It's the remnant of a supernova.
Also not seen in the near-infrared view is the loop of green light in the central cavity of Cas A that glowed in mid-infrared light, appropriately nicknamed the Green Monster by the research team. This feature was described as ‘challenging to understand’ by researchers at the time of their first look.
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. The circular holes visible in the MIRI image are faintly outlined in white and purple emission in the NIRCam image – this represents ionised gas. Researchers believe this is due to the supernova debris pushing through and sculpting gas left behind by the star before it exploded.
Researchers 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. The intricacy of the dust pattern, and Baby Cas A’s apparent proximity to Cas A itself, are particularly intriguing to researchers. In actuality, Baby Cas A is located about 170 light-years behind the supernova remnant.
