Editing Einstein ring

{{short description|Feature seen when light is gravitationally lensed by an object}}
[[File:Close-up of the Einstein ring around galaxy NGC 6505 ESA506346.jpg|thumb|Einstein ring, created by a nearer galaxy ([[NGC 6505]]) distorting light of a distant galaxy. Four images of the distant galaxy, corresponding to four bright spots, can be seen in this picture.]]
{{Gravitational Lensing}}

An '''Einstein ring''', also known as an '''Einstein–Chwolson ring''' or '''Chwolson ring''' (named for [[Orest Khvolson|Orest Chwolson]]), is created when light from a [[galaxy]] or [[star]] passes by a massive object en route to the Earth. Due to [[gravitational lens]]ing, the light is diverted, making it seem to come from different places. If source, lens, and observer are all in perfect alignment (''[[Syzygy (astronomy)|syzygy]]''), the light appears as a ring.

== Introduction ==
Gravitational lensing is predicted by [[Albert Einstein]]'s theory of [[general relativity]].<ref name="NYT-20150305">{{cite news |last=Overbye |first=Dennis 
|author-link=Dennis Overbye |title=Astronomers Observe Supernova and Find They're Watching Reruns |url=https://www.nytimes.com/2015/03/06/science/astronomers-observe-supernova-and-find-theyre-watching-reruns.html |date=March 5, 2015 |work=The New York Times |access-date=March 5, 2015 }}</ref> Instead of light from a source traveling in a straight line (in three dimensions), it is bent by the presence of a massive body, which distorts [[spacetime]]. An Einstein Ring is a special case of gravitational lensing, caused by the exact alignment of the source, lens, and observer. This results in symmetry around the lens, causing a ring-like structure.<ref name="NYT-20150305-video">{{cite news |last1=Drakeford |first1=Jason |last2=Corum |first2=Jonathan |last3=Overbye |first3=Dennis |date=March 5, 2015 |title=Einstein's Telescope – video (02:32) |url=https://www.nytimes.com/video/science/100000003552687/out-there-einsteins-telescope.html |access-date=December 27, 2015 |work=[[The New York Times]]}}</ref>

[[File:Einstein ring geometry.svg|upright=1.7|thumb|The geometry of a complete Einstein ring, as caused by a [[gravitational lens]]]]
The size of an Einstein ring is given by the [[Einstein radius]]. In [[radian]]s, it is

:<math>\theta_1 = \sqrt{\frac{4GM}{c^2}\;\frac{D_{LS}}{D_S D_L}},</math>

where

: <math>G</math> is the [[gravitational constant]],
: <math>M</math> is the mass of the lens,
: <math>c</math> is the [[speed of light]],
: <math>D_L</math> is the [[angular diameter distance]] to the lens,
: <math>D_S</math> is the [[angular diameter distance]] to the source, and
: <math>D_{LS}</math> is the [[angular diameter distance]] between the lens and the source.<ref>{{cite web |url=https://www.cfa.harvard.edu/~dfabricant/huchra/ay202/lectures/lecture12.pdf |title=Gravitational lensing |page=19 |last=Pritchard |first=Jonathan |publisher= Harvard and Smithsonian| access-date=21 December 2019 }}</ref>

Over cosmological distances <math>D_{LS}\ne D_S-D_L</math> in general.

== History ==
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.<ref name="Early History of Gravitational Lensing">{{cite web|url=http://www.nd.edu/~hps/turner.pdf|title=The Early History of Gravitational Lensing|first=Christina|last=Turner|date=February 14, 2006|url-status=dead|archive-url=https://web.archive.org/web/20080725085534/http://www.nd.edu/%7Ehps/turner.pdf|archive-date=July 25, 2008}}</ref> Einstein remarked upon this effect in 1936 in a paper prompted by a letter by a Czech engineer, R W Mandl,<ref>{{Cite web |last=Maurer |first=Stephen M. |title=IDEA MAN |url=https://www.slac.stanford.edu/pubs/beamline/31/1/31-1-maurer.pdf |access-date=4 November 2023 |website=www.slac.stanford.edu}}</ref> but stated

{{blockquote|Of course, there is no hope of observing this phenomenon directly. First, we shall scarcely ever approach closely enough to such a central line. Second, the angle β will defy the resolving power of our instruments.|''Science'' vol 84, p. 506. 1936}}
(In this statement, β is the Einstein Angle currently denoted by <math>\theta_1,</math> 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.<ref name="Bullseye">{{cite web | url=http://www.merlin.ac.uk/press/PR9801/press.html | title=A Bull's Eye for MERLIN and the Hubble | publisher=University of Manchester | date=27 March 1998}}</ref>

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.<ref>{{cite journal|last1=P. Kervella|display-authors=etal|title=Close stellar conjunctions of α Centauri A and B until 2050|journal=Astronomy & Astrophysics|volume=594|pages=A107|arxiv=1610.06079|date=Oct 19, 2016|doi=10.1051/0004-6361/201629201|bibcode=2016A&A...594A.107K|s2cid=55865290}}</ref>

=== Known Einstein rings ===
[[File:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg|thumb|"Smiley" or "Cheshire Cat" image of [[galaxy cluster]] (SDSS J1038+4849) and [[gravitational lensing]] (an "Einstein ring") discovered by an international team of scientists,<ref>{{cite journal |last1=Belokurov |first1=V. |display-authors=etal |title=Two new large-separation gravitational lenses from SDSS|journal=Monthly Notices of the Royal Astronomical Society |date=January 2009 |volume=392 |issue=1 |pages=104–112 |arxiv=0806.4188 |bibcode=2009MNRAS.392..104B |doi=10.1111/j.1365-2966.2008.14075.x |doi-access=free |s2cid=14154993 }}</ref> imaged with [[Hubble Space Telescope|HST]]<ref">{{cite web |last1=Loff |first1=Sarah |last2=Dunbar |first2=Brian |title=Hubble Sees A Smiling Lens |url=http://www.nasa.gov/content/hubble-sees-a-smiling-lens/ |date=10 February 2015 |work=[[NASA]] |access-date=10 February 2015 }}</ref>]]

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 [[Axial symmetry|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.<ref>{{cite web | url=http://www.nrao.edu/pr/2000/vla20/background/ering/ | title=Discovery of the First "Einstein Ring" Gravitational Lens | publisher=[[NRAO]] | date=2000 | access-date=2012-02-08}}</ref> These dual images are another possible effect of the source, lens, and observer not being perfectly aligned.
[[File:Webb Telescope Detects Universe’s Most Distant Organic Molecules (52958010034).jpg|thumb|[[James Webb Space Telescope|JWST]] 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]].<ref name="Bullseye"/><ref>{{cite news| url=https://query.nytimes.com/gst/fullpage.html?res=9906EFDF103BF932A05750C0A96E958260 | work=The New York Times | title='Einstein Ring' Caused by Space Warping Is Found | first=Malcolm W. | last=Browne | date=1998-03-31 | access-date=2010-05-01}}</ref> 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 galaxy|dwarf]] [[satellite galaxy]], which we would otherwise not be able to see with current technology.<ref>{{cite journal|last1=Vegetti|first1=Simona|display-authors=etal|title=Gravitational detection of a low-mass dark satellite at cosmological distance|journal=[[Nature (journal)|Nature]]|date=January 2012|volume=481|issue=7381|pages=341–343|arxiv = 1201.3643 |bibcode = 2012Natur.481..341V |doi = 10.1038/nature10669|pmid=22258612|s2cid=205227095}}</ref>

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,<ref>{{cite web|last1=Bolton|first1=A|display-authors=etal|title=Hubble, Sloan Quadruple Number of Known Optical Einstein Rings|url=http://hubblesite.org/newscenter/archive/releases/2005/32/|website=Hubblesite|access-date=2014-07-16}}</ref> 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.<ref>{{cite journal|last1=Auger|first1=Matt|display-authors=etal|title=The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies|journal=The Astrophysical Journal|date=November 2009|volume=705|issue=2|pages=1099–1115|arxiv = 0911.2471 |bibcode = 2009ApJ...705.1099A |doi = 10.1088/0004-637X/705/2/1099 |s2cid=118411085}}</ref> 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,<ref>{{cite journal |last1=Cabanac|first1=Remi |display-authors=etal |title=Discovery of a high-redshift Einstein ring|journal=Astronomy and Astrophysics|date=2005-04-27|volume=436|issue=2|pages=L21–L25|arxiv = astro-ph/0504585 |bibcode = 2005A&A...436L..21C |doi = 10.1051/0004-6361:200500115 |s2cid=15732993 }}</ref> 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.<ref>{{cite journal|last1=Belokurov|first1=V.|display-authors=etal|title=The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy|journal=[[The Astrophysical Journal]]|date=December 2007|volume=671|issue=1|pages=L9–L12|arxiv = 0706.2326 |bibcode = 2007ApJ...671L...9B |doi = 10.1086/524948 |s2cid=9908281}}</ref>
*[[SDSSJ0946+1006]], the "double Einstein ring" was discovered by Raphael Gavazzi and Tomasso Treu<ref>{{cite journal|last1=Gavazzi|first1=Raphael|display-authors=etal|title=The Sloan Lens ACS Survey. VI: Discovery and Analysis of a Double Einstein Ring|journal=The Astrophysical Journal|date=April 2008|volume=677|issue=2|pages=1046–1059|arxiv = 0801.1555 |bibcode = 2008ApJ...677.1046G |doi = 10.1086/529541 |s2cid=14271515}}</ref> 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 [[Einstein rings#Extra rings|extra rings]].
Another example is the radio/X-Ray Einstein ring around [[PKS 1830-211]], which is unusually strong in radio.<ref>{{cite journal|last1=Mathur|first1=Smita|last2=Nair|first2=Sunita|title=X-Ray Absorption toward the Einstein Ring Source PKS 1830-211|journal=The Astrophysical Journal|date=20 July 1997|volume=484|issue=1|pages=140–144|arxiv = astro-ph/9703015 |bibcode = 1997ApJ...484..140M |doi=10.1086/304327 |s2cid=11435604}}</ref> It was discovered in X-Ray by Varsha Gupta et al. at the Chandra X-Ray observatory<ref>{{cite web|last1=Gupta|first1=Varsha|title=Chandra Detection of AN X-Ray Einstein Ring in PKS 1830-211|url=https://www.researchgate.net/publication/252412929|website=ResearchGate.net|access-date=16 July 2014}}</ref> It is also notable for being the first case of a quasar being lensed by an almost face-on [[spiral galaxy]].<ref>{{cite journal|last1=Courbin|first1=Frederic|title=Cosmic alignment towards the radio Einstein ring PKS 1830-211 ?|journal=The Astrophysical Journal|date=August 2002|volume=575|issue=1|pages=95–102|arxiv = astro-ph/0202026 |bibcode = 2002ApJ...575...95C |doi = 10.1086/341261 |s2cid=13960111}}</ref>

Galaxy MG1654+1346 features a radio ring. The image in the ring is that of a quasar [[Radio galaxy|radio lobe]], discovered in 1989 by G.Langston et al.<ref>{{cite journal |last1=Langston |first1=G. I. |display-authors=etal |date=May 1989 |title=MG 1654+1346 – an Einstein Ring image of a quasar radio lobe |journal=Astronomical Journal |volume=97 |pages=1283–1290 |bibcode=1989AJ.....97.1283L |doi=10.1086/115071}}</ref>

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 compound|aromatic hydrocarbons]]).<ref>{{Cite journal |last1=Spilker |first1=Justin S. |last2=Phadke |first2=Kedar A. |date=June 2, 2023 |title=Spatial variations in aromatic hydrocarbon emission in a dust-rich galaxy |url=https://www.nature.com/articles/s41586-023-05998-6 |journal=Nature |volume=618 |issue=7966 |pages=708–711 |doi=10.1038/s41586-023-05998-6|pmid=37277615 |arxiv=2306.03152 |bibcode=2023Natur.618..708S |s2cid=259088774 }}</ref><ref>{{Cite news |last=Hutchins |first=Shana K. |date=June 5, 2023 |title=Webb Telescope Detects Universe's Most Distant Organic Molecules |work=Texas A&M Today |url=https://today.tamu.edu/2023/06/05/webb-telescope-detects-universes-most-distant-organic-molecules/ |access-date=June 29, 2023}}</ref>

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.<ref>{{Cite web |title=Euclid discovers a stunning Einstein ring |url=https://www.esa.int/Science_Exploration/Space_Science/Euclid/Euclid_discovers_a_stunning_Einstein_ring |access-date=2025-02-10 |website=www.esa.int |language=en}}</ref> The ring is located in a galaxy, NGC 6505, that is not too far from Earth–about 600-million light years away.<ref>{{Cite web |author1=Robert Lea |date=2025-02-10 |title=Euclid 'dark universe' telescope discovers stunning Einstein ring in warped space-time (image) |url=https://www.space.com/euclid-einstein-ring-dark-matter |access-date=2025-02-10 |website=Space.com |language=en}}</ref>

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.<ref>{{cite web |last=Strickland |first=Ashley |url=https://edition.cnn.com/2025/02/11/science/einstein-ring-euclid-telescope/index.html |title=Space telescope reveals rare 'Einstein ring' phenomenon with startling clarity |publisher=CNN |date=11 February 2025 }}</ref>

== Extra rings ==
[[File:SDSSJ0946+1006.jpg|thumb|SDSSJ0946+1006 is a Double Einstein Ring. Credit: [[Hubble Space Telescope|HST]]/[[NASA]]/[[ESA]]]]

Using the Hubble Space Telescope, a double ring has been found by Raphael Gavazzi of the [[STScI]] and Tommaso Treu of the [[University of California, Santa Barbara]]. This arises from the light from three galaxies at distances of 3, 6, and 11 billion light years. Such rings help in understanding the distribution of [[dark matter]], [[dark energy]], the nature of distant galaxies, and the [[curvature of the universe]]. The odds of finding such a double ring around a massive galaxy are 1 in 10,000. Sampling 50 suitable double rings would provide astronomers with a more accurate measurement of the dark matter content of the universe and the equation of state of the dark energy to within 10 percent precision.<ref>{{cite web |url=http://hubblesite.org/newscenter/archive/releases/2008/04/full/ |title=Hubble Finds Double Einstein Ring |access-date=2008-01-26 |work=Hubblesite.org |publisher=[[Space Telescope Science Institute]] }}</ref>

=== Simulation ===
Below in the Gallery section is a simulation depicting a zoom on a [[Schwarzschild metric|Schwarzschild black hole]] in the plane of the [[Milky Way]] between us and the centre of the galaxy. The first Einstein ring is the most distorted region of the picture and shows the [[Disc (galaxy)|galactic disc]]. The zoom then reveals a series of 4 extra rings, increasingly thinner and closer to the black hole shadow. They are multiple images of the galactic disk. The first and third correspond to points which are behind the black hole (from the observer's position) and correspond here to the bright yellow region of the galactic disc (close to the galactic center), whereas the second and fourth correspond to images of objects which are behind the observer, which appear bluer, since the corresponding part of the galactic disc is thinner and hence dimmer here.

== Gallery ==
<gallery widths="200px" heights="130px">
File:Euclid image of a bright Einstein ring around galaxy NGC 6505 ESA506345.tiff|Euclid image of a bright Einstein ring around galaxy NGC 6505
File:Einstein Rings (Cropped).jpg|Some observed Einstein rings by SLACS
File:Cosmic cloning SDSSJ0146-0929.jpg|Graceful arcs around SDSSJ0146-0929 are examples of an Einstein ring
File:BlackHole Lensing.gif|A simulated view of a black hole passing in front of a galaxy
File:Montage of the SDP.81 Einstein Ring and the lensed galaxy.jpg|Montage of the SDP.81 Einstein Ring and the lensed galaxy
File:EnsteinRingZoomOptimised.gif|Einstein rings close to a black hole
File:The Carousel Lens.jpg|The [[Carousel Lens]] shows 7 individual galaxies lensed by one core. Additional lenses are seen further out.<ref name="Sheu2024">{{Cite journal |last1=Sheu |first1=William |last2=Cikota |first2=Aleksandar |last3=Huang |first3=Xiaosheng |last4=Glazebrook |first4=Karl |last5=Storfer |first5=Christopher |last6=Agarwal |first6=Shrihan |last7=Schlegel |first7=David J. |last8=Suzuki |first8=Nao |last9=Barone |first9=Tania M. |last10=Bian |first10=Fuyan |last11=Jeltema |first11=Tesla |last12=Jones |first12=Tucker |last13=Kacprzak |first13=Glenn G. |last14=O'Donnell |first14=Jackson H. |last15=G. C. |first15=Keerthi Vasan |date=September 2024 |title=The Carousel Lens: A Well-modeled Strong Lens with Multiple Sources Spectroscopically Confirmed by VLT/MUSE |journal=The Astrophysical Journal |language=en |volume=973 |issue=1 |pages=3 |arxiv=2408.10320 |bibcode=2024ApJ...973....3S |doi=10.3847/1538-4357/ad65d3 |doi-access=free |issn=0004-637X}}</ref>
</gallery>


== See also ==
{{Commons category|Einstein Rings}}
* [[Einstein Cross]]
* [[Einstein radius]]
* [[Gravitational mirage]]

== References ==
{{Reflist}}

=== Journals ===
{{refbegin|30em}}
* {{cite journal |first=R. A. |last=Cabanac |display-authors=etal |title=Discovery of a high-redshift Einstein ring |date=2005 |journal=Astronomy and Astrophysics |volume=436 |issue=2 |pages=L21–L25 |doi=10.1051/0004-6361:200500115 |bibcode=2005A&A...436L..21C |arxiv=astro-ph/0504585 |s2cid=15732993}} (refers to FOR J0332-3357)
* {{cite journal |first=O. |last=Chwolson |title=Über eine mögliche Form fiktiver Doppelsterne |journal=Astronomische Nachrichten |volume=221 |issue=20 |pages=329–330 |date=1924 |bibcode=1924AN....221..329C |doi=10.1002/asna.19242212003}} (The first paper to propose rings)
* {{cite journal |first=Albert |last=Einstein |author-link=Albert Einstein |title=Lens-like Action of a Star by the Deviation of Light in the Gravitational Field |journal=Science |volume=84 |pages=506–507 |date=1936 |url=http://www.to.infn.it/~zaninett/projects/storia/einstein1936.pdf |doi=10.1126/science.84.2188.506 |pmid=17769014 |issue=2188 |bibcode=1936Sci....84..506E}}{{Dead link|date=October 2023|bot=InternetArchiveBot|fix-attempted=yes}} (The famous Einstein Ring paper)
* {{cite journal |first=J. |last=Hewitt |title=Unusual radio source MG1131+0456 – A possible Einstein ring |journal=Nature |volume=333 |issue=6173 |pages=537–540 |date=1988 |bibcode=1988Natur.333..537H |doi=10.1038/333537a0 |s2cid=23277001}}
* {{cite journal |first1=Jurgen |last1=Renn |first2=Tilman |last2=Sauer |first3=John |last3=Stachel |title=The Origin of Gravitational Lensing: A Postscript to Einstein's 1936 Science paper |journal=Science |volume=275 |pages=184–186 |date=1997 |doi=10.1126/science.275.5297.184 |pmid=8985006 |issue=5297 |bibcode=1997Sci...275..184R |s2cid=43449111}}
* {{cite journal |first=L. |last=King |title=A complete infrared Einstein ring in the gravitational lens system B1938 + 666 |journal=MNRAS |volume=295 |issue=2 |pages=L41–L44 |date=1998 |bibcode=1998MNRAS.295L..41K |arxiv=astro-ph/9710171 |doi=10.1046/j.1365-8711.1998.295241.x |doi-access=free |s2cid=15647305}}
{{refend}}

=== News ===
{{refbegin}}
* {{cite news |url=http://www.universetoday.com/am/publish/perfect_einstein_ring.html |title=Nearly perfect Einstein ring discovered |publisher=Universe Today |first=Jeff |last=Barbour |date=2005-04-29 |access-date=2006-06-15}} (refers to FOR J0332-3357)
* {{cite news |url=https://www.sciencedaily.com/releases/2008/01/080110102319.htm |title=Hubble Finds Double Einstein Ring |publisher=Science Daily |date=2008-01-12 |access-date=2008-01-14}}
{{refend}}

== Further reading ==
* {{cite journal | first1=C. S. | last1=Kochanek | first2=C. R. | last2=Keeton | first3=B. A. | last3=McLeod | title=The Importance of Einstein Rings | journal=The Astrophysical Journal | date=2001 | volume=547 | issue=1 | pages=50–59 | arxiv=astro-ph/0006116 | doi=10.1086/318350 | bibcode=2001ApJ...547...50K | s2cid=16122139 }}

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[[Category:Albert Einstein|Ring]]
[[Category:Effects of gravity]]
[[Category:Gravitational lensing]]
[[Category:Optical phenomena]]