Red giant star’s unexpected change in appearance puzzles astronomers

A whole set of carefully constructed models have been abandoned due to the 400 light-year distant CW Leonis’ odd behaviour


An artist’s impression of a red giant star like CW Leonis. Image Credit: ESO

New images of an intriguing red giant star, known as CW Leonis, have turned the usual astronomy narrative on its head, with scrutiny focused not only on the stars but also on the astronomers who study them. In just a couple of years, the 400 light-year distant CW Leo has changed its appearance completely, meaning a whole set of carefully constructed models have been abandoned.

Stars like our Sun become red giants near the end of their lives, after most of their available hydrogen fuel has been consumed. These huge objects – tens of times the size of our star – will eventually eject most of their atmosphere into space and create a planetary nebula, leaving behind a hot core that cools down over billions of years. CW Leo is nearing the end of its red giant stage and starting to throw out large amounts of matter.

“Although it is invisible to our eyes, to astronomers CW Leo is one of the most famous stars in the sky,” explains Stewart. “If we could see infrared light, it would be by far the brightest star in the sky. However the real excitement here is the extreme physics – it is a swollen luminous giant poised at the most self-destructive phase of its existence. It is literally tearing itself apart under its own glare, hurling dense clouds of dust and gas out into the galaxy – dying amidst its own glorious final fireworks display.”

The team of astronomers used images from the Keck and VLT telescopes, and the Cassini space probe, to study CW Leo over more than a decade. As might be expected from such a roiling cauldron of heat and dust, the star’s appearance evolves, but in the past this has been quite a stately affair. However the new images catch something more dramatic – in the last couple of years, it completely shed its familiar identity and adopted an entirely new visage. Such behaviour is a serious problem for the astronomers who have spent the decades studying this unique system.

A panel of images showing the turbulent clouds of stardust enveloping and concealing CW Leo. Image Credit: Paul Stewart

A panel of images showing the turbulent clouds of stardust enveloping and concealing CW Leo. Image Credit: Paul Stewart

“This is one of those humble moments when nature reminds us all who is boss,” reflects Peter Tuthill. “For the last 20 years many astronomers – and I count as one – have tried to put a skeleton underneath the clumpy images we see. I have seen complex models mathematically carving the nebula around the star up into cavities, plumes, and disks and halos. However, all along the star had decidedly other ideas.”

“The big problem with all the models is the scaffolding that they tried to impose onto the system,” explains Stewart. “When all the structures we thought we knew can completely melt away to be quickly replaced by new ones, then what are we left with?”

“It is pretty clear that the new images tell us is that CW Leo has just been ejecting clumps and plumes of hot dust at random all this time,” agrees Tuthill. “It is like a celestial version of the famous Rorschach Ink Blot Test in psychology. In trying to find underlying structure to the clumps and blobs, we have seen little more than our own preconceptions reflected back at us. Seeing rabbits or elephants in the clouds is okay for my 4-year old boy, but it seems that this time a dusty star in Leo has caught all the astronomers out daydreaming at their work.”

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