A-level physics students among us may well be aware of the photoelectric effect and the wave-particle duality of light, but fewer of us may be aware that light can exert a physical force on objects. Whilst too small to be noticeable day to day, this ‘radiation pressure’ can have a substantial impact. The cumulative effect of small forces from the radiation pressure can build up to mould the formation of huge gas and dust clouds. A rare interaction between two stars in our galaxy is allowing astronomers to investigate just how much of an effect light can have.
Two colliding stars, WR 140 and its companion in the cygnus constellation (around 5,600 light years away), are providing insights into the effect radiation pressure can have on the movement and formation of dust and gas clouds. The two colliding stars create dust as the gases released condense into soot, and this is propelled away from the stars at a reasonably swift 6.5 million kmh. By observing these dust clouds over 16 years, astronomer Yinuo Han reports in the scientific journal Nature that it is possible to observe the dust accelerating to over 10 million kmh. At that speed, the dust would be able to make the trip from the sun to Neptune and back within the time that Liz Truss spent as PM.
Radiation pressure comes from the transfer of momentum between an object and light. Whether you consider light as an electromagnetic wave or as individual photons, it always carries momentum, despite having zero mass. Whenever light then interacts with an object there must be a conservation of momentum, and as such light exerts a force of that object. You won’t be able to feel the light interacting with you right now, but the effect on small dust particles in deep space is many orders of magnitude larger. As such, we are able to view the dust clouds created by this pair of stars and measure just how large the impact is.
Until recently, it wasn’t possible to view the entirety of these dust clouds without the help of another innovation. The James Webb Space Telescope, which recently made headlines with its stunning new photos of deep space objects, allowed the University of Cambridge team to see more of the dust layers around the two companions. Soot is released in eight-year cycles as the two stars orbit each other, and this creates unique cone-shaped releases that allow for comparison over time. The position of the stars means we observe them from the ‘inside’ of the cone, such that the cones appear as concentric rings. These rings allow us to deduce the acceleration of the soot.
By observing the soot clouds, the researchers could see that the rate of dust generation varies throughout the orbit. Logically, when the pair were furthest away, the amount of the gas coming into contact with each other becomes too low to precipitate significant amounts of dust. Astronomers observed this as expected, but they also saw a second period of low dust production when the stars were at their closest. This surprised the team as the density of the colliding gas should be largest at this point.
The University of Cambridge team suggest that this shows there is a ‘goldilocks zone’ for dust condensation between stars. Andy Pollock, an astrophysicist at The University of Sheffield has conducted similar work on this pair of stars but looking at X-ray radiation emitted rather than dust. “Their Goldilocks zone is a new idea… A similar sort of thing happens in my field of X-rays”.
With his work at Sheffield, Pollock has observed a similar goldilocks effect with the number of X-rays emitted. The process of two gases colliding to form dust and the processes by which radiation is emitted are seemingly unconnected, but Pollock believes that “all of this must somehow fit together”. If there is indeed a link between the two goldilocks zones, finding the connection between them could reveal some deeper secrets of how stars in our galaxy interact with each other.
