The Webb Space Telescope discovers the hidden ‘green monster’ in a stunning high-resolution view of the exploding star

A new image taken by the James Webb Space Telescope of Cassiopeia A (Cas A) provides a detailed look at the supernova remnant, revealing new features and providing insight into its structure and complex history. The image, part of the holiday initiative, shows intricate details about the supernova, including the discovery of “Baby Cas A” and the “Green Monster.” Image credit: NASA, ESA, CSA, STScI, Dani Milisavljevic (Purdue University), Ilse De Luzzi (UGent), T. Timim (Princeton University)

Mysterious features hide in near infrared light

Objects in space reveal different aspects of their composition and behavior at different wavelengths of light. The Cassiopeia supernova remnant (Cas A) is one of the most well-studied objects in the world milky way across the wavelength spectrum. However, there were still secrets hidden within the shattered remains of the star.

Latest Tools is opened by one of the newest tools in the Researcher Toolkit, NASA‘s James Webb Space TelescopeWebb’s latest look at near-infrared light has stunned researchers.

Cassiopeia A (Webb NIRCam image)

A new high-resolution image from NASA’s James Webb Space Telescope’s NIRCam (near-infrared camera) reveals intricate details of the Cassiopeia A (Cas A) supernova remnant, showing the expanding shell of material colliding with the gas produced by the star before it explodes. . . Image credit: NASA, ESA, CSA, STScI, Dani Milisavljevic (Purdue University), Ilse De Luzzi (UGent), T. Timim (Princeton University)

Webb stuns NASA with a new, high-definition look at the exploding star

Like a shiny, round ornament ready to be placed in the perfect spot on the holiday tree, the remnant of the supernova Cassiopeia A (Cas A) shines in a new image from NASA’s James Webb Space Telescope.

as part of Holidays 2023 at the White House, First Lady of the United States Dr. Jill Biden launched the first-ever White House Advent Calendar. To showcase the “magic, wonder, and joy” of the holiday season, Dr. Biden and NASA are celebrating with this new image from Webb.

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While all is bright, this scene is not the proverbial silent night. Cas A’s Webb NIRCam (near infrared camera) displays this starburst at a resolution previously unattainable at these wavelengths. This high-resolution view reveals intricate details of the expanding shell of material colliding with gas released by the star before it exploded.

Cas A is one of the most well-studied supernova remnants in all of the universe. Over the years, ground-based and space-based observatories, including NASA Chandra X-ray Observatory, Hubble Space TelescopeAnd retired Spitzer Space Telescope They compiled a multi-wavelength image of the body’s remains.

However, astronomers have now entered a new era in the study of Cas A. In April 2023, Webb’s MIRI instrument (Mid-Infrared Instrument) was discovered. This chapter has begun, revealing new and unexpected features within the inner shell of the supernova remnant. Many of these features are not visible in the new NIRCam image, and astronomers are investigating why.

Webb Cassiopeia A close-ups (NIRCam image)

This image highlights several interesting features of the Cassiopeia A supernova remnant as seen using Webb’s NIRCam (near infrared camera):
1. NIRCam’s remarkable resolution is able to detect small nodes of gas, consisting of sulfur, oxygen, argon and neon, from the star itself. Some debris filaments are too small to be resolved even by Webb, meaning they are comparable to or less than 10 billion miles (about 100 astronomical units) across. Researchers say this represents how the star shattered like glass when it exploded.
2. The circular holes visible in the MIRI image inside the Green Monster, a ring of green light in the inner cavity of Cas A, is faintly outlined by white and purple emission in the NIRCam image – this represents ionized gas. Researchers believe this is due to supernova debris rushing through the gas and forming the gas left behind by the star before it exploded.
3. This is one of the few light echoes visible in the NIRCam image of Cas A. The light echo occurs when light from the star’s long-exploded explosion reaches, and heats, distant dust, which glows as it cools.
4. NIRCam captured a particularly large, complex light echo, which researchers have named Baby Cas A. It is actually located about 170 light-years behind the supernova remnant.
Image credit: NASA, ESA, CSA, STScI, Dani Milisavljevic (Purdue University), Ilse De Luzzi (UGent), T. Timim (Princeton University)

“Like shards of glass”

Infrared light is invisible to our eyes, so image processors and scientists translate these wavelengths of light into visible colors. In this newer image of Cas A, the colors are assigned to different NIRCam filters, and each of these colors indicates a different activity occurring within the object.

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At first glance, the NIRCam image may appear less colorful than the MIRI image. However, this is simply due to the wavelengths at which the material of the object emits its light.


This video tours Webb’s NIRCam (near infrared camera) image of the Cassiopeia A (Cas A) supernova remnant. NIRCam’s high resolution detects small nodes of gas remaining from the star’s explosion, as well as light echoes scattered across the field of view.

The most visible colors in Webb’s new image are the bright orange and light pink clumps that make up the inner crust of the supernova remnant. Webb’s sharp vision can detect the smallest knots of gas, made up of sulfur, oxygen, argon and neon, from the star itself. In this gas is a mixture of dust and particles, which will eventually become components of new stars and planetary systems. Some of the debris filaments are so small that even Loeb can’t resolve them, meaning they are comparable to or less than 10 billion miles (about 100 astronomical units) across. By comparison, the entirety of Cas A spans 10 light-years, or 60 trillion miles.

“Thanks to the resolution of NIRCam, we can now see how the dying star completely shattered as it exploded, leaving behind filaments that resemble tiny glass shards,” said Dani Milisavljevic of Purdue University, who leads the research team. β€œIt is truly incredible that after all these years of studying Cas A we can now resolve these details, which provide us with transformative insight into how this star exploded.”

Cassiopeia A (Webb NIRCam and MIRI side by side)

This image provides a side-by-side comparison of the Cassiopeia A (Cas A) supernova remnant as captured by the NIRCam (near infrared camera) and MIRI (middle infrared instrument) cameras of NASA’s James Webb Space Telescope.
At first glance, Webb’s NIRCam image appears less colorful than the MIRI image overall, however, this is only due to the wavelengths at which the material from the object emits its light. The NIRCam image appears slightly sharper than the MIRI image due to its increased resolution.
The edges of the main inner shell, which appeared dark orange and red in the MIRI image, look like smoke from a campfire in the NIRCam image. This refers to where the supernova’s explosion wave collides with the material surrounding the star. The dust in the material surrounding the star is too cold to be detected directly at near-infrared wavelengths, but it glows in the mid-infrared.
Also missing from the near-infrared image was the green light ring in Cas A’s central cavity that glowed in the mid-infrared, which the research team called the Green Monster. However, the circular holes visible in the MIRI image inside the Green Monster, are faintly outlined by white and purple emission in the NIRCam image.
Image credit: NASA, ESA, CSA, STScI, Dani Milisavljevic (Purdue University), Ilse De Luzzi (UGent), T. Timim (Princeton University)

The hidden green monster

When comparing Webb’s new near-infrared view of Cas A with the mid-infrared view, its inner cavity and outer shell are strangely colorless.

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The edges of the main inner shell, which appeared in deep orange and red in the MIRI image, now look like smoke from a campfire. This refers to where the supernova’s explosion wave collides with the material surrounding the star. The dust in the material surrounding the star is too cold to be detected directly at near-infrared wavelengths, but it glows in the mid-infrared.

The researchers say the white color is light from synchrotron radiation, which is emitted across the electromagnetic spectrum, including near infrared. They are generated by charged particles traveling at extremely high speeds and wrapping around magnetic field lines. Synchrotron radiation is also visible in the bubble-like shells in the lower half of the inner cavity.


This close-up video shows the relative position of the Cassiopeia A (Cas A) supernova remnant in the sky. It starts with a ground-based image taken by the late astrophotographer Akira Fujii. As it zooms in on smaller parts of the sky, it fades into an image from the Digital Sky Survey. It ends with a crossfade in a Cas A image from the NIRCam (Near Infrared Camera) on NASA’s James Webb Space Telescope, with additional strokes from NASA’s camera. Hubble Space Telescope picture.

Also missing from the near-infrared image was the green light ring in Cas A’s central cavity that glowed in the mid-infrared, which the research team called the Green Monster. This feature was described as “difficult to understand” by researchers upon first looking at them.

While the green color of the green monster is not visible in NIRCam, what remains of near-infrared light in that region could provide insight into this mysterious feature. The circular holes visible in the MIRI image are faintly outlined by white and purple emission in the NIRCam image – this represents ionized gas. Researchers believe this is due to supernova debris rushing through the gas and forming the gas left behind by the star before it exploded.

Cassiopeia A (Webb NIRCam compass image)

This image of the Cassiopeia supernova remnant, captured by Webb’s NIRCam (near infrared camera) shows the compass arrows, scale bar, and color key for reference.
The north and east compass arrows show the direction of the image in the sky.
The scale bar is stated in light years, which is the distance light travels in one Earth year. (It takes light 3 years to travel a distance equal to the length of the scale bar.) One light year is equal to about 5.88 trillion miles or 9.46 trillion kilometers.
This image shows invisible wavelengths of near-infrared light translated into the colors of visible light. The color key shows which NIRCam filters were used when collecting the light. The color of each filter name is the color of visible light used to represent the infrared light passing through that filter.
Image credit: NASA, ESA, CSA, STScI, Dani Milisavljevic (Purdue University), Ilse De Luzzi (UGent), T. Timim (Princeton University)

Baby Cass A

The researchers were also amazed by a remarkable feature in the lower right corner of NIRCam’s field of view. They call that big, striped blob Baby Cas A, because it looks like the descendant of a major supernova.

This is a light echo, where the light from the star’s explosion arrived long ago, heating up the distant dust, which glows as it cools. The complexity of the dust pattern, and the apparent closeness of Baby Cas A to Cas A itself, particularly interests researchers. In fact, Baby Cas A is located about 170 light-years beyond the supernova remnant.

There are also many smaller light echoes scattered throughout the new Webb image.

The Cas A supernova remnant is located 11,000 light-years away in the constellation Cassiopeia. It is estimated to have exploded about 340 years ago from our perspective.

The James Webb Space Telescope is the world’s leading space science observatory. Webb solves the mysteries of our solar system, looks beyond the distant worlds around other stars, and explores the mysterious structures and origins of our universe and our place in it. WEB is an international program led by NASA with its partners the European Space Agency (ESA).European Space Agency) and the Canadian Space Agency.

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