The intricate whorls and striations of dust that drift between the stars have just been revealed in stupefying detail.
In new images from JWST, we’re finally seeing exquisite details of the flow and turbulence of the interstellar medium in the vicinity of a star we saw explode just a few hundred years ago.
As light from the explosion, which we now call Cassiopeia A, expanded outwards, it reflected off and warmed the tenuous dust it passed through, generating a faint, red lambency.
So tenuous is the material, and so faint the glow, that its true complexity has largely eluded us. Now, thanks to the power of JWST to see dim, red light, we’re finally gaining a fuller understanding of the interstellar medium’s structure.
What’s even more incredible is that we can see changes in the structure on the scale of days. JWST took multiple images of a wisp in the dust cloud near Cassiopeia A in August and September 2024, and saw significant changes as light moved through the wood-grain-like striations, producing a phenomenon known as a light echo.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen=”allowfullscreen”>
“We see layers like an onion,” says astronomer Josh Peek of the Space Telescope Science Institute in the US. “We think every dense, dusty region that we see, and most of the ones we don’t see, look like this on the inside. We just have never been able to look inside them before.”
Light echoes can produce some of the most beautiful sights in the galaxy. They occur when something produces a flash of light that radiates out into space.
If that light encounters a physical barrier, such as clouds of cosmic dust, it will reflect, arriving at a different time from the initial burst; just like a sound echo, but with light. We can use these light echoes to help map and understand space, and the objects within it.
To date, most of the light echoes we’ve detected tend to be from very bright events or very thick dust close to the light source, as we see in the star V838 Monocerotis. Thinner dust, farther from the source, is a lot harder to see.
But, well, seeing things we couldn’t see before is what JWST does. The infrared telescope is optimized to see the faint red light other instruments can’t pick up; so astronomers turned it to a wisp of dust near and behind, but unrelated to, Cassiopeia A, a star humanity saw explode from 11,000 light-years away in the 1670s.
This wisp had been identified as a light echo by NASA’s now-retired Spitzer space telescope. But Spitzer didn’t have the resolution of JWST.
Even so, “we were pretty shocked to see this level of detail,” says astronomer Jacob Jencson of the California Institute of Technology.
Perhaps most surprising was the discovery that the medium is arranged in densely packed sheets of material, with knots and whorls, a bit like the knots you might see in the grain of a tree. The researchers could see details in these sheets down to scales of around 400 astronomical units, or 400 times the distance between Earth and the Sun.
These structures, the researchers believe, could be related to magnetic field lines running through space. If this is the case, studying the evolution of light echoes opens a new window into the study of magnetized turbulence.
“This is the astronomical equivalent of a medical CT scan,” says astronomer Armin Rest of the Space Telescope Science Institute. “We have three slices taken at three different times, which will allow us to study the true 3D structure. It will completely change the way we study the interstellar medium.”
Further, in-depth analyses of the observations are no doubt forthcoming.
Meanwhile, two presentations, one led by Jencson, the other by Peek, have taken place at the 245th Meeting of the American Astronomical Society. You can find the abstracts here and here.
