The remnant of a supernova exploded 11,000 light-years away. The light from the exploding star is thought to have reached Earth around 1670. The optical light didn't reach Earth, so there are no records of it.
One of the most studied objects in deep space is the Cass A nebula, which ripples with energy and light from the ancient explosion. When its progenitor star exploded, it blasted an expanding gas shell into space.
Astronomers think they know why Cass isn't expanding evenly.
A supernova explosion is one of the most significant events in the universe. A gas shell is blasted into space by the explosion. The material can be sent into space at a rate of ten percent of the speed of light. The explosion sweeps up more material when it slams into the ISM. The center of the expanding cloud contains a neutron star that was left behind by the supernova explosion. The cloud of gas is superheated to 30 million degrees Celsius and still glowing with the energy of the explosion.
A new study shows that the inner nebula is not expanding evenly. Jacco Vink is an associate professor at the University of Amsterdam.
The drama began long before the explosion. Massive stars run out of hydrogen as they age. This causes them to pulse several times as they strain to reach equilibrium. Matter is sent out into space.
There are rings in the Cass A nebula. The outer ring of the nebula contains some of the material that was cast off from the star. There are both forward and reverse shock waves. The reverse shock waves come from the explosion slamming into the ISM and bouncing towards the center.
Two jet-like structures and a lump of denser material called high-density knots are located opposite each other. The authors of the study describe ring-like features aspeculiar morphological features, which are bubbles of radioactive material pushing aside other material as they expand.
A quick look at the multi-wavelength image of the nebula makes it clear that there is a lot going on.
![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 there's a lot going on in this complex object. Image Credit: NASA/JPL-Caltech [via Wikimedia]](https://www.universetoday.com/wp-content/uploads/2022/04/Cassiopeia_A-credit-NASA-JPL-Caltech-1024x779.jpg)
Chandra X-ray observations show that the western side of Cass A isn't expanding. The regions are moving inward. The outer shock wave is more rapid in the west than in the east.
The paper says that the reverse shock is moving in the eastern part of the SNR. There is a negative expansion rate for the range of 260 to 300. Astronomers call the right side of the image the west.
This is not normal for supernova remnants. Astronomers have observed and modelled SNRs extensively. The authors write that 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. There are two explanations for the reverse shock's inward motion.
The lead author said that the backward movement in the west can mean two things. The nebula has collided with something.
The models showed that a collision was most likely. Vink said that the models showed that the shock wave would decrease in speed after the collision.
A motion toward the interior can be seen in the dashed circle. Vink et al.
The early evolution of supernova remnants is determined by the density structure of the progenitor's circumstellar medium.
That seems to be made clear by the morphology of the man. The reverse shock wave appears to have collided with a dense region of the SNR's ejecta, bouncing back toward the middle and giving the person a weird appearance.
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