Nature's Boost: Gravitational Lenses

The gravity of massive objects warps space and time, creating a lens that provides astronomers with a powerful tool to extend our view into previously unseen regions of the universe.

Black background is dotted with distant galaxies. Image center holds a cluster of galaxies, many of the appear to be large elliptical galaxies. Curved orange-yellow lines appear to cut through the galaxy cluster. These are more distant galaxies gravitationally lensed by the cluster.

In the early twentieth century, three years before publishing his General Theory of Relativity in 1915, Albert Einstein recorded his thoughts on how light might behave as it moved through a strong gravitational field. He reasoned that gravity would act as a lens, deflecting light from more distant objects.

Einstein’s work hinted at a cosmic quirk of nature that boosts our view of distant astronomical objects. This phenomenon, called a gravitational lens, is a region of space that holds an enormous object whose gravitational field bends space and time. Any light passing through this field is distorted and amplified in a way that is similar to how light passes through an optical lens. Along with distorting and magnifying, the gravitational lens can produce multiple images of the same object. As the distant object’s light passes through the gravitational lens, it may take different paths. When that light emerges from the lens, we see several contorted images of a single object.

Lower left corner: Hubble sits looking toward the upper-right corner where there is a spiral galaxy. Between the two is an image of a large galaxy cluster. Lines drawn from the spiral at upper-right to Hubble illustrate the gravitational lens created by the galaxy cluster.
This sketch shows paths of light from a distant galaxy that is being gravitationally lensed by a foreground cluster.
Hubble's Senior Project Scientist, Dr. Jennifer Wiseman, explains gravitational lensing. Credit: NASA GSFC, Producer: James Leigh

It wasn’t until 1919 that scientists tested and verified Einstein’s General Theory of Relativity. British astronomer, Sir Arthur Eddington, designed an experiment to measure if and by how much the Sun’s gravity would deflect the path of starlight passing near the Sun. He accomplished this by photographing stars near the limb of the Sun during a total solar eclipse. It would be another 70 years before radio astronomers announced the discovery of a double quasar, designated Q0957+561, that provided observational evidence for Einstein’s theory.

Today, astronomers regularly use gravitational lenses created by large galaxies and galaxy clusters as a way of extending our view, and Hubble’s sensitivity and high resolution allow it to see the faint lensed images of distant objects that ground-based telescopes are unable to detect because their images are blurred by Earth's atmosphere.

graphic showing a quasar's light, warped to appear like four quasars to Hubble because of a massive galaxy in between
This graphic illustrates how a distant quasar's light is altered by a massive foreground galaxy. The galaxy's powerful gravity warps and magnifies the quasar's light, producing four distorted images of the quasar.
NASA, ESA, and D. Player (STScI)

Hubble's View of Gravitational Lenses

Five months after its April 1990 launch, Hubble captured four images of a quasar in the gravitational lens, G2237+0305. Called an Einstein Cross, the image reveals a distant quasar gravitationally lensed four times by a relatively nearby galaxy.

By 1999, Hubble observations had significantly increased the number of known optical gravitational lenses, and when astronauts installed Hubble's Advanced Camera for Surveys during Servicing Mission 3B in 2002, it returned spectacular images of galaxy clusters gravitationally lensing some of the most distant objects we have ever seen. The Advanced Camera for Surveys is five times more sensitive and provides pictures that are twice as sharp as the previous work-horse Hubble cameras. It offers us exquisite views that continue to demonstrate Albert Einstein's prediction that gravity warps space and time, magnifying and distorting the light of more distant objects.

Five bright points of blue-white light. The center point is the galaxy. The four other points are above, below, and left, and right of the center.
Hubble’s Faint Object Camera provided astronomers with a detailed view of this multi-imaged quasar, a classic example of an Einstein Cross. The quasar is about 8 billion light-years away. The central galaxy creating the lens is 20 times closer to Earth, some 400 million light-years away.


Galaxies and quasars aren't the only gravitationally lensed objects that Hubble has captured. The observatory has seen several gravitationally lensed supernovae, and astronomers were able to directly calculate the mass of a white dwarf star using Hubble's observations of a gravitational lens.

Hubble's best known gravitationally lensed stars include the two most distant stars ever seen. Hubble captured the first one in 2018. The enormous blue star, nicknamed Icarus, is so far away that its light took 9 billion years to reach Earth. Because the star’s light had such a long distance to travel, Hubble's observations captured the star it as it appeared when the universe was about 30 percent of its current age.

Just four years after the discovery of Icarus, Hubble broke its own record by finding a more distant star called Earendel. Earendel is so far away that its light took 12.9 billion years to reach Earth travelling at 186 thousand miles per second. Hubble’s image of Earendel captured the star as it appeared when the universe was only seven percent of its current age. The observation established an extraordinary new benchmark: detecting the light of a star that existed within the first billion years after the universe’s birth in the Big Bang (at a redshift of 6.2) – making it the most distant individual star ever seen. The star is positioned along a ripple in spacetime that gives it extreme magnification, allowing it to emerge into view from its host galaxy, which appears as a red smear across the sky. The discovery of both Icarus and Earendel were made possible by powerful gravitational lenses that boosted Hubble’s view.

background galaxies, a faint, red arc holds 3 bright dots, the center one is Earendel
This detailed view highlights the star Earendel's position along a ripple in space-time (dotted line) that magnified it, making it possible for Hubble to detect the star.
NASA, ESA, Brian Welch (JHU), Dan Coe (STScI); Image Processing: Alyssa Pagan (STScI)
Dr. Brian Welch explains the discovery of Earendel while outlining how important Hubble is to exploring the mysteries of the universe. Credit: NASA GSFC, Producer: James Leigh

In 2023, Hubble and NASA's James Webb Space Telescope combined their observations to study an enormous galaxy cluster some 4.3 billion light-years from Earth. This galaxy group, called MACS0416, is actually a pair of colliding galaxy clusters that will eventually merge into an extremely massive cluster.

Across the field of view, astronomers identified 14 objects whose light varies over time. Twelve of those are located in three galaxies that are highly magnified by gravitational lensing, and are likely individual stars or multiple-star systems that are briefly, very-highly magnified. The remaining two are within more moderately-magnified background galaxies and are likely supernovae.

A field of galaxies on the black background of space. In the middle, stretching from left to right, is a collection of dozens of yellowish spiral and elliptical galaxies that form a foreground galaxy cluster. Among them are distorted linear features, which mostly appear to follow invisible concentric circles curving around the center of the image.
This view of galaxy cluster MACS0416 combines infrared observations from Webb with visible-light data from Hubble. This gravitational lens allowed astronomers to identify magnified supernovae and even very highly magnified individual stars. The bluest galaxies are relatively nearby and often show intense star formation, as best detected by Hubble, while the redder galaxies tend to be more distant, or else contain copious amount of dust, as detected by Webb. The image reveals details that are only possible to capture by combining the power of both Hubble and Webb.
NASA, ESA, CSA, STScI, Jose M. Diego (IFCA), Jordan C. J. D'Silva (UWA), Anton M. Koekemoer (STScI), Jake Summers (ASU), Rogier Windhorst (ASU), Haojing Yan (University of Missouri)

Einstein Rings

Einstein's theory also predicts a special case of gravitational lensing called an Einstein Ring. When a distant light source is perfectly aligned with a powerful gravitational lens and the observer, the light source can curve symmetrically around the gravitational lens, causing a ring-like structure called an Einstein Ring.

Hubble's observations helped to more than quadruple the number of known, visible Einstein Rings.

Black background dotted with galaxies. A reddish galaxy at image center with a bluish-white ring curving almost completely around it.
The blue horseshoe is a distant galaxy magnified and distorted by the strong gravitational pull of the massive foreground Luminous Red Galaxy, which has roughly ten times the mass of the Milky Way. The blue galaxy’s redshift is approximately 2.4, meaning we see it as it was about 3 billion years after the Big Bang. This is a visible and infrared light image taken with Hubble's Wide Field Camera 3.
Two bright orange galaxies seem to form eyes, and streaks of curved, bent and magnified light form the circle of a face and a happy smile. A smaller bright galaxy serves as the nose in this smiley face. Other galaxies dot the picture.
The Einstein Ring around galaxy cluster SDSS J1038+4849 makes it appear to be smiling.
NASA, ESA, Michael Gladders (University of Chicago); Acknowledgement: Judy Schmidt
A black background dotted with galaxies. Image center holds a cluster of galaxies with a distinct arc nearly surrounding it.
GAL-CLUS-022058s is nicknamed the "Molten Ring."
NASA, ESA, Anastasio Díaz-Sánchez (Universidad Politécnica de Cartagena), Saurabh Jha (Rutgers, The State University of New Jersey)


Hubble teamed up with the W. M. Keck Observatory in Hawaii to discover a Uranus-sized planet around a distant star. They used a technique, called microlensing, that can find more distant and colder planets in long-period orbits that other methods cannot detect. Microlensing uses the gravitational pull of a foreground star to amplify the light of a background star that momentarily aligned with it. If the foreground star has planets, then the planets may also amplify the light of the background star, but for a much shorter period of time than their host star. The exact timing and amount of light amplification can reveal clues to the nature of the foreground star and its accompanying planets.

The Hubble and Keck Observatory microlensing observations revealed that the system, cataloged as OGLE-2005-BLG-169, holds a Uranus-sized planet orbiting about 370 million miles from its parent star, slightly less than the distance between Jupiter and the Sun. The host star, however, is about 70 percent as massive as our Sun.

One way Hubble finds planets thousands of light-years from Earth is by using a gravitational microlensing technique. Credit: NASA, ESA, D. Bennett (GSFC), Wiggle Puppy Productions

Dark Matter

Astronomers also use gravitational lenses to study the non-luminous stuff of the universe we call dark matter. Visible matter reveals itself by shining brightly, but dark matter is only detected by its influence on the light we see. Large galaxy clusters, and the powerful gravitational lenses they create, offer astronomers a way of seeing dark matter's signature. By studying the magnified and distorted images lensed by enormous clusters of galaxies, astronomers can piece together the gravitational signature of dark matter.

To learn more about Hubble's dark matter discoveries, see Shining a Light on Dark Matter.

Gravitational lenses boost our vision, allowing us to see distant objects we would otherwise have missed. Hubble's pioneering work in observing these natural zoom lenses helped prove Einstein's theories while providing astronomers with the tools to probe these massive galaxies and model their lensing effects, allowing us to peer further into the early universe than ever before. Hubble also set the stage for NASA's James Webb Space Telescope and Nancy Grace Roman Space Telescope to see deeper, bringing us even closer to the beginning of the universe.

A cluster of galaxies fills the frame. A purple glow around the largest concentrations of galaxies indicates the distribution of dark matter.
Abell 1689 is an immense cluster of galaxies located 2.2 billion light-years away. Astronomers map the cluster's dark matter by plotting the arcs produced by light from background galaxies that are warped by the foreground cluster's gravitational field.
NASA, ESA, E. Jullo (JPL), P. Natarajan (Yale University), J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), T. Broadhurst (Tel Aviv University)
Galaxies dot a black background with only one bright foreground star just above and to the right of image center. Swaths of purple and purple through the center of the galaxy cluster.
This composite image shows the galaxy cluster 1E 0657-556, the "bullet cluster." Two pink clumps are hot gas detected by Chandra in X-rays. They contain most of the "normal" matter in the two clusters. The bullet-shaped clump on the right is hot gas from one cluster, which passed through the hot gas from the other, larger cluster during the collision. An optical image combining Hubble and the ground-based data from the twin Magellan telescopes shows the galaxies in orange and white. The blue areas in this image depict where astronomers find most of the mass in the clusters, as determined by analyzing the effect of gravitational lensing. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in these clusters is dark.
X-ray: NASA/CXC/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.