Astronomy Picture of the Day
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July 11, 1995
Microlensing of the Einstein Cross
Picture Credit: Geraint Lewis and Michael Irwin, William Hershel Telescope
The famous "Einstein Cross" is a case where a single object is seen four times.
Here a very distant
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.
Tomorrow's picture: Eta Carinae Before Explosion
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