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Focus On Einstein's Cross: A Gravitational Lens

For telescopes 18" and larger

Einstein's cross is so difficult to observe successfully that some people believe that it can't be done in amateur instruments. Yet, the allure of seeing a gravitational lens with your own eyes--a direct visual confirmation of Einstein's General Relativity--is compelling.

Einstein's General Relativity is a modern description of gravity. Building on the work of Newton and Galileo before him, Einstein took our understanding of gravity one step further. He described gravity as an acceleration due to a curvature in spacetime rather than as a force. So what exactly, you say, does that mean?

Let's first begin with an analogy. Imagine a being for whom there was no up or down; only north-south and east-west. This being would have no height at all and no direct knowledge of the "3rd" up-down dimension. Now place this being on the surface of the earth and have him walk very far in one direction. After a long time he would completely circumnavigate the globe and return to where he started from. "How could this be?", he would rightly ask. After all, he never deviated from a straight line from his point of view. But from our point of view the solution is easy. The earth is curved in the 3rd dimension and this curvature eventually led the 2-D being back to where he started from.

The basic idea behind General Relativity is that there exists another dimension that we cannot directly observe. Any object with mass, such as earth, will bend space and time around it--cause it to curve--in that unseen other dimension.

The moon, for instance, is actually going in a straight line as it orbits the earth. That is, when you take all dimensions into account. That's why no rockets are needed to keep it up there. But from our point of view the moon follows a curved path around the earth. That's because we can't see the curvature around the earth in the other dimension. If you throw a ball it follows this curvature as well, at least until it hits the ground. The beauty of this idea is that it alleviates the need for a "force" of gravity which somehow magically operates instantly over a distance and without any sort of medium to carry it.

A key prediction of Einstein's theory is that light will also follow a curved path around massive objects. Classical gravity theory predicts that light, being massless, will not be affected by gravity. One test of this idea is to observe the position of a star near the edge of the sun. If the light does follow a curved path near the sun we will see it as a shift in the star's position. And in fact, this shift is observed in precisely the amount predicted.  In this same way the light from a  distant quasar can be bent around a more nearby galaxy. The closer galaxy acts like a lens, bending the light to produce multiple images of the distant quasar. Einstein's cross is one of the best examples of this effect.

Even moderate sized scopes will reveal the galaxy that is doing the lensing. This galaxy is the 14-15th magnitude PGC 69457 which is some 400 million light years distant. Once you find this tiny smudge of light you can begin to look for the cross.

PGC 69457
Right Ascension Declination Magnitude Con Distance From Earth
22h40m30.3s 03o21'31" 14 Peg 400 million lys

Einstein's cross is located near the Pegasus/Aquarius border. PGC 69457 is the lensing galaxy. 

The field in an 18-inch at 94x; the circular field of view is 44'. The labeled HD stars are 8-9th magnitude. 

North is down and east is to the right.

The quasar appears as four distinct objects all fainter than 17th magnitude. In order to see these objects you will need at least an 18" scope, the darkest skies imaginable, and excellent seeing. You will also need to be fully dark adapted, which means that your eyes must not be exposed to light for at least 30 minutes. As always with difficult objects, observe from the most relaxing position possible.

Use as much magnification as the conditions will permit.

Experienced observers report that the cross looks like four smudges of light embedded in the galaxy. Try looking for the brightest component (A) first. In apertures greater than 24" the individual components may be briefly visible separately.

Magnitudes of components: A=17.4, B=17.4, C=18.4, D=18.7. 

The above long-exposure photograph shows the position of the lensed images of the quasar. North is down and east is to the right, matching the view in a Newtonian. The separation between A and B is less than 2".