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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

Credit: NASA
Source: http://commons.wikimedia.org/wiki/File:Einstein_cross.jpg

Sitting behind ZW 2237+030, the Einstein Cross refers to the four images of the same quasar. It serves as an excellent example of the gravitational lensing phenomenon.

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Credit: ESO/M. Kornmesser
Source: http://www.eso.org/public/images/eso1122a/

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

• A quasar is an energetic and active galactic nucleus that normally shows a high redshift. They are also recognized as the most luminous objects in the entire universe.
• The four images seen in the Einstein Cross are 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. 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:

#### Explore More

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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