Baffling ‘spiderweb’ star is a nesting binary – not an alien megastructure

A bizarre image of the distant star known as WR140 surrounded by concentric geometric ripples, captured by the James Webb Space Telescope (JWST) in July, has baffled astronomers worldwide – even triggering frenzied internet speculation that it might be evidence of an alien megastructure light-years across.

The puzzling image was captured shortly after JWST started science operations and released its first full batch of images. It quickly provoked spirited discussion online, with the wild corners of the internet theorising the giant ripples might have alien origins. Mark McCaughrean, a senior adviser for science and exploration at the European Space Agency and a member of the James Webb Space Telescope Science Working Group, described the image as “bonkers”.

But in two companion papers, both published today in Nature and Nature Astronomy, two Australian astronomers explain that the 17 concentric rings seen girdling the star are actually a series of mammoth dust shells created by the cyclic interaction between a pair of hot stars, one of them a dying Wolf-Rayet, locked together in a tight orbit.

“Like clockwork, WR140 puffs out a sculpted smoke ring every eight years, which is then inflated in the stellar wind like a balloon,” said Professor Peter Tuthill from the Sydney Institute for Astronomy at the University of Sydney, a co-author in both papers. “Eight years later, as the binary returns in its orbit, another ring appears, the same as the one before, streaming out into space inside the bubble of the previous one, like a set of giant nested Russian dolls.”

The WR140 binary is comprised of a huge Wolf-Rayet star and an even bigger blue supergiant star, gravitationally bound in an eight-year orbit. And while all stars generate stellar winds, those from Wolf-Rayet stars can be more likened to a stellar hurricane. Elements such as carbon in the wind condense out as soot, which remains hot enough to glow bright in the infrared. Like smoke captured by wind, the dust clouds give telescopes something to observe, following the flow.

Because the two stars are in elliptical rather than circular orbits, dust production turns on and off as WR140’s binary companion nears and then departs the point of closest approach. Based on data collected with other telescopes since 2006, Professor Tuthill and his former student Yinuo Han – now at the University of Cambridge’s Institute of Astronomy – created a three-dimensional model of the dust plume’s geometry.

That model, created for the Nature paper of which Han is the lead author, turned out to perfectly explain the bizarre results obtained by the JWST in July. Thanks to this and other contributions, both Han and Professor Tuthill also became co-authors of the Nature Astronomy paper with the new Webb data.

What’s more, in their Nature paper, Han and Professor Tuthill showed – for the first time – direct evidence of intense starlight driving into matter and accelerating it, after tracking titanic plumes of dust generated by the violent interactions between two colossal stars over 16 years. 

It’s known that starlight carries momentum, exerting a push on matter known as ‘radiation pressure’. Astronomers often see the aftermath of this in the form of matter coasting at high speed around the cosmos, but have never caught the process in the act. Direct observation of acceleration due to forces other than gravity is rarely witnessed, and never in a stellar environment like this. 

“It's hard to see starlight causing acceleration because the force fades with distance, and other forces quickly take over,” said Han. “To witness acceleration at the level that it becomes measurable, the material needs to be reasonably close to the star or the source of the radiation pressure needs to be extra strong. WR140 is a binary star whose ferocious radiation field supercharges these effects, placing them within reach of our high-precision data.”

Using imaging technology known as interferometry, which was able to act like a zoom lens for the 10-metre mirror of the Keck telescope in Hawaii, the Australians were able to recover sufficiently sharp images of WR140 for the study. 

They discovered that the dust does not stream out from the star with the wind forming a hazy ball, as had been thought. Instead, the dust condenses adjacent to where the winds from the two stars collide, on the surface of a cone-shaped shock front between them. Because the orbiting binary star is in constant motion, the shock front also rotates. The sooty plume gets wrapped into a spiral, in the same way that droplets form a spiral in a garden sprinkler.

“In the absence of external forces, each dust spiral should expand at a constant speed,” said Han. “We were puzzled at first because we could not get our model to fit the observations until we finally realised that we were seeing something new. The data did not fit because the expansion speed wasn’t constant, but rather that it was accelerating. We’d caught that for the first time on camera.”

Once they added the acceleration of dust by starlight into their three-dimensional model of the WR140 binary, it explained their observational data perfectly. And also ended up explaining the strange concentric rings later spotted with JWST. 

“In one sense, we always knew this must be the reason for the outflow, but I never dreamed we’d be able to see the physics at work like this,” said Professor Tuthill. “When I look at the data now, I see WR140’s plume unfurling a like giant sail made of dust. When it catches the photon wind streaming from the star, like a yacht catching a gust, it makes a sudden leap forward.”

With JWST now in operation, researchers will be able to learn much more about WR140 and similar systems. “The Webb telescope offers new extremes of stability and sensitivity,” said Dr Ryan Lau, Assistant Astronomer at the U.S. National Optical-Infrared Astronomy Research Laboratory and lead author of the JWST study published in Nature Astronomy. “We’ll now be able to make observations like this much more easily than from the ground, opening a new window into the world of Wolf-Rayet physics.”