Astronomy Picture Of the Day
Publications with keywords: microlensing - Einstein cross
Publications with words: microlensing - Einstein cross