For those who really like blowing things up, an alternative career path to Mythbusters host or a minerals geologist has opened up. Astrophysicists have replicated the supernova explosion that caused Cassiopeia A, a project so challenging it required contributions from 12 institutions to make it happen.
“The laws of physics are the same everywhere, and physical processes can be scaled from one to the other in the same way that waves in a bucket are comparable to waves in the ocean,” says Oxford University's Professor Gianluca Gregori, one of the authors of the report published in Nature Physics. “So our experiments can complement observations of events such as the Cassiopeia A supernova explosion.”
While having “replicated a supernova” on your CV has its attractions, the motivation for the study was the anomaly of the magnetic fields around Cassiopeia A, and the behavior of the material ejected in the explosion. As the paper notes, “Observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium.”
Light from the supernova that caused Cassiopeia A should have reached Earth 300 years ago, which would have made it the only supernova to be seen in the Milky Way since the invention of the telescope. However, there are no records of observations at the time. Astronomers have been making up for lost time, with the expanding nebula having attracted much attention since. TAs the brightest radio object outside the solar system, the supernova remnant attracts plenty of attention.
The explosion threw off a shell of gasses expanding at a speed of 4000-6000km/s with a temperature of 30 million °C but these are not spreading out evenly. Instead twisting shapes are produced. It is thought that the progenitor star for the explosion was so large that it threw off clouds of materials prior to the main explosion, just as giant star Eta Carinae has done more recently. The faster moving material from the explosion itself is running into the uneven circumstellar cloud, creating turbulence as it encounters denser clumps. Some other supernova remnants look similar, while others are much more even, presumably because they did not throw off the pre-explosion clouds.
Keen to test the theory Gregori and his co-authors fired three laser beams 60 trillion times the power of a laser pointer at a thin carbon rod in a chamber of gas replicating the possible surroundings into which the supernova exploded. They heated the rod to millions of degrees, causing it to explode.
“The experiment demonstrated that as the blast of the explosion passes through the grid it becomes irregular and turbulent just like the images from Cassiopeia,” said Gregori. “We found that the magnetic field is higher with the grid than without it.”
“The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena,” the authors note. This could have wider significance. The magnetic field in interstellar space is much stronger than theoretical models suggest it should be. The amplification of the field in this case may provide insight into how this could be occurring throughout the galaxy.
An artist's impression of the Cassiopeia A explosion