The Einstein Cross

The righthand figure shows another example of a gravitational lens called the Einstein Cross. In this image, a single object appears as four objects. A very distant quasar is thought to be positioned behind a massive galaxy. The gravitational effect of the galaxy has created multiple images through gravitational lensing on the light from the quasar. The individual stars in the foreground galaxy may also be acting as gravitational lenses, causing the images to change their relative brightness in these two images taken three years apart as stars change position in the lensing galaxy.

The Lensing Galaxy
This interpretation of the Einstein Cross is bolstered by the adjacent left image, which shows in faint outline the foreground lensing galaxy surrounding the bright central nucleus of the spiral and the four quasar images. The lensing galaxy is a relatively nearby barred spiral. Both the spiral arms and the central bar of the foreground galaxy can be seen if one looks carefully (click on the "Show Labels" button to annotate these features). This image is due to University of Alabama astronomer Bill Keel and is a composite of five Hubble Space Telescope images taken in visible light at 5400 Angstroms. The intensity has been displayed on a logarithmic scale so that the very bright quasar images and the extremely faint bar and arms of the lensing galaxy can be seen at the same time.

True Position of the Quasar
The true image of the quasar would be almost perfectly aligned with the nucleus of the spiral if the gravity of the foreground galaxy could be turned off. The quasar is about 20 times further away than the spiral galaxy. Calculations based on the lensing observations indicate that it is aligned within 0.05 arc seconds of the nucleus of the spiral.