There’s an odd quirk to a tremendous shock wave going through a gas cloud left behind after a star’s catastrophic death: recent research shows that part of it is flowing in the wrong way.
Study authors name this “reverse shock” because one component of the shock wave is falling back toward the supernova’s core, accelerating at a different pace than the rest of the shock wave.
In the Cassiopeia constellation, the nebula Cassiopeia A, or gas cloud, left by a supernova, is about 11,000 light-years from Earth. The 16-light-year-wide nebula is composed of gas (mostly hydrogen) ejected before as well as throughout the explosion that shattered the parent star. Still, a shock wave has been generated, and theoretical models predict that this shock wave will continue to grow equally, like a flawlessly round balloon being continuously filled.
NASA’s Chandra X-ray Observatory, an orbiting observatory, provided the X-ray photos used in the current study, which tracked the shock wave’s path across space. An analysis of 19 years of data revealed that a portion of the shockwave’s western area was really retreating the other way, creating what is known as a reverse shock.
Reverse shock wave
Even more shocking was the discovery that parts of the supernova’s shock wave were still rushing away from the supernova’s center.
In Cassiopeia A, the expanding gas is moving at an average speed of around 13.4 million miles per hour (21.6 million kilometers per hour), according to Vink. Because Cassiopeia A’s light just reached Earth in 1970, this is mostly due to its youth. Shock waves, on the other hand, lose energy as they interact with their environment and eventually slow down.
Cassiopeia A is composed of an inner and an outer shell, both of which are expanding bands of gas. Across the majority of the nebula, the inner and outer shells are moving at the same speed and in the same direction since they are two half of the same shock wave. A strange thing is happening out west: while the outer shell continues to grow outward, the interior shell is retreating toward the location of the exploding star.
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