The supernova remnant of Cassiopeia A (Cas A) sparkles in a image recently captured by the James Webb Space Telescope (CSA). Webb’s NIRCam (Near Infrared Camera) shows a very strong explosion with a resolution that was previously unavailable at these wavelengths.
This high-resolution image reveals the intricate details of the shell of expanding material that crashes into the gas emitted by the star before it explodes.
A new era of Cas A research
Cass A is one of the best-studied supernova remnants in all of space. For many years, ground-based and space-based observatories, including the Hubble Space Telescope, have been collecting a multi-wavelength picture of the object’s torn-up remnants.
However, astronomers have now entered a new era in the study of Cas A. In April 2023, the Webb Infrared Instrument (MIRI) kicked off this story by revealing new and unexpected features in the inner shell of the supernova remnant. But many of these features are invisible in the new NIRCam image, and astronomers are investigating why this is.
Infrared light is invisible to our eyes, so image processors and scientists represent these wavelengths of light with visible colors. In this last image of Cas A, colors have been assigned to different NIRCam filters, and each of these colors hints at different activity occurring in the object.
At first glance, a NIRCam image may appear to be less colorful than a MIRI image. However, this does not mean that it contains less information: it simply boils down to the wavelengths at which the object’s material emits light.
Compare images
The most prominent colors in Webb’s latest image are the clumps of bright orange and light pink that make up the inner shell of the supernova remnant. Webb’s razor-sharp eyes can see the tiniest nodules of gas composed of sulfur, oxygen, argon, and neon from the star itself. This gas contains a mixture of dust and molecules that later form new stars and planetary systems. Some of the debris filaments are too small to be distinguished even with Webb, which means that they are no larger than 16 billion kilometers (about 100 AU). For comparison, the entirety of Cas A covers 10 light-years, or about 96 trillion kilometers.
Comparing Webb’s new near-infrared image of Cass A with the mid-infrared image, we can see that its inner cavity and outer shell are strangely devoid of color. The edges of the main inner shell, which looked dark orange and red in the MIRI image, now look like smoke from a campfire. This marks the place where the supernova’s blast wave crashes into the surrounding circumstellar material. The dust in the circumstellar material is too cold to be detected directly in the near-infrared, but it glows in the mid-infrared.
Radiation spectra
The researchers concluded that the white color is light from synchrotron radiation, which is emitted across the entire electromagnetic spectrum, including the near infrared region. It is created by charged particles moving at extremely high speeds and spiraling around the magnetic field lines. Synchrotron radiation is also visible in the bubble shells in the lower half of the inner cavity.
Also invisible in the near-infrared was a loop of green light in the central cavity of Cas A that glowed in the mid-infrared, which the research team called the “Green Monster.” The researchers called this feature “difficult to understand” when they first studied it.
Although the “green” color of the Green Monster is not visible in the NIRCam image, what remains in the near-infrared in this area may provide insight into the mysterious feature. The round holes visible in the MIRI image are faintly illuminated by white and purple radiation in the NIRCam image, indicating ionized gas. The researchers believe that this is due to the pushing through of supernova debris and the formation of gas left over from the star’s explosion.
Credit:
NASA, ESA, CSA, STScI, D. Milisavljevic (Purdue University), T. Temim (Princeton University), I. De Looze (University of Gent)
Echoes of Cassiopeia
The researchers were also absolutely stunned by one fascinating feature in the lower right corner of the NIRCam field of view. They call this large, streaky blob Baby Cas A – because it looks like the spawn of a main supernova.
This is a light echo. The light from the star’s ancient explosion reached and heated the distant dust, which glows as it cools. The complexity of the dust’s structure and the apparent proximity of Baby Cas A to Cas A itself are particularly intriguing to researchers. In fact, Baby Cas A is about 170 light-years behind the supernova remnant.
There are also several other, smaller light echoes scattered throughout Webb’s new portrait.
The Cas A supernova remnant is located 11,000 light-years away in the constellation Cassiopeia. From our perspective, it exploded about 340 years ago.
