Objects in space reveal different aspects of their composition and behavior at different wavelengths of light. A supernova remnant called Cassiopeia A is one of the most studied objects in the Milky Way across the wavelength spectrum. However, there are still secrets hidden within the star’s tattered remains. The latest are being unlocked by the Near-Infrared Camera (NIRCam) instrument aboard the NASA/ESA/CSA James Webb Space Telescope.
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This high-definition image from Webb’s NIRCam instrument unveils intricate details of the supernova remnant Cassiopeia A; it shows the expanding shell of material slamming into the gas shed by the star before it exploded. Image credit: NASA / ESA / CSA / STScI / D. Milisavljevic, Purdue University / T. Temim, Princeton University / I. De Looze, University of Gent.
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.
Cassiopeia A is one of the best-studied supernova remnants in all the cosmos.
Over the years, ground-based and space-based observatories, including the NASA/ESA Hubble Space Telescope, have collectively assembled a multiwavelength picture of the object’s tattered remains.
However, astronomers have now entered a new era in the study of Cassiopeia A.
In April 2023, Webb’s Mid-Infrared Instrument (MIRI) started this story, revealing new and unexpected features within the inner shell of the supernova remnant.
But many of those features are invisible in the new NIRCam image, and astronomers are investigating why that is.
“Infrared light is invisible to our eyes, so image processors and scientists represent these wavelengths of light with visible colors,” Webb astronomers said.
“In this newest image of Cassiopeia A, colors were assigned to NIRCam’s different filters, and each of those colors hints at different activity occurring within the object.”
“At first glance, the NIRCam image may appear less colorful than the MIRI image. However, this does not mean there is less information: it simply comes down to the wavelengths in which the material in the object is emitting its light.”
“The most noticeable colors in the newest image are clumps of bright orange and light pink that make up the inner shell of the supernova remnant.”
“Webb’s razor-sharp view can detect the tiniest knots of gas, composed of sulfur, oxygen, argon, and neon from the star itself.”
“Embedded in this gas is a mixture of dust and molecules, which will eventually be incorporated into new stars and planetary systems.”
“Some filaments of debris are too tiny to be resolved, even by Webb, meaning that they are comparable to or less than 16 billion km across (around 100 astronomical units).”
“In comparison, the entirety of Cassiopeia A spans 10 light-years, or roughly 96 trillion km.”
“When comparing Webb’s new near-infrared view of Cassiopeia A with the mid-infrared view, its inner cavity and outermost shell are curiously devoid of color.”
“The outskirts of the main inner shell, which appeared as a deep orange and red in the MIRI image, now look like smoke from a campfire.”
“This marks where the supernova blast wave is ramming into the surrounding circumstellar material.”
“The dust in the circumstellar material is too cool to be detected directly at near-infrared wavelengths, but lights up in the mid-infrared.”
This image highlights several interesting features of Cassiopeia A, as seen with Webb’s NIRCam instrument. Image credit: NASA / ESA / CSA / STScI / D. Milisavljevic, Purdue University / T. Temim, Princeton University / I. De Looze, University of Gent.
According to the astronomers, the white color is light from synchrotron radiation, which is emitted across the electromagnetic spectrum, including the near-infrared.
It’s generated by charged particles traveling at extremely high speeds and spiraling around magnetic field lines.
Synchrotron radiation is also visible in the bubble-like shells in the lower half of the inner cavity.
Also not seen in the near-infrared view is the loop of green light in the central cavity of Cassiopeia A that glowed in mid-infrared light, appropriately nicknamed the Green Monster by the researchers.
“While the ‘green’ of the Green Monster is not visible in NIRCam, what’s left over in the near-infrared in that region can provide insight into the mysterious feature,” they said.
“The circular holes visible in the MIRI image are faintly outlined in white and purple emission in the NIRCam image — this represents ionized gas.”
The authors believe this is due to the supernova debris pushing through and sculpting gas left behind by the star before it exploded.
The scientists were also absolutely stunned by one fascinating feature at the bottom right corner of NIRCam’s field of view.
They’re calling that large, striated blob Baby Cas A — because it appears like an offspring of the main supernova.
“This is a light echo. Light from the star’s long-ago explosion has reached, and is warming, distant dust, which glows as it cools down,” they said.
“The intricacy of the dust pattern, and Baby Cas A’s apparent proximity to Cassiopeia A itself, are particularly intriguing.”
“In actuality, Baby Cas A is located about 170 light-years behind the supernova remnant.”
“There are also several other, smaller light echoes scattered throughout Webb’s new portrait.”
