The bending of light by a gravitational body was predicted by
Albert Einstein in 1912, a few years before the publication of
general relativity in 1916 (Renn et al. 1997). The ring effect was first mentioned in the academic literature by
Orest Khvolson in a short article in 1924, in which he mentioned the “halo effect” of gravitation when the source, lens, and observer are in near-perfect alignment. Einstein remarked upon this effect in 1936 in a paper prompted by a letter by a Czech engineer, R W Mandl, but stated (In this statement, β is the Einstein Angle currently denoted by \theta_1, as in the expression above.) However, Einstein was only considering the chance of observing Einstein rings produced by stars, which is low – the chance of observing those produced by larger lenses such as galaxies or black holes is higher since the angular size of an Einstein ring increases with the mass of the lens. The first complete Einstein ring, designated B1938+666, was discovered by collaboration between astronomers at the
University of Manchester and
NASA's
Hubble Space Telescope in 1998. There have apparently not been any observations of a star forming an Einstein ring with another star, but there is a 45% chance of this happening in early May, 2028 when
Alpha Centauri A passes between us and a distant red star.
Known Einstein rings (SDSS J1038+4849) and
gravitational lensing (an "Einstein ring") discovered by an international team of scientists, imaged with
HST Hundreds of gravitational lenses are currently known. About half a dozen of them are partial Einstein rings with diameters up to an
arcsecond, although as either the mass distribution of the lenses is not perfectly
axially symmetrical, or the source, lens, and observer are not perfectly aligned, we have yet to see a perfect Einstein ring. Most rings have been discovered in the radio range. The degree of completeness needed for an image seen through a gravitational lens to qualify as an Einstein ring is yet to be defined. The first Einstein ring was discovered by Hewitt et al. (1988), who observed the radio source
MG1131+0456 using the
Very Large Array. This observation saw a
quasar lensed by a nearer galaxy into two separate but very similar images of the same object, the images stretched round the lens into an almost complete ring. These dual images are another possible effect of the source, lens, and observer not being perfectly aligned. false-color image of
SPT0418-47, a high-redshift galaxy rich in organic molecules, which appears as a nearly-perfect Einstein ring The first complete Einstein ring to be discovered was
B1938+666, which was found by King et al. (1998) via optical follow-up with the Hubble Space Telescope of a gravitational lens imaged with
MERLIN. The galaxy causing the lens at B1938+666 is an ancient
elliptical galaxy, and the image we see through the lens is a dark
dwarf satellite galaxy, which we would otherwise not be able to see with current technology. In 2005, the combined power of the
Sloan Digital Sky Survey (SDSS) with the Hubble Space Telescope was used in the Sloan Lens ACS (SLACS) Survey to find 19 new gravitational lenses, 8 of which showed Einstein rings, these are the 8 shown in the adjacent image. As of 2009, this survey has found 85 confirmed gravitational lenses but there is not yet a number for how many show Einstein rings. This survey is responsible for most of the recent discoveries of Einstein rings in the optical range, following are some examples which were found: •
FOR J0332-3557, discovered by
Remi Cabanac et al. in 2005, notable for its high
redshift which allows us to use it to make observations about the
early universe. • The "
Cosmic Horseshoe" is a partial Einstein ring which was observed through the gravitational lens of LRG 3-757, a distinctively large Luminous Red Galaxy. It was discovered in 2007 by V. Belokurov et al. •
SDSSJ0946+1006, the "double Einstein ring" was discovered by Raphael Gavazzi and Tommaso Treu in 2008, notable for the presence of multiple rings observed through the same gravitational lens, the significance of which is explained in the next section on
extra rings. Another example is the radio/X-Ray Einstein ring around
PKS 1830-211, which is unusually strong in radio. It was discovered in X-Ray by Varsha Gupta et al. at the Chandra X-Ray observatory It is also notable for being the first case of a quasar being lensed by an almost face-on
spiral galaxy. Galaxy MG1654+1346 features a radio ring. The image in the ring is that of a quasar
radio lobe, discovered in 1989 by G.Langston et al. In June 2023, a team of astronomers led by Justin Spilker announced their discovery of an Einstein ring of distant galaxy rich in organic molecules (
aromatic hydrocarbons). In September 2023, a scientist named Bruno Altieri saw a hint of an Einstein ring in the data coming back from the Euclid space telescope. The ring is located in a galaxy, NGC 6505, that is not too far from Earth–about 600-million light years away. In February 2025, the
Euclid space telescope captured a nearly perfect Einstein ring around galaxy
NGC 6505, about 590 million light-years away. This gravitational lensing effect bent light from a background galaxy 4.42 billion light-years away. == Extra rings ==