Stars are messy, especially in death. While mid-mass stars simply puff away their atmosphere, leaving the material in a gently expanding cloud of gas, the largest stars instead explode as supernovae and spew materials at high velocity in either a sphere or an hourglass shape. Initially, we think, these explosions should be symmetric, but just like with the planetary nebulae, environmental conditions can affect how the resulting nebulae are shaped.

The Cassiopeia A supernova remnant was formed in a stellar explosion vaguely around 1690. There is no definitive record of the event, although a note on August 16, 1680, by John Flamsteed may be of the initial explosion.

But we’re not worried about that exact event. What matters is that the explosion, on cosmological scales, was super recent, and we have been observing it with cameras since cameras started to be used in astronomy. With several decades of detailed images, astronomers have the ability to measure in detail how the system is expanding against the background of stars.

Or not.

It turns out that some regions of the nebula are expanding out but a few are either stopped or actually moving inwards. In a new paper led by Jacco Vink and published in The Astrophysical Journal, researchers map out the distribution motions of knots of gas and determine, and here I quote Vink: The backward movement… can mean two things. Either there is a hole somewhere, a kind of vacuum, in the supernova material, causing the hot shell to suddenly move inwards locally. Or the nebula has collided with something. 

The inward motion best matches models for a collision, which predict the kind of speed changes being seen.

Space is lumpy and bumpy and full of things that are waiting darkly to be collided with. Just as material from a firecracker can bounce off something in its surroundings, supernova remnants, we now know, can also bounce off things in their surroundings.

More Information

NOVA press release

“The forward and reverse shock dynamics of Cassiopeia A,” Jacco Vink, Daniel J. Patnaude, and Daniel Castro, to be published in The Astrophysical Journal (preprint on arxiv.org)