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History of Astronomy

Early explorations in astronomy and how they link to math and geometrical models.

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Einstein's Cross

Einstein's Cross

License: CC BY-NC 3.0

Sitting behind ZW 2237+030, the Einstein Cross seen as four images of the same quasar, is an example of a gravitationally lensed quasar.

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Credit: ESO/M. Kornmesser
Source: http://www.eso.org/public/images/eso1122a/
License: CC BY-NC 3.0

Quasars are known to be the brighest objects in the universe [Figure2]

  • The Einstein Cross is an example of a gravitationally lensed quasar. A quasar is an energetic and active galactic nucleus that normally shows a very high red shift. Believed to be powered by the accretion of matter into black holes, quasars are known to be the most luminous objects in the universe.
  • The four images seen in the Einstein Cross is a result of a galaxy in the foreground that is bending the light that is coming from the quasar. The bending of light due to a massive object is one of the concepts involved in general relativity. This means that the light given off by an object on the other side of a massive object will be bent towards your eye like a lens.
  • View an animation of the Einstein Cross at the link below: 


  • Learn how quasars are created at the video below: 


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Using the information provided above, answer the following questions.

  1. What exists at the center of a quasar?
  2. Describe gravitational lensing.
  3. If a star is observed to have a red shift, does this tell you what direction the star is moving relative to you? If so, what direction is it moving?

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Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ Credit: ESO/M. Kornmesser; Source: http://www.eso.org/public/images/eso1122a/; License: CC BY-NC 3.0

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