Astronomers have long marveled at the strange and beautiful phenomena that unfold across the cosmos, and the “Cheshire Cat” group of galaxies is no exception. Named for its resemblance to the iconic grinning feline from Alice in Wonderland, this galactic ensemble showcases the bizarre and wondrous effects of gravitational lensing — a striking consequence of Einstein’s general theory of relativity. In this system, the light from distant background galaxies is stretched and curved into arcs and rings by the immense gravity of a foreground galaxy cluster, most of whose mass is composed of invisible dark matter detectable only through its gravitational effects.
The grinning face is anchored by two brilliant “eyes” — the massive elliptical galaxies SDSS J103843.58+484917.7 and SDSS J103842.68+484920.2 — located at redshifts z = 0.426 and z = 0.433, respectively. Between them lies a smaller “nose” galaxy, and surrounding them are multiple arc-like features: distorted images of background galaxies, some at redshifts as high as z = 2.78 — representing objects seen as they were billions of years ago. These arcs are magnified by the gravitational lensing effect, allowing astronomers to peer further into the universe than otherwise possible. These features were captured in breathtaking optical detail by NASA’s Hubble Space Telescope. Complementary X-ray data from NASA’s Chandra X-ray Observatory reveal glowing, million-degree gas and an active supermassive black hole within one of the “eyes” — signs of a high-speed galactic collision underway.
This composite image shows a galaxy group nicknamed the ‘Cheshire Cat’ for its resemblance to a grinning feline face. Optical data from the Hubble Space Telescope reveal the ‘eyes’ and ‘smile,’ while X-ray observations from NASA’s Chandra X-ray Observatory (shown in purple) highlight hot gas heated by merging galaxies.
What makes the Cheshire Cat group especially compelling is that it may represent a fossil group progenitor — a rare, transitional phase in galaxy evolution where smaller galaxy groups merge into one massive elliptical galaxy surrounded by fainter companions (Irwin et al., 2015). While fossil groups are believed to be a common outcome of galaxy mergers, catching one in the act is unusual. The Cheshire Cat offers a unique opportunity to study this process in real time, shedding light on the dynamic forces that shape galaxy clusters over cosmic time.
The precise alignment of galaxies in this system also produces a rare and beautiful phenomenon: an Einstein Ring — a nearly perfect circle of light formed when the background galaxy, lensing mass, and observer are aligned just right. These naturally occurring lenses serve as powerful tools in astronomy, enabling the study of galaxies otherwise too faint or distant to observe. In the case of the Cheshire Cat, Einstein’s century-old theory continues to help us see further, deeper, and with more wonder than ever before.
A gravitational lens occurs when the gravity of a massive object bends the path of light from a more distant source located behind it. This bending results from the warping of spacetime, as predicted by Einstein’s General Relativity [Einstein_1936]. In 1937, Fritz Zwicky proposed that entire galaxies or galaxy clusters could serve as gravitational lenses, deflecting the light of background objects [Zwicky_1937]. He suggested that this phenomenon could help detect invisible matter and measure galactic mass—an idea that laid the foundation for a powerful tool in astrophysics. Today, gravitational lensing plays a key role in studying dark matter, galaxy evolution, and the large-scale structure of the universe.
Strong lensing events can produce dramatic features such as Einstein rings and Einstein crosses. These configurations offer natural magnification of faint, distant sources like early galaxies or supernovae [Congdon_2018]. The photo below is a lens called the Cosmic Seahorse and in it you can see several arcs created by a galaxy cluster bending light from background objects.
The Cosmic Seahorse image, captured by NIRCam on the James Webb Space Telescope (JWST), showcases various lensing arcs caused by the galaxy cluster SDSS J1226+2152 in the constellation Coma Berenices. This cluster is located approximately 1.9 billion parsecs from Earth. (Credit: ESA/Webb, NASA and CSA, J. Rigby)
The first confirmed gravitational lens was discovered in 1979 with the observation of the quasar QSO 0957+561 A/B—commonly called the Twin Quasar [1980_Young]. This system was the first large-scale example of gravitational lensing by a galaxy. Much earlier, in 1919, Arthur Eddington provided the first observational evidence for Einstein’s theory by measuring the deflection of starlight during a solar eclipse.
The Twin Quasar appears as two nearly identical images of the same quasar, created by the lensing effect of the intervening galaxy YGKOW-G1. The photo below shows this famous system.
The Twin Quasar QSO 0957+561 is the two bright objects seen in the center of this image. (Credit: ESA/Hubble and NASA)
How a Gravitational Lens Works
Strong gravitational lenses are rare, requiring near-perfect alignment among the source, lens, and observer. This alignment is depicted below, where light from the red point source bends around an elliptical galaxy and focuses toward the observer.
Simply put, what is happening is the large galaxy’s mass is warping spacetime around it, as predicted by Einstein’s theory of General Relativity. The light from the distant star travels along what it perceives as a straight line, but in fact, the space around the galaxy is distorted such that the light curves around it and comes out the other side in a way that it can be focused towards us, the observers on Earth. In doing so, more light is concentrated at the focal point, hence magnifying the faint and distant star. This is a very simplified explanation but it should be enough to explain the gist of what is happening. For a more thorough treatment, with the math and all, refer to the book, “Principles of Gravitational Lensing: Light Deflection as a Probe of Astrophysics and Cosmology” by Congdon and Keeton.
Schematic of gravitational lensing of distant star-forming galaxies. This diagram illustrates how a strong gravitational lens operates: light from a distant source object is deflected by the foreground lensing object, resulting in brighter, distorted, and magnified images. (Credit: ALMA (ESO/NRAO/NAOJ), L. Calcada (ESO), Y. Hezaveh, et al.)
The wine glass stem experiment shown here is a good demonstration of how gravitational lensing appears. In this experiment, you can look down the stem and observe a light across the room bending around the base of the stem. Although this demonstration relies on the glass acting as a lens, it is similar in appearance and function to gravitational lenses. However, with gravitational lenses, the effect is a result of space around the galaxy being distorted. It is actual spacetime warping around the massive galaxy that creates the lensing effect, providing an amazing confirmation of Einstein’s theory.
A wine glass simulation of the gravitational lens effect. The source is a light at the end of the room and the stem/base of the glass acts as the lens. As the glass is moved around, you observe the arc and ring of light bending around the glass lens. The effect is similar to that of a gravitational lens in space.
The light patterns observed in the glass, such as arcs and rings, are analogous to the Einstein rings seen in actual gravitational lensing. As the glass is moved off the central axis, you can observe points of light referred to as an Einstein cross. These typically appear as pairs, triplets, or quadruplets of concentrated light. When the light, glass, and observer are aligned along the central axis, you see a full or nearly full circle of concentrated light as it is focused from all around the lens and comes to a focus at your position.
The concept that gravity can bend light was once revolutionary—Einstein’s general relativity predicted it, but the world needed proof. That proof came during a solar eclipse in 1919, and it opened the door to one of the most elegant phenomena in astrophysics: gravitational lensing. Today, these cosmic distortions don’t just prove Einstein right—they reveal some of the most mesmerizing structures in the universe.
1. Eddington’s 1919 Eclipse Expedition
Eddington’s eclipse plates showing star shifts near the Sun (between the horizontal dashes).
British astronomers Frank W. Dyson, Arthur S. Eddington, and Mr. C. Davidson organized an expedition in 1919 to observe a total solar eclipse and test Einstein’s prediction that gravity bends light. During the eclipse, they measured the apparent positions of stars near the Sun and found that the starlight was indeed deflected—just as general relativity predicted. While no arcs or rings were visible, this experiment, now known as the Eddington experiment, laid the foundation for gravitational lensing as a real and measurable phenomenon.
The Twin Quasar QSO 0957+561 is the two bright objects seen in the center of this image. (Credit: ESA/Hubble and NASA
The Twin Quasar, designated Q0957+561, was the first confirmed case of gravitational lensing. Astronomers observed two nearly identical quasar images and later confirmed they were actually the same object, whose light was split by a massive galaxy positioned between it and Earth. This marked the first observation of gravitational lensing on a cosmic scale. The quasar lies at a redshift of z = 1.41 (about 8.7 billion light-years away), while the lensing galaxy sits at z = 0.355 (around 3.7 billion light-years).
Close up showing four quasar images arrayed around a foreground galaxy PGC-69457. The photo was taken with the European Space Agency’s Faint Object Camera on board NASA’s Hubble Space Telescope. (source: NASA, ESA, and STScI).
The Einstein Cross is a stunning configuration of four lensed images of a single quasar, perfectly arrayed around the core of a foreground galaxy. From Earth, it resembles a cosmic cross—beautiful, symmetrical, and precise. Its striking appearance and the clarity of the multiple images helped bring gravitational lensing into the public eye and made it a textbook example of strong lensing.
The quasar, located about 8 billion light-years from Earth (redshift z ≈ 1.7), sits directly behind a galaxy roughly 400 million light-years away. This foreground galaxy, known as Huchra’s Lens, bends the light of the quasar into four distinct images. The peculiar cross shape arises because the lensing galaxy is slightly oblong and the alignment is not perfectly centered.
Found in the constellation Pegasus, the Einstein Cross (Q2237+0305) is one of the few gravitational lenses that amateur astronomers can potentially observe—though it requires a large telescope, exceptionally dark skies, and excellent viewing conditions.
Photo of Einstein’s Cross (Q2237+030) taken with my Slooh.com account using the Chile One Wide-Field Telescope, part of Slooh’s southern hemisphere observatory.
A massive blue arc nearly wraps a red galaxy core. The Cosmic Horseshoe taken using the wide field camera on the Hubble Space Telescope (ESA/Hubble & NASA).
The Cosmic Horseshoe (SDSS J1148+1930) is a striking gravitational lens system located in the constellation Leo, formed by the near-perfect alignment of two galaxies. A massive foreground galaxy lies directly along the line of sight to a much more distant background galaxy. The intense gravity of the foreground galaxy bends and magnifies the light from the background galaxy into a vivid, horseshoe-shaped arc.
While a perfect alignment would produce a full Einstein ring, this near-perfect setup results in a partial arc—making the image both rare and visually stunning. These kinds of alignments are exceedingly uncommon, which makes the Cosmic Horseshoe especially notable among gravitational lensing discoveries.
The system was discovered in 2007 by an international team of astronomers using data from the Sloan Digital Sky Survey (SDSS) and you can see the image on SkyserverSDSS.
ALMA’s Long Baseline Campaign produced one of the most spectacular and detailed images of a gravitational lens ever captured. This system, known as SDP.81, features a distant galaxy more than 12 billion light-years away, whose light is bent into a nearly perfect Einstein ring by a massive foreground galaxy.
What makes this observation extraordinary is not just the ring’s symmetry—it’s the clarity of internal structure revealed within the distant galaxy. Thanks to gravitational lensing’s natural magnification and ALMA’s unprecedented resolution at submillimeter wavelengths, astronomers were able to resolve clumps of star-forming regions—akin to giant versions of the Orion Nebula—in a galaxy from the early universe. This had never been done with such detail at such a distance.
The ALMA Einstein Ring, a perfect ring formed not in visible light, but submillimeter wavelengths. Credit:ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University)
The image is presented in three panels:
Left: A Hubble Space Telescope view of the system in visible light, where the foreground galaxy is clearly visible but the ring is faint.
Center: ALMA’s sharp submillimeter image of the ring, where the foreground lens is invisible to ALMA due to its lack of visible-light sensitivity.
Right: A reconstructed image of the background galaxy, produced using models of the lensing effect, revealing giant cold molecular clouds—the likely birthplaces of stars and planets.
This observation not only provided a beautiful image, but also valuable data to help map dark matter distributions and study star formation in the early universe.
