New near-infrared observations from the Webb telescope reveal intricate strands of debris from the exploded star.

round, expanding shell with intricate, knotty filaments
A new near-infrared image from NASA’s James Webb Space Telescope’s NIRCam of the supernova remnant Cassiopeia A. In the bottom right corner is a light echo , created by light from the star’s long-ago explosion off surrounding dust.
NASA / ESA / CSA / STScI / Danny Milisavljevic (Purdue University) / Ilse De Looze (UGent) / Tea Temim (Princeton University)

This new infrared image from the James Webb Space Telescope reveals the intricate knots of debris inside the supernova remnant Cassiopeia A. Cas A lies about 11,000 light-years from Earth and formed more than 300 years ago when a massive star went kablooey.

Spectroscopic study of the explosion’s light echo — the reflection of the flash off surrounding dust grains — has previously revealed that the event was a Type IIb supernova, the death of a big star stripped of most of its hydrogen shell.

JWST astronomers released a different image of Cas A earlier this year (see below). That one was assembled from mid-infrared data and highlighted in reddish orange where the expanding blast wave is ramming into material surrounding the dead star. In the new image, the outer regions have instead been colored white. This is not merely an aesthetic choice but one made to highlight that we’re looking at different kinds of emission: In mid-infrared, we were detecting glowing dust; in the near-infrared image, we’re seeing emission from electrons corkscrewing along magnetic field lines at breakneck speeds.

On the remnant’s exterior, particularly at the top and left, lie curtains of material appearing orange and red due to emission from warm dust. This marks where ejected material from the exploded star is ramming into surrounding circumstellar material.   Interior to this outer shell lie mottled filaments of bright pink studded with clumps and knots. This represents material from the star itself, and likely shines due to a mix of various heavy elements and dust emission. The stellar material can also be seen as fainter wisps near the cavity’s interior.  A loop represented in green extends across the right side of the central cavity. Its shape and complexity are unexpected and challenging for scientists to understand.
Cassiopeia A, as revealed by JWST's mid-infrared camera. This image combines various filters with the color red assigned to 25.5 microns, orange-red to 21 microns, orange to 18 microns, yellow to 12.8 microns, green to 11.3 microns, cyan to 10 microns, light blue to 7.7 microns, and blue to 5.6 microns.
NASA / ESA / CSA / Danny Milisavljevic (Purdue University) / Tea Temim (Princeton University) / Ilse De Looze (UGent); Image processing: Joseph DePasquale (STScI)

The most eye-catching part of the first, near-infrared image is the pinkish festoons. These strands are debris from the now-dead star and comprise sulfur, oxygen, argon, and neon. Dust sprinkles the mix. Cas A spans some 10 light-years, but some of these ejecta filaments are so small that they evade JWST’s resolution at this distance, meaning they’re at most 100 astronomical units across — roughly twice the size of the solar system, if you include the main part of the Kuiper Belt outside Neptune’s orbit.  

Dust is a major player in stellar evolution. It helps cool gas, enabling it to collapse and form stars. Astronomers still question what the universe’s primary source of dust is. Some dust comes from aging, puffy giants that are sloughing off their outer layers as winds, but these don’t form rapidly enough to explain the high quantities of dust found in the early universe.

Supernovae also create dust, which forms in the cooling ejecta. The problem is, supernovae destroy the same dust they create: The shock wave created when the ejecta slam into surrounding material rebounds back into the remnant’s interior, heating and destroying dust as it goes.

The highly clumpy nature of ejecta that JWST is revealing could explain how dust survive this process: Grains may shelter deep inside the clumps, away from the shock wave’s destructive effects.

We’ll talk more about Cas A and other supernova remnants in our May 2024 issue. Subscribe now!


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