Supernova remnant Cassiopeia A isn't expanding evenly. Its reverse shock wave has bounced off of something and is travelling inward.
A supernova explosion is one of the most cataclysmic events in the Universe. The explosion releases an enormous amount of energy, blasting a gas shell into space at high velocities. Supernovae can send several solar masses of material into space at velocities reaching ten percent of the speed of light. The explosion sends an expanding shock wave into space, where it slams into the interstellar medium and sweeps up more material.
The Cass A nebula has discrete expanding rings. Some of the material in the nebula was cast off from the star in the pulses preceding the supernova explosion, and that material is contained in the outer ring. Cass A also has both forward and reverse shock waves. The reverse shock waves are matter from the explosion slamming into the ISM and bouncing inwards towards Cass A’s center.
This coloured image of Cassiopeia A based on data from the space telescopes Hubble, Spitzer and Chandra. You don’t need to be an astronomer to see that a lot is going on in this complex object. Image Credit: NASA/JPL-Caltech [via Wikimedia]observations of Cass A. Those observations show that the western side of Cass A’s inner regions isn’t expanding. Instead, those regions are moving inward. Observations also show that the outer shock wave accelerates in the west rather than decelerates.
This is very unusual for supernova remnants. Astronomers have observed and modelled SNRs extensively. “The inward motion of the reverse shock in the western part does not agree with the evolutionary model for an SNR expanding into a dense wind at less than 1000 yr,” the authors write. The authors say there are a couple of explanations for the inward motion of the reverse shock.
These two images from the study help explain the results. The image on the left shows the annuli that were used for selecting the forward and reverse shock regions . The small green circles with a red line indicate regions excluded from the analysis due to ambiguity. The image on the right is the same as the left, but with a spider diagram overlayed that visualizes the expansion rate as a function of position angle.
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