Cassiopeia A. Image: Hubble Site
How exactly can one study a cosmic event as enormous as a supernova? Why, by using super powerful lasers to recreate a star's death in a lab, of course.
It may sound incredible, but an international team of scientists has found a way to study high energy supernova explosions inside a lab using a laser array. But the lasers aren’t exactly ordinary lasers; they’re 60,000 billion times more powerful than the average laser pointer.
In nature, supernova explosions are triggered when a star’s fuel reignites or its core collapses. They are the largest explosions yet discovered in nature, and they send shockwaves rippling light years through the universe. The phenomena is more or less the same every time, but the effects differ. Sometimes the explosion is regular and uniform; the energy expands evenly. Other times, like in the case of the Cassiopeia A supernova, the explosion emits energy in strange twisting shapes.
The Cassiopeia A supernova, which occurred about 11,000 light years away in the constellation Cassiopeia, was first seen by astronomers about 300 years ago. Optical images of the event reveal knotty-looking features along with intense radio and X-ray emissions. It's unclear why the Cassiopeia A supernova looks as gnarled as it does, although one possibility is that this is just what happens when a supernova blast passes through dense clumps or clouds of gas in interstellar space.
Because modeling a supernova in the field is impossible (for now!), researchers set out to understand Cassiopeia A's shape by recreating the events in a laboratory.
Recreating supernova explosions in a laboratory is actually less crazy than it sounds. “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,” said Professor Gianluca Gregori, of Oxford University's Department of Physics, who led the study published in Nature Physics. “So our experiments can complement observations of events such as the Cassiopeia A supernova explosion.”
The Oxford University-based team used the Vulcan laser facility at the UK's Science and Technology Facilities Council's Rutherford Appleton Lab. "Our team began by focusing three laser beams onto a carbon rod target, not much thicker than a strand of hair, in a low density gas-filled chamber," said Jena Meinecke, an Oxford graduate student who led the experimental team.
This series of figures from the paper diagrams how the experiment worked (A and C), as well as a Schlieren image of shock wave propagation without (B) and with (D) the plastic grid.
These three laser beams generated a lot of heat—more than a few million degrees Celsius—causing the rod to explode in a blast that expanded through the test environment. A plastic grid designed to disturb the shock front acted as a proxy for the dense dust and gas that surrounds an exploding star in nature.
“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.”
A higher magnetic field implies a more efficient generation of radio and X-ray photons, so that the magnetic field in this experiment is high is consistent with observations and numerical models of a supernova shockwave passing through a 'clumpy' interstellar material.
"Magnetic fields are ubiquitous in the universe," said Don Lamb of the University of Chicago. "We're pretty sure that the fields didn't exist at the beginning, at the Big Bang. So there's this fundamental question: how did magnetic fields arise?"
Lamb's is a really core question, and it highlights the significance of the Oxford team’s experiment. This results of this experiment are helping astronomers figure out how magnetic fields first developed in our universe. This is the first experimental proof that turbulence amplifies magnetic fields in interstellar plasma.
Going forward, this result will continue to be a key piece of the puzzle. As Petros Tzeferacos of the University of Chicago, a study co-author, said, these results will give scientists a benchmark against which future observations can be measured. Slowly but surely, scientists are unraveling the mysteries of the universe around us.