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New Details Revealed in the “Green Monster”

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Cas A supernova remnant as seen by JWST, revealing the structure known as the Green Monster
This image of the famous supernova remnant Cassiopeia A combines X-ray, optical, and infrared observations. The new infrared data from JWST revealed fascinating structures. [X-ray: NASA/CXC/SAO; Optical: NASA/ESA/STScl; IR: NASA/ESA/CSA/STScl/Milisavljevic et al., NASA/JPL/CalTech; Image Processing: NASA/CXC/SAO/J. Schmidt and K. Arcand]

JWST images of the supernova remnant Cassiopeia A (Cas A) revealed for the first time a perforated, filamentary structure named the “Green Monster” for its resemblance to the home-run-thwarting wall at Fenway Park. Recently, researchers reported on this feature and the cause of its curious spots.

Studying a Star’s Life from Its Remains

Cas A supernova remnant

This perspective on Cas A comes from the Hubble Space Telescope. Each of the tiny blobs of gas near the top of the shell is tens of times wider than the solar system. [NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: R. Fesen (Dartmouth) and J. Morse (Univ. of Colorado)]

Stars much more massive than the Sun end their lives spectacularly. In the finale of their brief, brilliant lives, these stars are no longer able to produce enough energy to stave off the crushing pressure of gravity, causing them to collapse. In an explosion seen galaxies away, their outer layers rebound off their shrunken cores, leaving behind an intricate and colorful supernova remnant that slowly expands to span hundreds of light-years.

Among the most famous and well-studied remnants of a core-collapse supernova is Cas A, which has posed for countless astronomical portraits since its discovery in 1948. Recently, JWST data revealed a never-before-seen feature stretching across Cas A. Because of its green appearance in representative-color images from JWST, this feature was dubbed the Green Monster.

How the Green Monster Got Its Spots

In a recent article, Ilse De Looze (Ghent University) and collaborators focused on one particularly intriguing aspect of the Green Monster: multiple complete and partial circles that appear to be torn in the fabric of the structure. The team measured more than two dozen of these circles, each with diameters of about 1–3 arcseconds. If the Sun were placed at the center of one of these circles, the boundary of the circle would lie close to the inner edge of the Oort Cloud.

closeup of the Green Monster holes

A closeup of the Green Monster, showing the small holes investigated in this study. The orange arrows indicate the location of an ejecta filament that might be associated with the hole labeled “P2.” Click to enlarge. [De Looze et al. 2024]

What could create these shapes? De Looze’s team proposed several scenarios, each of which draws on the complicated interactions between the fast-moving ejecta from the exploded star, the surrounding circumstellar material, and shock waves that propagate outward and inward as the various materials collide.

In the scenario deemed most likely, the Green Monster is made up of circumstellar material that was lost before the star exploded as a supernova. This material now sits in front of the supernova remnant, from our perspective, and was impacted by a shock wave. The holes are created where ejected material has poked through the Green Monster.

What’s not yet clear is the timeline: did the collision between the outward-moving ejecta and the Green Monster happen before or after the Green Monster was struck by the shock wave? As De Looze’s team showed, either order is possible, and more data and simulations are needed to explore the sequence of events and the timescales involved.

Mapping Mass Loss

Cas A with locations of quasi-stationary flocculi labeled.

Locations of quasi-stationary flocculi, outlined in cyan, relative to the position of the Green Monster. Click to enlarge. [Adapted from De Looze et al. 2024]

Combined with Cas A’s other intricate structures, the Green Monster gives researchers clues to the tumultuous final years of this exploded star’s life. Currently, evidence points to a massive asymmetric mass-loss episode roughly 30,000–100,000 years before the supernova — in astronomical terms, just briefly before the ultimate explosion. The similarities between the Green Monster and previously discovered structures called quasi-stationary flocculi — dense, slow-moving clumps of circumstellar material — suggest that they may have arisen from the same mass-loss episode.

JWST has already given astronomers an entirely new view of Cas A, but there are surely more discoveries yet to come: future observations will sample the spectra of multiple regions of the Green Monster and several quasi-stationary flocculi, helping to reconstruct the final years of Cas A’s progenitor star’s life.

Citation

“The Green Monster Hiding in Front of Cas A: JWST Reveals a Dense and Dusty Circumstellar Structure Pockmarked by Ejecta Interactions,” Ilse De Looze et al 2024 ApJL 976 L4. doi:10.3847/2041-8213/ad855d