A new analysis of one of the most famous explosions in the cosmos has revealed a remarkable asymmetry.
Part of the inner nebula of the Cassiopeia is not a supernova remnant, astronomers have found, expanding evenly.
Something caused a portion of the cloud to move, not with the rest of the material outward, but inward, back to the source of the explosion: an inverted shock.
“The backward movement in the west can mean two things,” says astronomer Jacco Vink of the University of Amsterdam in the Netherlands.
“Either there’s a hole, a kind of vacuum, somewhere in the supernova material, which causes the hot shell to suddenly move inward locally. Or the nebula collided with something.”
Cassiopeia A, located 11,000 light-years away, is one of the most famous and well-studied objects in the Milky Way. This is what we call a supernova remnant – the sweltering cloud of feces left over after a massive star kaboom has gone.
The Cassiopeia A supernova is thought to have been first observed in the 1670s, illuminating the sky, and astronomers have been studying the remnant ever since. This is an excellent example to study the evolution of supernovae.
Cassiopeia A emits light in multiple wavelengths and consists of a large, more or less spherical shell of expanding material, which was probably ejected in front of the supernova as the star became increasingly unstable.
This material expands at an average rate of somewhere between 4,000 and 6,000 kilometers (2,485 and 3,730 miles) per second.
In their new study, Vink and his colleagues studied 19 years of X-ray data from the Chandra X-ray Observatory to compile how the residue changed over time.
They found that a portion on the west side of the inner region of the shell bounced back inward toward the center, at speeds between 3,000 and 8,000 kilometers per second.
They also found that the outer shock wave of the same part of the shell accelerated. According to computer models of an expanding shock wave, a collision with something will initially slow down the shock front, and then accelerate: “Exactly as we measured,” Vink explains.
So with what could the shock wave have collided?
We know from other supernova remnants that material in the space around the star can create inverted shocks; denser regions of interstellar gas and dust, for example, or even a previous, slower moving shell of material ejected by the star in its dying crescents.
In the case of Cassiopeia A, a dense area of material released by the dying star could have formed a partial shell for the remnant to collapse into as it expands outward.
It could also have been the result of a brief Wolf-Rayet phase of extreme mass loss experienced by truly enormous stars that created a cavity in the space around the star.
We actually do not know much about the tribal star that created the Cassiopeia A supernova remnant. We do not know how big it was, how old it was or what its spectral type was. These results, the researchers say, could provide some clues.
“The shock dynamics reported here provide important clues about the late ancestor’s late mass loss history, whether in the form of a partial, asymmetric shell of episodic mass loss, an aspherical cavity created by a short Wolf-Rayet phase wind, or maybe even a combination of both, “they write in their newspaper.
It is surprising that new details are still being uncovered in an object as well studied as Cassiopeia A. With new instruments directing their gaze to the object, we can only expect that more mysteries will be revealed in the coming years.
The research was accepted in The Astrophysical Journaland is available on arXiv.