March 3, 2024

Astronomers Solve Mystery of “Green Monster” in Supernova Remnant

Written by AiBot

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Jan 10, 2024

Astronomers have finally uncovered the identity of a mysterious green glow emanating from the Cassiopeia A supernova remnant. Using observations from NASA’s powerful new James Webb Space Telescope (JWST) and Chandra X-ray Observatory, researchers determined that the emission is coming from negatively charged chlorine ions produced when the shockwave from the stellar explosion slammed into surrounding material.

New Images Reveal Stunning Details

Cassiopeia A is the remains of a star approximately 11,000 lightyears away that exploded around 350 years ago. The supernova would have been visible from Earth at the time, but no records exist of it being spotted.

JWST’s sensitive infrared vision has now provided an unprecedented view of the aftermath. Paired with X-ray data from Chandra that shows the supernova shockwave, the composite image reveals glorious tendrils of glowing gas:

Cassiopeia A composite

JWST and Chandra composite image of Cassiopeia A. The green emission comes from doubly ionized chlorine. (Image credit: NASA/ESA/CSA/STScI/Chandra)

The bright green region traces where the shockwave is plowing into a ring of gas shed by the star before it exploded. This is causing the gas to light up. But the precise emission mechanism has been unknown until now.

“Previous infrared observations couldn’t pinpoint the molecular composition of the green filaments seen swirling around the supernova remnant,” said William Blair of Johns Hopkins University. “Thanks to Webb’s high-resolution spectra, we now know that these filaments are largely made up of doubly ionized chlorine.”

Solving a Decades-Old Mystery

First spotted in visible light images taken in the 1970s, the green glow has been captivating astronomers for over 40 years. But lacked the capability to analyze the emission in detail. JWST has finally lifted the veil.

“We’ve discovered that the remnant’s bright green filaments are produced by extremely high velocity oxygen-rich debris from the supernova slamming into a surrounding ring of gas at thousands of kilometers per second,” said Jeonghee Rho of the SETI Institute. “The violent collision ionizes the debris, causing it to glow.”

Specifically, JWST’s Near-Infrared Spectrograph (NIRSpec) shows that shock velocities exceed 2,000 km/s in some locations. This extreme acceleration strips electrons from chlorine atoms, turning neutral chlorine into highly ionized Cl2+ ions. Their transition back to lower energy states causes the radiant green light.

“Our spectroscopic observations precisely match what theoretical models have predicted is happening,” said Rho. “Seeing this agreement between observation and theory is important – it means our understanding of supernova remnants and shock physics is on the right track.”

Looking to the Future

With the chlorine mystery now solved, astronomers will turn their instruments to new questions about Cassiopeia A’s tumultuous history. Further observations are expected to reveal insights into the progenitor star – what it was made of, what fueled the cataclysmic explosion, and how the debris is altering the surrounding interstellar medium.

“JWST will undoubtedly help address some long-standing mysteries about supernova remnants,” said Blair. “But it will also reveal new questions we didn’t even know we should be asking. This is what progress in scientific understanding looks like – we build on what came before to find surprises we never expected.”

Researchers also anticipate finding clues to the origins of crucial elements dispersed by such explosions. Supernovae are responsible for producing many of the heavy elements in the cosmos critical to forming new stars, planets, and even life.

“Each teaspoon of Cassiopeia A holds as much gold, platinum and uranium as you’d find in the richest Earth mines,” noted Robert Kirshner of the Harvard–Smithsonian Center for Astrophysics. “JWST allows us to closely study element production right at their source.”

So while one secret of Cassiopeia A has been revealed, the unfolding story of this long-dead star will keep astronomers busy for many years to come thanks to JWST’s game-changing perspective.

“It feels like a Renaissance in supernova studies right now,” said Rho. “JWST is giving us a front row seat to phenomena we could previously only imagine.”

More About Cassiopeia A and JWST

Cassiopeia A Supernova Remnant

  • Formed by a supernova explosion around 1680
  • One of the youngest known supernova remnants in the Milky Way
  • Located approximately 11,000 lightyears away in the Cassiopeia constellation
  • Shockwave expanding at about 5 million mph (2,300 km/s)
  • Visible debris rich in newly synthesized elements like silicon, sulfur, calcium and iron

The James Webb Space Telescope

JWST is the largest, most complex, and most powerful space telescope ever constructed. Key facts:

  • Joint project of NASA, ESA and CSA launched on December 25, 2021
  • Infrared optimized 6.5 meter primary mirror made of beryllium
  • Operates 1 million miles from Earth at the Sun-Earth Lagrange point L2
  • Carries four state-of-the-art science instruments:
    • Near-Infrared Camera (NIRCam)
    • Near-Infrared Spectrograph (NIRSpec)
    • Mid-Infrared Instrument (MIRI)
    • Fine Guidance Sensor and Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS)
  • Early release observations showcase unprecedented infrared imaging and analysis of the early universe and objects within the Milky Way galaxy


The first scientific results from James Webb Space Telescope observations are already fundamentally transforming astronomers’ understanding of cosmic phenomena like supernova remnants. Expect many more astonishing discoveries about Cassiopeia A and other intriguing subjects in the years ahead as researchers exploit JWST’s game-changing capabilities. The future of space astronomy looks brighter than ever!




AiBot scans breaking news and distills multiple news articles into a concise, easy-to-understand summary which reads just like a news story, saving users time while keeping them well-informed.

To err is human, but AI does it too. Whilst factual data is used in the production of these articles, the content is written entirely by AI. Double check any facts you intend to rely on with another source.

By AiBot

AiBot scans breaking news and distills multiple news articles into a concise, easy-to-understand summary which reads just like a news story, saving users time while keeping them well-informed.

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