Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

December 15, 1996
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Microlensing of the Einstein Cross
Credit: G. Lewis (IOA), M. Irwin (RGO), William Hershel Telescope

Explanation: 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 an empty wine 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.

Tomorrow's picture: Nebula Nova Cygni Turns On

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