For the first time, astronomers saw dust in space being pushed by starlight

A pair of stars in our galaxy is revealing how light pushes around matter. It’s the first time anyone has directly seen how the pressure of light from stars changes the flow of dust in space.

Such radiation pressure influences how dust clears from the regions near young stars and guides the formation of gas clouds around dying stars (SN: 9/22/20). The dust pattern surrounding a stellar pair 5,600 light-years away in the Cygnus constellation is providing a rare laboratory to observe the effect in action, astronomer Yinuo Han and colleagues report in the Oct. 13 Nature. 

Astronomers have long known that the dust emerging from the star WR 140 and its companion is formed by gas from these two stars colliding and condensing into soot. But images of the pair taken over the course of 16 years show that the dust is accelerating as it travels away from the stars.

Dust initially departs the stars at about 6.5 million kilometers per hour, the researchers report. That’s fast enough to make a trip from Earth to the moon in a little over half a day. Over the course of a year, the dust speeds up to nearly 10 million km/h.

The revelation came from comparing the positions of concentric dust shells year to year and deducing a speed. The researchers’ calculations show that the force accelerating the dust is the pressure exerted by light radiated from the stars, says Han, of the University of Cambridge. “Radiation pressure [becomes apparent] only when we put all the images next to each other.”  

Not only are those layers of dust feeling light’s push, they also extend out farther than any telescope could see — until this year. Images from the James Webb Space Telescope, or JWST, depict more of the dusty layers around WR 140 and its companion than ever seen before, Han and another team report October 12 in Nature Astronomy.

At first glance, the intricate patterns surrounding the stars resemble a gigantic spiderweb. But the researchers’ analysis reveals that they are actually enormous, expanding, cone-shaped dust shells. They’re nested inside each other, with a new one forming every eight years as the stars complete another journey around their orbits. In the new images, the shells look like sections of rings because we observe them from the side, Han says.  

A computer simulation that takes radiation pressure from starlight into account shows how a dust plume (expanding arc and line) emerges from a pair of orbiting stars (not visible).Y. Han/Univ. of Cambridge

The patterns don’t completely surround the stars because the distance between the stars changes as they orbit one another. When the stars are far apart, the density of the colliding gas is too low to condense to dust — an effect the researchers expected. 

What surprised them is that the gas doesn’t condense well when the stars are closest together either. That suggests there’s a “Goldilocks zone” for dust formation: Dust forms only when the separation between the stars is just right, creating a series of concentric dust shells rippling away from the duo.

“Their Goldilocks zone is a new idea,” says astrophysicist Andy Pollock of the University of Sheffield in England, who was not part of either study. “A similar sort of thing happens in my field of X-rays.”

In his work, Pollock has observed that WR 140 and its partner emit more X-rays as the stars approach each other, but then fewer as they get very close together, suggesting there’s a Goldilocks zone for X-rays coming from the stars as well. “It would be interesting to see if there’s any connection” between the two types of Goldilocks zones, he says. “All of this must somehow fit together.”