A powerful shock wave traveling through a cloud of gas left behind by the explosion of a star has a strange quirk.
In the study, researchers found that the shock wave is speeding at different rates, with one section collapsing back toward the origin of the stellar explosion.
One of the closest supernova remnants is the gas cloud called Cassiopeia A, which is 11,000 light-years from Earth. The original star was torn apart by an explosion that ejected gas and hydrogen from it. A shock wave from that explosion is rippling through the gas, and theoretical models show that this shock wave should be expanding evenly, like a perfectly rounded balloon.
The researchers found that this was not the case.
Jacco Vink, an astronomer at the University of Amsterdam in the Netherlands, told Live Science that for a long time, they suspected something weird was going on. He said that previous studies had shown that the internal motions within the nebula were chaotic and that the western region of the shock wave might be going in the wrong direction.
There are great images of star explosions.
The researchers analyzed the movement of the shock wave using images from the Chandra X-ray Observatory. The data collected over 19 years showed that part of the western region of the shock wave was retreating in the opposite direction.
They discovered that parts of the same region were still moving away from the center of the shock wave.
One of the fastest shock waves ever seen in a supernova remnant can be seen in the current average speed of the expanding gas. Light from the remnant reached Earth in 1970. Over time, shock waves lose their strength and speed.
There are two main expanding bands of gas in the form of an inner and an outer shell. The inner and outer shells are traveling in the same direction across most of the nebula. In the western region, the outer shell is still expanding, but the inner shell is moving back to where the exploding star would have been.
The reverse shock is retreating at a slower rate than the rest of the nebula. The researchers were puzzled by how fast the outer shell was expanding compared to the retreating inner shell. The researchers had expected the outer shell to expand at a slower rate than the rest of the shock wave, but they found that it was actually speeding up.
Vink said that the expansion within the western region does not match up with theoretical models and that something happened to the shock wave in the aftermath of the stellar explosion.
The most likely explanation is that the shock wave hit another shell of gas that was ejected from the star before it exploded. The inner shell may have been pushed back toward the center as the shock wave slowed down. The outer shell may have been forced through this obstruction and begun to accelerate again on the other side, Vink said.
The researchers think that the unique way the star died could explain the shock wave. Vink said that the star exploded after it had almost completely shed its outer layers.
X-ray estimates suggest that the star was four to six times the mass of the sun during the explosion, but the star most likely had a mass of around 18 times the sun when it was born. Vink said that the star lost around two-thirds of its mass, most of which would have been hydrogen, before it exploded.
There are many theories as to why the mass of Cassiopeia A was lost before it exploded. A team of researchers proposed in September 2020 that the original star was part of a system of two stars. Live Science previously reported that the companion star could have gone supernova before the star, and blasted off the star's hydrogen skin.
The authors of the new study are not convinced by the theory.
No one knows what is causing the shock wave.
The study was accepted for publication in The Astrophysical Journal after it was published online.
It was originally published on Live Science.
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