When NASA's Hubble Space Telescope snapped a picture of an elliptical galaxy, it caught something else in the frame: A bright, red arc of light wrapped around it.
That glowing curve isn't the sign of a broken telescope, but in fact another galaxy — about 19.5 billion light-years from Earth in space. It's much farther than the elliptical galaxy, seen as the central dot in the image at the top of this story, roughly 2.7 billion light-years away.
The strange, bent shape of the extremely remote galaxy, called HerS 020941.1+001557, is caused by a quirk of nature called "gravitational lensing," something predicted in Albert Einstein's General Theory of Relativity over a century ago. But the gravitational lensing here displays a special type of phenomenon, known as an Einstein ring.
Gravitational lensing happens when a celestial object has such a massive gravitational pull that it warps the time and space around it. NASA often uses the analogy of a bowling ball placed on a foam mattress or trampoline to illustrate how the fabric of spacetime bends. Light that would otherwise travel straight curves and gets distorted as it passes through the warped spacetime.
In this scene, the nearer galaxy SDSS J020941.27+001558.4, is acting like a colossal magnifying glass in the sky. This allows scientists to see the even more distant galaxy, making it appear brighter.
But gravitational lenses are known to play tricks on the eyes: They have the power to even replicate objects, the way a funhouse mirror can create multiple irregular images.
Astronomers are now adept at spotting the telltale effects of gravitational lensing, but that wasn't always the case. Four decades ago, the concentric arcs of light and stretched celestial objects could be downright confusing.
In 1987, an enormous blue arc, thought to be hundreds of trillions of miles long, was first considered one of the largest objects ever detected in space. The arc was discovered near the galaxy cluster Abell 370, with another similar object near galaxy cluster 2242-02.
Later that year, scientists at Stanford University and the National Optical Astronomy Observatory in Arizona sorted out that they were, in fact, optical illusions, distorted by Abell 370. The New York Times published a story about the "bizarre" implication of Einstein's theory, titled "Vast Cosmic Object Downgraded to a Mirage."
Einstein rings happen when a lensed object lines up with its magnifying object. Depending on how precisely they align, an Einstein ring can either look like a full or partial circle around the object in the foreground.
This particular Einstein ring was discovered by a citizen scientist, someone who volunteered time to help review images, through SPACE WARPS. The project relies on crowdsourcing to search for gravitational lenses in space images.
Gravitational lenses have become a handy tool for studying galaxies otherwise too faint or distant to detect, extending the range of telescopes. Because light from this distant source takes time to reach Hubble, astronomers are seeing HerS 020941.1+001557 as it was about 11 billion years ago, long before the universe gave birth to the sun and Earth.
And — surprise — there's actually a third galaxy photo-bombing this picture. It's called SDSS J020941.23+001600.7, and it appears to be intersecting part of that red curve.
