Supernova secrets seen in X-rays

Jessica Crump | 2/19/2014, 12:30 p.m.
Cassiopeia A was a star more than eight times the mass of our sun before it exploded in the cataclysmic, ...
This is the first map of radioactivity in a supernova remnant, the blown-out bits and pieces of a massive star that exploded. The blue color shows radioactive material mapped in high-energy X-rays using NuSTAR. Heated, non-radioactive elements previously imaged by Chandra using low-energy X-rays are shown in red, yellow and green.

By Elizabeth Landau


Cassiopeia A was a star more than eight times the mass of our sun before it exploded in the cataclysmic, fiery death astronomers call a supernova.

And thanks to NASA space telescopes, scientists are learning more than ever about exactly how it happened.

The NuSTAR space telescope array is the first to map the radioactive material from a supernova explosion. The results were published Wednesday in the journal Nature.

NuSTAR's observations of Cassiopeia A showed scientists the location and distribution of radioactive titanium-44, an unstable isotope with a half-life of about 60 years. The supernova explosion's light arrived on Earth about 350 years ago, but even today there's still plenty of titanium-44 to be observed.

Each atom of titanium-44 decays to calcium, as well as two particles of light at particular frequencies that NuSTAR can detect. NuSTAR, which launched in June 2012, consists of an instrument with two telescopes that focus high energy X-ray light.

There's currently no solid model for how the supernova explosion process actually works, said Brian Grefenstette, lead author and research scientist at California Institute of Technology. Scientists would like to know more, especially because the components of our planet came from a supernova that blew up about 5 billion years ago.

"People should care about supernova explosions because that's where all the stuff that makes us comes from," Grefenstette said. "All of the iron in your blood and calcium in your bones and teeth, and gold in your wedding band, that all comes from the center of a supernova explosion."

The map of the radioactive ash that NuSTAR studied is akin to a "fossil record," Grefenstette said. It is an imprint of what happened in the explosion, and it is helping astronomers rule out previous ideas about how stars explode.

So here's what scientists think is happening: At the center of the supernova, an intense amount of pressure builds up. Neutrinos, tiny particles, are produced and heat up the gas in the center.

"What you get is just like when you're boiling water on your stove top: You get hot bubbles at the bottom that try to rise up through the cold material above it, and the whole thing starts to slosh around," Grefenstette said.

Big "bubbles" form in this process, and the whole thing starts to fall apart, he said. "It lets the shock wave out, and that's how you get the star to explode," he said.

The NuSTAR observations suggest that these large bubbles are present at the center of the star, a phenomenon that had been thought about through computer models but never observed.

The new study finds evidence for the bubbles forming in the first fraction of a second of the explosion.

"It's like you blink your eyes twice, and the whole thing has exploded, and we're seeing it three or four hundred years later, preserved in the radioactive ash," Grefenstette said.

Researchers describe this explosion process as "asymmetrical" because according to their modeling, temperatures and densities must be different around the explosion in order for the "bubbles" to escape and let the shock wave out.