In April 2023, infrared data from the NASA James Webb space telescope detected an unusual structure. It was dubbed the "Green Monster" because of its resemblance to the wall in left field at Fenway Park. However, researchers think they have found the source of the Green Monster by combining Webb data with X-rays from NASA Chandra Xray.

When a massive star exploded about 340 years ago, creating Cassiopeia A (also known as Cas A), it created a ball of matter and light that expands outwards as seen from Earth. The debris from the star is seen by Chandra because it is heated to tens of millions of degrees by shock waves similar to sonic booms from a supersonic aircraft. Webb can see some material unaffected by the shock waves, which can be called "pristine" debris. Most of this lies behind the Green Monster. The combination of Webb and Chandra data therefore gives a better idea of the total debris of the exploding star.

Image description:

This image of Cassiopeia A resembles a disk of electric light with red clouds, glowing white streaks, red and orange flames, and a circular region of green lightning near the centre of the remnant. The X-rays from Chandra are blue and mostly reveal hot gas from the supernova debris of the destroyed star, and contain elements such as silicon and iron. X-rays are also present as thin arcs in the outer regions of the remnant.

The infrared data from Webb are red, green and blue. Webb highlights infrared emission from dust that is heated because it is embedded in the hot gas seen by Chandra, and from much cooler supernova debris. The Hubble data show a large number of stars penetrating the field of view.