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

 

Multi-wavelength image of Cassiopeia A. (NASA/JPL-Caltech)

A new study of one of the universe's most renowned explosions has found a strange asymmetry.

The Cassiopeia nebula's inner nebula Astronomers have discovered that a supernova remnant does not expand evenly.

A reverse shock has forced a piece of the cloud to move inwards, back towards the source of the explosion, rather than outwards with the rest of the material.

According to astronomer Jacco Vink of the University of Amsterdam in the Netherlands, "the backward movement in the west can signify two things."

"Either there is a hole in the supernova material, a form of vacuum, forcing the hot shell to suddenly slide inwards locally. Alternatively, the nebula may have collided with something."

Cassiopeia A, which is 11,000 light-years away, is one of the Milky Way's most famous and well-studied objects. It's a supernova remnant, which is an expanding cloud of ejecta left over after a huge star explodes.

The Cassiopeia A supernova was initially spotted in the 1670s, brightening the sky, and astronomers have been studying the remnant since since. It's a fantastic sample for understanding supernova evolution.

Cassiopeia A produces light in a variety of wavelengths and is made up of a massive, approximately spherical shell of expanding material that was ejected before to the supernova as the star became more unstable.

This material is expanding at a rate of between 4,000 and 6,000 kilometres per second (2,485 and 3,730 miles).

Vink and his colleagues used 19 years of X-ray data from the Chandra X-ray Observatory in their new study to put together how the relic has changed over time.

They discovered that a part of the inner shell's west side is bouncing back inwards towards the core at speeds ranging from 3,000 to 8,000 kilometres per second.

They also discovered that the same part of shell's outer shock wave is speeding up. A collision with something causes the shock front to decelerate at first, then accelerate, according to computer models of expanding shock waves: "Exactly as we have measured," Vink explains.

A map of Cassiopeia A showing its measured expansion. (J.Vink/astronomie.nl)

So, what may have collided with the shock wave?

Material in the space around the star, such as thicker regions of interstellar gas and dust, or even a prior, slower-moving shell of material ejected by the star in its death throes, has been known to cause reverse shocks in other supernova remnants.

A dense region of material ejected by Cassiopeia A could have created a partial shell for the remnant to slam into as it expands outwards.

It could potentially have been caused by a brief Wolf-Rayet phase of high mass loss experienced by truly massive stars, which resulted in a cavity in the space around the star.

We don't know anything about the progenitor star that triggered the supernova remnant Cassiopeia A. We have no idea how huge it was, how ancient it was, or what spectral class it belonged to. According to the researchers, these findings may provide some clues.

"The shock dynamics reported here provide important hints on the progenitor's late mass-loss history," they write in their paper, "whether in the form of a partial, asymmetric shell from episodic mass loss, an aspherical cavity created by a brief Wolf-Rayet phase wind, or perhaps even a combination of both."

It's incredible that even after all these years, fresh facts about Cassiopeia A are still being discovered. We can only expect more mysteries to be uncovered in the years ahead as new instruments shift their sight to the object.

References:

• High Energy Astrophysical Phenomena https://doi.org/10.48550/arXiv.2201.08911