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.