The curious structure was first identified in Webb’s infrared data from April 2023. The origin of this feature, dubbed the “Green Monster” because of its resemblance to the wall in the left field of Fenway Park, was not clear.
However, by combining the Webb data with X-rays from Chandra, researchers think they have hunted down the source of the Green Monster.
While the astronomers already suspected the Green Monster was created by a blast wave from the exploded star slamming into material surrounding it, the Chandra data helped clinch the case. They showed that the properties of the X-rays from the Green Monster are similar to the X-rays from the blast wave rather than from the supernova debris.
When a massive star exploded to create Cas A about 340 years ago, from Earth’s perspective, it created a ball of matter and light that expanded outward. In the outer parts of Cas A the blast wave is striking surrounding gas that was ejected by the star between about 10,000 and 100,000 years before the explosion.
Chandra detects debris from the star because it is heated to tens of millions of degrees by shock waves, akin to sonic booms from a supersonic plane. Webb can see some material that has not been affected by shock waves, what can be called “pristine” debris. Much of this lies behind the Green Monster. This means the combination of Webb and Chandra data gives a fuller census of debris from the exploded star.
Astronomers will continue to use every tool available — including Chandra and Webb — to study this object that has fascinated them for years.
Revised: January 18, 2024