A serpentine figure captured by the Hubble Space Telescope, the “Cosmic Snake,” is a real galaxy whose light has traveled for billions of years to reach us. Its elongated and warped appearance is an astronomical illusion, a trick of light and gravity caused by its light passing through a massive galaxy cluster. This chance alignment provides astronomers with an opportunity to study an object that would otherwise be too faint and distant to observe in such detail.
The Gravitational Lens Effect
The phenomenon responsible for the Cosmic Snake’s distorted shape is known as gravitational lensing, a consequence of Albert Einstein’s theory of general relativity. The theory describes gravity not as a force, but as a curvature of spacetime caused by mass. In space, a massive object, such as a galaxy cluster, warps the spacetime around it. When light from a more distant object passes through this warped region, its path is bent.
From our perspective on Earth, this bending of light can magnify, distort, and even create multiple images of the background source. It is as if we are looking at the distant galaxy through the imperfect base of a giant cosmic wine glass, which splits and stretches the image into the shape we observe.
The Cosmic Snake is an example of “strong” gravitational lensing, where the alignment between Earth, the lensing object, and the background source is nearly perfect. This alignment causes light to travel along several paths around the lens, resulting in multiple, distorted images of the same galaxy. These separate images are arranged in such a way that they merge into the continuous arc that gives the Cosmic Snake its name.
The Galaxy Behind the Illusion
The Cosmic Snake itself is an image of a galaxy located far behind the massive galaxy cluster MACSJ1206.2-0847. This background galaxy is so distant that its light provides a snapshot of what galaxies were like in the early universe. The stretched image reveals distinct features that would otherwise be invisible.
Bright knots and clumps are visible along the length of the snake’s body, which astronomers have identified as large regions of star formation. These stellar nurseries are where vast clouds of gas are collapsing to form new stars at a high rate. What we see as the Cosmic Snake is composed of at least four separate, lensed images of the background galaxy, twisted and connected end-to-end, with a fainter fifth image appearing separately from the main arc.
What the Cosmic Snake Reveals
The precise way the Cosmic Snake’s light is bent provides a powerful tool for probing the cosmos.
Mapping Dark Matter
One primary application is mapping the distribution of dark matter within the foreground galaxy cluster, MACS J1206.2-0847. Dark matter is an invisible substance that does not emit or reflect light, but its gravitational influence can be measured. By analyzing the warping of the snake’s image, astronomers calculate how much mass must be present in the cluster to create the observed lensing effect. This calculation reveals that the visible matter in the cluster’s galaxies is not enough to account for the intense gravitational bending. The discrepancy points to the existence of a massive halo of dark matter surrounding the cluster, and the multiple images allow for a detailed map of its distribution.
A Closer Look at Star Formation
The gravitational lens also acts as a “natural telescope,” magnifying the distant galaxy beyond the capabilities of current technology. This cosmic magnification allows for a detailed view of star formation in the early universe. Early studies of distant galaxies suggested their star-forming clumps were much larger than those in our local galactic neighborhood. The different images of the Cosmic Snake are magnified by different amounts, providing a unique natural experiment. By comparing the less-magnified images with the highly-magnified ones, astronomers discovered that these giant clumps are not single entities but are composed of a complex substructure of many smaller star-forming regions, suggesting that star formation in the early universe may be more similar to processes seen today than previously thought.