The Mind-Bending Phenomenon of the Einstein Ring

Studying a Mysterious Image: A Multitude of Galaxies
What are we looking at when we study this Hubble Space Telescope image? A very distant galaxy that lies 19.5 billion light-years from Earth? Or a much closer luminous red galaxy that is a (relatively) small 2.7 billion light-years away? Or a third galaxy that appears to be fairly close to the second?

The answer, perhaps confusingly, is that we are looking at all three. More precisely, we are looking at light emitted from all of those galaxies, even though the most distant galaxy lies directly behind the first as seen from Earth. In fact, it is that very alignment that makes the particular visuals in this image possible.

Identifying the Galaxies: SDSS and HerS
The central bright dot in this image is one of the closer galaxies, known by the lengthy — but informative — name of SDSS J020941.27+001558.4 (galaxy names in this format provide precise information about their location in the sky). The other bright dot above it — that appears to be intersecting a curving crescent of light — is SDSS J020941.23+001600.7, the second closer galaxy. And finally, that curving crescent of light itself is the ‘lensed’ light from the very distant galaxy. This is known as HerS J020941.1+001557, and it is also an interesting example of a phenomenon known as an Einstein ring.