Unexpectedly, some of the material left over from a supernova explosion does not move outwards, but inwards.

Light must have reached Earth in 1672 from a supernova explosion that happened some 11,000 years ago — but based on historical records, no one seems to have seen it at the time. The remnant of that supernova, Cassiopeia A or Cas A, however, has been closely studied by terrestrial astronomers over the past few decades. astronomer Jacco Vink of the University of Amsterdam is now doing a new money in the bag†

From the observations that this trio reviewed, it appears that not all the material that was propelled into the universe during the explosion is still moving outwards. A part goes the other way, so inwards. And that really shouldn’t have happened for a few thousand years.

forward shock wave

A supernova is what you get when a star that is at least ten times as massive as our sun has run out of fuel. It then blows off its outer layers at high speed, while the inner ones collapse into a neutron star or black hole.

The aftermath of such an explosion is quite a complicated process. “The material from the exploded star cools down quickly,” says Vink. “But its outer parts collide with surrounding material, causing a forward impact. That shock heats up the surrounding material and sets it in motion. You get such a hot skin.”

Reverse shock wave

That hot shell then surrounds material from the supernova itself that has already cooled. “Due to the high pressure in the hot shell, a second shock wave runs through the cooled material,” continues Vink. “That is then heated again, and added to the skin.”

To get that second shock wave, the reverse shock or reverse shock wave, here it goes. You might expect it to always be inward – but it isn’t. For the first few millennia after the explosion, this shock wave moves outward. This is because the supernova material inside the shell moves faster than the shell around it, explains Vink. Only after about 2500 years does the reverse shock wave live up to its name, does it really move inwards.

Acceleration instead of deceleration

But not with Cas A. Vink and colleagues analyzed images from the X-ray satellite Chandra, made over a span of nineteen years. They concluded that in the west of the supernova remnant, part of the reverse shock wave is already directed inwards now – 350 years after the explosion.

Meanwhile, the material on the outside of the remnant accelerates along with the forward shock wave. And that’s crazy too, because that stuff should actually slow down at this stage.

Crash

Where does that come from? According to Vink and his team, the most likely option is that the supernova material collided with something on its way out. “Then the forward shock wave slows down and then accelerates again,” says Vink. “Meanwhile, the reverse shock wave is getting stronger and moving inwards. And we both measured that.”

The article about the discovery will be published soon in the scientific journal The Astrophysical Journal† On the site ArXiv.org there is a near-final version of the text can be read (for free).