Astronomy Picture of the Day

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

December 20, 1995
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A Galaxy Gravitational Lens
Credit: NASA, HST, K. Ratnatunga. Myungshin Im (JHU)

Explanation: Sometimes it takes a keen eye to see a good mirage. Around the center of the red galaxy image in the above photograph lie four blue "smudges." Each smudge is actually a different image of the same background quasar. The central galaxy happens to fall directly in the light path of the quasar. Consequently, the huge mass of the galaxy is able to pull separate images of the quasar around it - an effect called gravitational lensing. Hence we see a gravitational mirage! Astronomers have hopes of using light differences between these quasar images to not only "weigh" the central galaxy but even provide clues about the expansion rate and composition of the universe.

Tomorrow's picture: Hot Stars in the Trifid Nebula

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Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
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