Two rare stars whipping around each other in a wide, wild tango have given astronomers a unique opportunity to study the gentle slap of light on their dusty skirts.
The binary object called WR 140 is surrounded by a series of nested shells of dust that are slowly being pushed into space, not only by the binary’s stellar winds of charged particles, but by the glow of radiation emitted by the stars themselves.
For the first time, scientists have been able to directly observe this radiation pressure in action, using infrared observations from the Keck Observatory to track a giant plume as it expanded through space over a 16-year period.
This helps explain what we see in a recent image from the James Webb Space Telescope (JWST), the subject of a second paper, that shows the burning binary nestled among a profusion of glowing dust shells.
“It’s hard to see starlight causing acceleration because the force weakens with distance and other forces quickly take over,” says astronomer Yinuo Han of the University of Cambridge.
“To observe acceleration at the level that becomes measurable, the material must be close enough to the star, or the source of the radiation pressure must be very strong. WR 140 is a binary star whose fierce radiation field supercharges these effects. , placing it close to our high-precision data.”
WR 140 is located about 5,600 light-years away in the constellation Cygnus and is a rare among the rare. It is what is known as a colliding wind binary system, consisting of an extremely rare Wolf-Rayet star, and a blue O-type supergiant companion – another rare object.
As we’ve explained before, Wolf-Rayet stars are very hot, very luminous, and very old, burning out at the end of their main-sequence lifetimes. They are significantly depleted in hydrogen, rich in nitrogen or carbon, and lose mass at a very high rate. This lost mass is also high in carbon, which absorbs radiation from the stars and re-emits it as infrared light.
O-type stars, on the other hand, are among the most massive stars known, also very hot and luminous. Because they are so massive, their lifespans are incredibly short, blinking after a few million years.
Both stars in the WR 140 system have fast stellar winds, blasting through space at about 3,000 kilometers (1,864 miles) per second. Therefore, both lose mass at a fairly furious rate. This is actually quite normal. But the stars orbit each other in an elliptical or oval shape, meaning they don’t orbit uniformly. They come together for a close approach (periastro) and then separate again at a long distance (apastron).
In the periaster, their strong stellar winds collide, creating shocks and a huge puff of dust that expands outwards, creating a dust shell. The stars orbit each other once every 7.94 years, meaning that each new shell is formed 7.94 years after the last. This predictability means that objects like WR 140 are exciting objects to study dust production and acceleration.
But you may have noticed that the shape of the shells is odd, with one side stretched out, creating what has been described as a ‘squirrel’ shape. This is difficult to explain by stellar winds alone.
“In the absence of external forces, each dust spiral should expand at a constant speed,” says Han.
“We were confused at first because we couldn’t get our model to match the observations, until we finally realized we were seeing something new. The data did not match because the rate of expansion was not constant, but rather accelerating. We caught it for the first time on camera.”
But there is another explanation: radiation pressure. Electromagnetic radiation – light – exerts a tiny, microscopic pressure on whatever it hits, due to the transfer of momentum from the photon to the surface. Photons are so small and massless that this is not going to affect your daily life, but stars emit very strong radiation. Unfiltered and in the vacuum of space, it can actually push matter. This is the principle behind lightsail technology.
When the team included radiation pressure in their WR 140 models, they were able to reproduce the strange shape of the shells moving around the binary system.
“In a sense, we’ve always known that this must be the reason for the outflow, but I never dreamed that we could see the physics working like this,” says astrophysicist Peter Tuthill of the University of Sydney in Australia.
“When I look at the data now, I see the plume of WR140 unfurling like a giant sail made of dust. When it catches the photon wind rushing from the star, like a yacht catching a gust, it takes a sudden leap forward.”
The Universe is, indeed, full of wonders.
The team’s research was published in Natureand the second paper on JWST observations at Astronomy of Nature.