In the remains of this exploded star lurks a strange structure dubbed the "Green Monster". Combining data from the Webb and NASA Chandra Xray telescopes, scientists may have found the source of this structure: a blast wave from the star.

The "Green Monster" (named after its resemblance to the wall in left field at Fenway Park) was first seen by Webb in April 2023 in the central region of supernova remnant Cassiopeia A. Chandra's X-ray data found that the blast wave filaments in the outer part of Cassiopeia A have properties matching those of the "Green Monster". This supports earlier suggestions from the Webb data that the "Green Monster" was formed when the blast wave struck the surrounding material.

Image descriptions:
1. 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 reveal hot gas, mostly 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. 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.

2. Same image, but annotated to show a sketch of the "Green Monster" in Cassiopeia A. The scattered bright green lines in the centre right form a roughly square shape.