One of Our Galaxy's Most Famous Explosions May Have Collided With Something

A new analysis of one of the most famous explosions in the world has shown a strange discrepancy.

Astronomers have found that the part of the inner nebula that is not a supernova remnant is expanding evenly.

Something has caused a section of the cloud to move back towards the source of the explosion: a reverse shock.

The backward movement in the west can mean two things, according to astronomer Jacco Vink of the University of Amsterdam.

Either there is a hole somewhere, a kind of vacuum in the supernova material, causing the hot shell to suddenly move inwards. The nebula has collided with something.

One of the most famous and well-studied objects in the Milky Way is Cassiopeia A, located 11,000 light-years away. It is called a supernova remnant because it is the expanding cloud of ejecta left over after a massive star has gone kaboom.

Astronomers have been studying the remnant of the supernova since it was first observed in the 1670s. It is an excellent sample for studying the evolution of supernovae.

A multi-wavelength image of a plant.

The star became unstable and ejected a large spherical shell of expanding material, which emits light in multiple wavelengths.

The material is expanding between 4,000 and 6,000 kilometers per second.

Vink and his colleagues studied 19 years of data from the Chandra X-ray Observatory to understand how the remnant has changed over time.

A section on the west side of the shell is bouncing back towards the center at speeds of up to 8,000 kilometers per second.

The outer shock wave of the shell is speeding up. According to computer models of an expanding shock wave, a collision with something will cause the shock front to decelerate and then accelerate.

The map shows the measured expansion. J.Vink is the astronomer.

What could have caused the shock wave to hit?

We know from other supernova remnants that material in the space around the star can create reverse shocks; denser regions of interstellar gas and dust, for example, or even a previous, more slowly moving shell of material ejected by the star in its dying throes.

A dense region of material emitted by the dying star could have created a partial shell for the remnant to slam into.

It could have been the result of a brief Wolf-Rayet phase of mass loss experienced by enormous stars that created a space around the star.

We don't know a lot about the progenitor star. We don't know how big it was, how old it was, or what type it was. Some clues could be provided by these results.

The shock dynamics reported here provide important hints on the late mass-loss history of the progenitor, be it in the form of a partial, asymmetric shell from a brief Wolf-Rayet phase wind, or even a combination.

We can expect more mysteries to be revealed in the years to come, with new instruments turning their gaze to the object.

The research is available on arXiv.