New research suggests that the Green Monster — a curious structure first spotted in Webb data in April 2023 — came from that blast wave slamming into material surrounding the Cassiopeia A supernova remnant.
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This composite image shows the Green Monster structure within the supernova remnant Cassiopeia A. Image credit: NASA / CXC / SAO / ESA / STScI / CSA / Milisavljevic et al. / JPL / Caltech / J. Schmidt / K. Arcand.
Cassiopeia A is located about 11,000 light-years away from Earth in the constellation of Cassiopeia.
Also known as Cas A, SNR G111.7-02.1 or NRAO 711, the object spans approximately 10 light-years.
When the original star ran out of fuel, it collapsed onto itself and blew up as a supernova, possibly briefly becoming one of the brightest objects in the sky.
Although astronomers think that this happened around the year 1680, there are no verifiable historical records to confirm this.
A curious structure dubbed the Green Monster was first identified in infrared data from the NASA/ESA/CSA James Webb Space Telescope in April 2023, but its origin was not clear.
However, by combining the Webb data with X-rays from NASA’s Chandra X-ray Observatory, astronomers think they have hunted down the source of this feature.
“We already suspected the Green Monster was created by a blast wave from the exploded star slamming into material surrounding it. Chandra helped us clinch the case,” said Dr. Jacco Vink, an astronomer at the University of Amsterdam.
When a massive star exploded to create Cassiopeia A about 340 years ago, from Earth’s perspective, it created a ball of matter and light that expanded outward. In the outer parts of Cassiopeia A the blast wave is striking surrounding gas that was ejected by the star between about 10,000 and 100,000 years before the explosion.
That formed a favorable environment for dust formation after the ejected stellar material cooled down.
The Chandra data reveal hot gas, mostly from supernova debris including elements like silicon and iron, but also from energetic electrons spiraling around magnetic field lines in the blast wave.
These electrons light up as thin arcs near the blast wave, and also show up in parts of the interior.
Webb highlights infrared emission from dust that is warmed up because it is embedded in the hot gas seen by Chandra, and from much cooler supernova debris.
Despite this chaotic stellar scene, the Green Monster clearly stood out in the original Webb image.
While analyzing Chandra data of the remnant, Dr. Vink and colleagues found that filaments in the outer part of Cassiopeia A, from the blast wave, closely matched the X-ray properties of the Green Monster, including less iron and silicon than in the supernova debris.
This implies a common origin for the Green Monster and blast wave.
The Chandra data also show that the material in the Green Monster is all moving towards us, indicating that it is plowing into the star’s ejected gas on the near side of Cassiopeia A.
Its speed is about half the average speed of the blast wave, suggesting that the density of material in the Green Monster is much higher than the average density of material surrounding Cassiopeia A.
This result can help reconstruct the complicated history of mass lost by the star before it exploded.
“We concluded that the Green Monster is also part of the blast wave and is photobombing the central part of Cassiopeia A rather than being part of it,” said Dr. Ilse De Looze, an astronomer at Ghent University.
“We then digitally removed the Green Monster from the rest of the image to learn more about what is behind it.”
“It’s like we were handed a completed, 3D jigsaw puzzle, and we were able to take parts out to see what’s on the inside.”
The debris from the star is seen by Chandra because it is heated to tens of millions of degrees by shock waves, akin to sonic booms from a supersonic plane.
Webb can see some material that has not been affected by shock waves, what can be called pristine debris. Much of this lies behind the Green Monster.
The combination of Webb and Chandra data therefore gives a fuller census of debris from the exploded star.
“We’ve made the first map of the web-shaped, pristine debris in the center of this supernova remnant,” said Dr. Dan Milisavljevic, an astronomer at Purdue University.
“No one has ever seen structures like this before in an exploded star.”
To learn more about the supernova explosion, the astronomers compared the Webb view of the destroyed star’s pristine debris with X-ray maps of radioactive elements that were created in the supernova.
They used NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) data to map radioactive titanium — still visible today — and Chandra to map where radioactive nickel — which decays to form iron — was by measuring the locations of iron.
Two aspects stood out in this comparison. Some filaments of pristine debris near the center of Cassiopeia A, seen with Webb, are connected to the iron seen with Chandra farther out. Radioactive titanium is seen where pristine debris is relatively weak.
These comparisons suggest that radioactive material seen in X-rays has helped shape the pristine debris near the center of the remnant seen with Webb, forming cavities.
The fine structures in the pristine debris were most likely formed when the star’s inner layers were violently mixed with hot, radioactive matter produced during collapse of the star’s core under gravity.
“These Webb survey data and initial findings, supported by other telescopes like Chandra, help address unresolved questions about massive star explosions that have broad implications for the formation and evolution of stellar populations, and the metal and dust enrichment of galaxies,” said Dr. Tea Temim, an astronomer at Princeton University.
The results will appear in two papers (paper #1 and paper #2) in the Astrophysical Journal Letters.
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Dan Milisavljevic et al. 2024. A JWST Survey of the Supernova Remnant Cassiopeia A. ApJL, in press; arXiv: 2401.02477
Jacco Vink et al. 2024. X-ray diagnostics of Cassiopeia A’s ‘Green Monster:’ evidence for dense shocked circumstellar plasma. ApJL, in press; arXiv: 2401.02491
