What is Einstein ring? « Back to Questions List

Einstein ring is the phenomenon that happens when light from a source such as galaxy or star gets deformed create the shape of a ring. This happens due to gravitational lensing  of the light by a large mass like black hole or another galaxy.  The large mass exists between the source of light and the observer.  When the two galaxies and the observer are exactly aligned, the image of the distant galaxy looks like an almost perfect circular shape. 

The phenomenon got the name Einstein ring because the principle of gravitational lensing is rooted in Einstein’s Theory of General Relativity.  The bending of light was predicted by him in 1912. 

Many gravitational lenses are currently known.  Half a dozen of them are partial Einstein rings. Since the source, gravitational lens and observer have not been perfectly aligned, majority of them are partial Einstein ring.  The first Einstein ring was discovered in 1988 by Hewitt Et. Al. The first complete Einstein ring was discovered in 1988 by King and team.  This was identified using Hubble Space Telescope.  High resolution and sensitivity of Hubble telescope has been behind the discovery of many of these rings.  Ground based telescopes cannot be used to locate such rings as the images gets blurred by the Earths atmosphere.  In 2005, experiments with the combined power of the Sloan Digital Sky Survey (SDSS) with the Hubble Space Telescope  found 19 new gravitational lenses, 8 of which produced  Einstein rings.

In the above picture,  the glowing ring at the centre of the image is called the Einstein ring.  The light originated from the source galaxy is diverted and distorted by the massive object and the light is forced to travel towards the observer in earth along many different light paths.  The mass of the object is large enough to severely distort the light.  The gravitational lensing sometimes causes  multiple images of the original galaxy each with a characteristically distorted banana-like shape or  rings. 
 

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