APOD 4.2 The Einstein Cross Gravitational Lens

If you look at the center of this galaxy you might think it has four nuclei. Actually, the nucleus of this galaxy cannot be seen in this image. What you see is the light from a distant pulsar directly behind the central nucleus. The gravitational field of the galaxy bends the light into the mirage. The bending of light due to gravity is called "gravitational lensing," the image this particular lensing creates is calledthe "Einstein Cross." Different images of the "Einstein Cross" shows that its brightness varies; due to the gravitational effect of other nearby stars in the galaxy.