NASA’s NuSTAR Unravels Mystery of How Stars Explode in Supernova Explosions

A team of international astrophysicists successfully unraveled the biggest mystery in astronomy. A new study explains how stars explode in supernova explosions.

Using NASA's high energy X-ray observatory,  the 'Nuclear Spectroscopic Telescope Array', NuSTAR, the astrophysicists in collaboration with Lawerence Livemore National Laboratory have created the first map of radioactive material in a supernova remnant dubbed Cassiopeia A (Cas A). The results from Cas A show how shock waves tear apart the huge dying stars and ultimately end their cosmic journey.

"Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power," Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology (Caltech) in Pasadena, said in a statement. "Our new results show how the explosion's heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating."  

A dying star explodes into what is called a supernova, which is extremely bright and throws radiation that briefly outshines an entire galaxy before disappearing from view. During the explosion, most of the star's material is expelled at a velocity that is 10 percent the speed of light.  During this a shock wave is passed on to the nearby interstellar medium that grabs the cloud of gas and dust, which is  referred as the supernova remnant.

One such remnant is Cas A that was formed when a massive star blew up into a supernova some 11,000  years ago, leaving behind thick stellar debris and its emitted materials. Due to the massive distance between the supernova and the Earth, the light from the explosion reached the Earth a few hundred years ago. This shows that the remnant was fresh and young when it was studied.

Supernova explosion produces heavy elements like gold, the calcium present in the bones and iron in the blood. Generally, small stars like our Sun, have a less violent death.  But the stars that are eight times the mass of our Sun, have a supernova explosion. It is during this explosion that the high temperatures and particles trigger the fusion of lighter elements into heavier ones.

The space agency NuSTAR is the first telescope that successfully produced maps of radioactive elements present in the remnants of supernova. In the map of Cas A the element identified was titanium-44, which contains an unstable nucleus that is produced at the center of the exploding star.

NuSTAR's map of Cas A reveals clusters of concentrated titanium in the center and offers clue to demise of the star. On simulating the supernova blasts with computers, the researchers noticed that as the massive star dies and collapses the shock wave often freezes and the star does not break apart.

This finding suggests that the exploding star splattered around re-energizing the stalled shock waves and permitting the star to explode its outer layers.

"With NuSTAR we have a new forensic tool to investigate the explosion," said the paper's lead author, Brian Grefenstette of Caltech. "Previously, it was hard to interpret what was going on in Cas A because the material that we could see only glows in X-rays when it's heated up. Now that we can see the radioactive material, which glows in X-rays no matter what, we are getting a more complete picture of what was going on at the core of the explosion."

This new map challenges the other models of supernova explosion that show the star rapidly rotates before dying and emits a narrow stream of gas that drives the stellar blast. The researchers continue to examine Cas's explosion.

The finding was documented in the journal Nature.