Supernova has a strange 'reverse shock wave'

A new study reveals that a powerful shock wave traveling through a cloud of gas left behind by a star's exploding death has an oddity: Part of it is heading in the wrong direction.

The researchers found that the shock wave was accelerating at different speeds, with part collapsing back to the origin of the supernova explosion, which the study authors call a "reverse shock" ".

Cassiopeia A is a nebula, or gas cloud, left behind by a supernova in the constellation Cassiopeia, about 11,000 light-years from Earth, making it one of the closest supernova remnants.

The supernova was observed with NASA's Hubble space telescope.

This nebula, about 16 light-years across, is made up of gas (mostly hydrogen) that was ejected both before and during the explosion that tore apart the original star. A shock wave from that explosion is still traveling through the gas, and theoretical models suggest that the shock wave would expand uniformly, like a continuously inflated round balloon.

Lead author Jacco Vink, an astronomer at the University of Amsterdam in the Netherlands, said: 'For a long time we suspected something strange was happening inside Cassiopeia A. Previous research has shown that the motions inside the nebula are "quite chaotic" and highlighted that the region west of the shock wave moving through the gas cloud could even go in the wrong direction'.

In the new study, the researchers analyzed the motion of the shock waves, using X-ray images collected by NASA's Chandra X-ray Observatory, a telescope orbiting the Earth. Data collected over 19 years has confirmed that part of the area west of the shock wave is, in fact, contracting in the opposite direction.

But they discovered something even more surprising: Parts of the same region are still accelerating away from the supernova's epicenter, just like the rest of the shock wave.

Vink says the current average speed of the expanding gas at Cassiopeia A is about 21.6 million km/h, making it one of the fastest shock waves ever seen in supernova remnants. This is mainly because the remnants are so young; light from Cassiopeia A reached Earth in 1970. But over time, the shock wave loses momentum with respect to its surroundings and slows down.

Cassiopeia A consists of two major gaseous extensions: the inner shell and the outer mantle. These two shells are halves of the same shock wave, and on most nebulae, the inner and outer shells are traveling at the same speed and in the same direction. But in the western sector, the two crusts are moving in opposite directions: The outer shell is still extending outward, but the inner shell is moving back to where the exploding star should have been.

What really baffles researchers, however, is how rapidly the outer crust is expanding compared to the retreating inner crust in this region. The researchers expected the outer shell to expand at a reduced rate compared to the rest of the shock wave, but they found that it was actually accelerating faster than some other regions of the shock wave. 'It was a complete surprise,' Vink said.

The unusual expansion in the western region of Cassiopeia A does not match the theoretical supernova models and suggests that something must have happened to the shock wave after the stellar explosion.

The researchers also suggest that the unique way in which the star originally died could account for the uneven shock wave. Cassiopeia A is the result of a Type IIb supernova, in which a massive star exploded after it had almost completely shed its outer layers.

Vink said: 'X-ray estimates suggest the star was four to six times the mass of the sun during the explosion, but it is most likely that the star was about 18 times the mass of the sun when it was born. This means that the star lost about two-thirds of its mass, most of which is hydrogen, before it exploded; the shock wave may have collided with this gas."

At present, no one knows exactly what is driving Cassiopeia A's uneven shock wave.