Most galaxies have only one nucleus, so why does the galaxy shown in this image appear to have four?
To solve this strange puzzle, astronomers concluded that the galaxy's own nucleus—whose spiral arms can be seen faintly in the image—is not actually visible at all in this photograph, taken by a telescope in southern Arizona. What appears as a four-leaf-clover shape at the center of the galaxy is instead light from a distant quasar[1] located behind it. The gravitational field of the foreground galaxy's central region splits the quasar's light into four separate images.
The quasar must lie almost directly behind the center of this massive galaxy for such a beautiful cosmic mirage to be created. This phenomenon, which was mentioned in last week's post, is generally known as gravitational lensing. In this particular case, because of its distinctive shape, it is known as the Einstein Cross.
The Einstein Cross continues to be a subject of active research among cosmologists. In different observations, its images can show somewhat different brightnesses and shapes, sometimes influenced by additional gravitational microlensing caused by stars in the foreground galaxy, producing interesting changes in the observed images.
[1] A quasar is an extremely luminous and distant active galactic nucleus associated with a young galaxy. Despite various historical debates about the nature of these objects, the accepted scientific picture is that a quasar is powered by matter concentrated around the supermassive black hole at the center of a galaxy. (Source: Wikipedia.)