James Webb Telescope captures starlight nudging dust from a dying star into space

The James Webb Space telescope has captured an image of intense light from a star pushing multiple dust plumes into space. 

The propulsive effect of the starlight is what's known as radiation pressure. Radiation pressure is one of the factors preventing stars from collapsing under their own gravity and creates the bright smudged tails of comets as they pass close to the sun. But the new image is the most complete picture of the phenomenon taking place around a star. 

The strange image, which was first released in July by citizen scientist Judy Schmidt, shows a pair of stars in WR140, located 5,600 light-years away in the Cygnus constellation. The binary star system is encircled by an onion-like shell of almost 20 concentric ripples. Upon its release, the image generated a lot of online speculation over what could be causing the effect, now another team of researchers working closely with the first has finally provided the answers in a paper published Oct. 12 in the journal Nature. 

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The ripples are great plumes of glowing dust and soot spewed out as a pair of leaky stars in WR140 swing closely past each other in an elliptical orbit that they complete roughly every eight years. 

As the two approach, their 1,864 miles per second (3,000 kilometer per second) solar winds smash into each other, arcing a plume of material across space that slowly expands to form rings. As the plumes are only ejected when the stars are near each other, the spacing of the rings is set by their orbital period. This means the dust is made in regular intervals, and the cloud's rings can be counted like tree rings to find the age of the outermost ripple — with 20 visible rings amounting to 160 years of dust.

But these ripples are not expanding outwards at a constant speed. Rather, they are accelerating, pushed on by periodic peltings of photons, or light particles, from the stars nearby. It is this acceleration that changes the spacing of the gaps between the rings.

"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," study co-author Peter Tuthill, an astrophysicist at the University of Sydney in Australia, said in a statement. "When I look at the data now, I see WR140's plume unfurling like a 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."

One of the stars in the duo is a Wolf-Rayet star, a type of rare, slowly-dying star that has lost its outer shell of hydrogen, leaving it to spew out gosts of ionized helium, carbon and nitrogen from its insides. These stars will explode as supernovas one day, but until then the radiation pressure produced by the light unfurls their burst contents, stretching them out like giant phantom jellyfish in the night sky. Ejected superheated elements, especially the carbon that is transformed into soot, stay hot enough to glow bright in the infrared spectrum. 

The other member of the pair is an O-type blue supergiant, one of the most massive classes of stars. Hot, bright, and enormous, the supergiant is also leaking gas and destined to supernova. When the two stars fly close to each other, their solar winds are combined into a giant cone of material which is shot out into space.

"Like clockwork, this star puffs out sculpted smoke rings every eight years, with all this wonderful physics written then inflated in the wind like a banner for us to read," Tuthill said. "Eight years later as the binary returns in its orbit, another 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 highly predictable timings of the puffs and their expansion over large distances gave the astronomers a unique opportunity to study the underlying physics of the ejections.

To detail the glowing rings of infrared soot, the astronomers first turned to one of the world's largest optical telescopes — the Keck Observatory in Hawaii and its 32 foot (10-meters) mirror. By training the telescope's infrared camera on the distant rings, the researchers tracked them as they were pushed outwards and slowly grew over the course of 16 years. Then, following up on their work, the scientists teamed up with a second group to snap another image with the James Webb space telescope that showed all twenty rings in crystal clear definition. 

After trying and failing to model what they had seen, the astronomers were initially confused. 

"In the absence of external forces, each dust spiral should expand at a constant speed," first author Yinuo Han, an astronomer at the Institute of Astronomy in Cambridge, England, said in the statement. "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."

The dust rings were accelerating due to periodic shoves from starlight, which, like all light, carries momentum. According to the researchers, astronomers have often indirectly seen the fingerprints of this effect in the inexplicably high speeds of some matter in the universe, but starlight's radiation pressure has never been directly measured observed acting on dust before now. This is because close to stars, where the radiation pressure is strongest, the shoves it produces are often masked by extremely powerful gravitational and magnetic fields.

The researchers say that with the James Webb Space telescope now in full operation, they will be able to take an even deeper look into WR140 and other weird systems where new physics may lurk.

"The Webb telescope offers new extremes of stability and sensitivity," Ryan Lau, an infrared astronomer at the National Science Foundation who led the James Webb section of the research, said in the statement. "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."