A quasar whose image has been split into two by the gravitational lens effect of a massive galaxy or cluster of galaxies along the line of sight. The first example found, 0957+561 A and B, is a quasar at a redshift of 1.41 whose twin images lie 6″ apart. In this case the lensing is caused by a cluster at a redshift of 0.36, one of whose galaxies is visible 1″ away from one of the quasar images. The discovery of this double quasar in 1979 was the first confirmation (apart from measurements at eclipses of small deflections of light by the Sun) of the existence of gravitational lenses. More complex gravitational lens effects produce more than two images.
Subjects: Astronomy and Astrophysics.