Einstein Cross (G2237+030)
The Einstein Cross (G2237+030) is the most spectacular of the 20 or so well-attested instances of gravitational
lenses that involve a quasar and foreground
galaxy. The Einstein Cross consists of four symmetrically placed images
of the same quasar, which lies 8 billion light-years away and almost directly
behind the nucleus of a galaxy that is only 500 million light-years away.
The alignment is so close that if the lens effect could be removed, the
quasar would appear within 0.05" of the galaxy's nucleus. The system was
discovered in the course of a redshift survey by the Smithsonian Center
for Astrophysics by John Huchra (1948–), and is sometimes referred
as the Huchra Lens. Einstein crosses form when source and lens are in alignment
with the observer but the lensing mass is unevenly distributed. When there
is alignment plus even distribution of the lensing mass the result is an Einstein ring.