Light from a distant galaxy travels toward us for billions of years, but along the way it can run into a massive galaxy cluster. Its gravity acts like curved glass: it bends the path of rays, so instead of one image, we see several — sometimes arcs, sometimes points. Usually such 'lenses' produce simple patterns, but in the cluster MS 0451.6-0305, the James Webb Space Telescope captured something trickier.
In two different places behind the cluster, light bent in a way that produced shapes resembling a forked swallowtail. Astronomers call this a 'swallowtail'—a rare type of gravitational lens where the source image is multiplied not into two or four, but into a larger number of copies, arranged in a characteristic pattern. In the first system, at a distance corresponding to a universe age of about 2.5 billion years, a small part of a galaxy with two dense clumps of stars appeared within such a pattern. These clumps are visible in four locations, and in two of them the light is magnified by more than 300 times. This allowed details smaller than one parsec to be resolved, as if we could see individual city blocks in a city on the other side of the Earth.
The second system is even farther: the light came from a galaxy we see as it was just a billion years after the Big Bang. There, the pattern is even more complex, so astronomers used only the most reliable image matches to build a lens model. It turned out that different clumps are magnified from 20 to 200 times, and their sizes range from fractions of a parsec to 18 parsecs. This is the first case where two 'swallowtails' have been found in a single cluster, and it shows that such rare lenses can act as natural microscopes, revealing tiny details in galaxies from the earliest times.
The cluster MS 0451.6-0305 acted as a giant magnifying glass that didn't just smear light into arcs but focused it into several bright points. And in those points, we saw what would otherwise remain indistinguishable: individual star clusters the size of a small part of a galaxy, but so distant that without the lens their light would have merged into a single faint blob.
A gravitational lens can magnify the light of a distant object hundreds of times, but it also distorts its shape, like looking at a lightbulb through uneven glass and seeing multiple reflections.
In the novel 'Light of Distant Stars,' astronomers use a gravitational lens to examine the surface of a planet in another galaxy — much like the 'swallowtails' in this article work.