This high-resolution view reveals intricate details of the expanding shell of material that crashed into the gas projected by the star before it exploded.
CAS-A is one of the best-studied supernova remnants in the entire universe. It is located 11,000 light years away in the constellation Cassiopeia. It is estimated that it exploded about 340 years ago from our perspective.
Over the years, ground-based and space-based observatories, including the NASA/ESA Hubble Space Telescope, have collectively gathered a multi-wavelength image of the object’s remnants.
However, astronomers are now entering a new era in the study of CAS A. In April 2023, Webb’s MIRI intermediate-infrared instrument began this story, revealing new and unexpected features inside the inner shell of the supernova left over. But many of those features can’t be seen in the new Nircam image, and astronomers are investigating why.
Infrared light is invisible to our eyes, so image processors and scientists represent these wavelengths of light with visible colors. In this recent CAS A image, colors have been assigned to different Nircam filters, and each of those colors suggests different activity within the object.
At first glance, the Nircam image may appear less colorful than the Miri image. However, this does not mean that there is less information; it boils down only to the wavelengths where the material of the object emits its light, NASA reports.
The most prominent colors in the new Webb image are the clusters of bright orange and light pink that make up the inner layer of the supernova remnant. Webb’s sophisticated vision 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 eventually become incorporated into new stars and planetary systems. Some filaments of debris are too small to be resolved, even by Webb, meaning they are comparable to or smaller than 16 billion km (about 100 astronomical units). In comparison, the entirety of CAS A spans 10 light years, or approximately 96 trillion km.
Comparing Webb’s new near-infrared view of CAS A with the mid-infrared view, its inner cavity and outermost shell are remarkably colorless. The exterior of the main inner shell, which showed a deep orange and red in the MIRI image, now looks like smoke from a campfire. This marks where the supernova explosion wave started in the surrounding circumstellar material. Dust in circumstellar material is too cool to be detected directly at near-infrared wavelengths, but it shines in the mid-infrared.
The researchers concluded that the white color is light from synchrotron radiation, which is emitted throughout the electromagnetic spectrum, including the near-infrared. This is done by charged particles traveling at high speeds and orbiting magnetic field lines. Synchrotron radiation is also seen in the foam shells in the lower half of the internal cavity.
Also not visible in the near-infrared view is a loop of green light in CAS A’s central cavity that glows with mid-infrared light, aptly dubbed the green monster by the research team. Researchers described this feature as “difficult to understand” at the time of its initial observation.
While the “green” of the green monster is invisible to Nircam, what is left of the near-infrared in that region may provide insight into the mysterious side. The round holes seen in the Miri image are slightly outlined by the white and purple emission in the Nircam image; it represents ionized gas. Researchers believe this is because the supernova debris pushed and carved up the gas left behind by the star before it exploded.
The researchers were also surprised by an interesting feature in the lower right corner of Nircam’s field of view. They named the large, striated baby Cas A because it appears to be a descendant of the parent supernova.
This is a light echo. The light from the exploding star reaches and heats the distant dust, which glows as it cools. The complexity of the dust pattern and the apparent proximity of Baby Cas A to CAS A itself are of particular interest to researchers. In fact, Baby Cas A is located about 170 light years behind the supernova remnant.