Scientists have finally solved the mystery surrounding a glowing blue ring of light that was unlike anything astronomers had seen before.
Astronomers have spent years attempting to understand why the mysterious object in space had a circle of blue light around it, analysing images taken with telescopes both on the ground and in space.
Now scientists believe they have worked out how the blue light formed, bringing an end to the 16-year-old mystery and detailing the history of a spectacular and dramatic object in distant space.
The astronomers argue that the blue ring is not actually a ring at all, but a cone. The cloud of fluorescing debris probably formed after a sun-like star swallowed a smaller companion, and because one of the cones is facing directly at Earth, it looks from here as if it is a ring.
(The ring is not strictly blue, either, but the colour is instead a way of representing the otherwise invisible light that surrounds the object.)
The observation is the first time that astronomers have seen a rare phase of the evolution of stars that occurs just a few thousand years after they began, and lasts only perhaps thousands of years, a short period at the scale of stars.
The two stars started life floating in space, but as the sun-like star expanded and came closer to the other star, the smaller of the two began siphoning off material from its larger sibling.
Eventually, as the smaller star was consumed, the collision launched a cloud of debris that was bisected by a disk of gas from the smaller star - hence creating the two cone-shaped debris clouds.
Hydrogen molecules in the debris were then excited by the shock wave, causing them to glow with ultraviolet light, giving the cloud its titular hue.
"The merging of two stars is fairly common, but they quickly become obscured by lots of dust as the ejecta from them expands and cools in space, which means we can't see what has actually happened," says lead study author Keri Hoadley, the David and Ellen Lee Postdoctoral Scholar in Physics at Caltech.
"We think this object represents a late stage of these transient events, when the dust finally clears and we have a good view," Hoadley says. "But we also caught the process before it was too far along; after time, the nebula will dissolve into the interstellar medium, and we would not be able to tell anything happened at all."
The mystery had baffled scientists for years since it was spotted. "Every time we thought we had this thing figured out, something would tell us 'No, that's not right,'" Mark Seibert, an astrophysicist with the Carnegie Institution for Science and a member of the GALEX team, said.
"That's a scary thing as a scientist. But I also love how unique this object is, and the effort that so many people put in to figure it out."
While attempting to understand what caused the star, scientists suggested numerous ideas. Using Caltech's Hale Telescope at Palomar Observatory, near San Diego, and the W. M. Keck Observatory in Hawaii, they tried to find evidence of a shock wave around the star that would suggest a cloud of gas had been sent into space.
Later, it was hypothesied that the star could be destroying a nearby, unseen planet, but data from the Habitable Zone Planet Finder that was released in 2017 showed that there was no such object orbiting the star.
Although it took a surprisingly lengthy amount of time to discover the cause of the blue rings, the use of new technology means that older programs are still revealing new information.
"Whenever you survey the sky at new wavelengths, you inevitably get new discoveries years later and beyond," Christopher Martin, professor of physics at Caltech and the former principal investigator of GALEX, said.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies