Staring into the dramatic corpse of a dead star known as Cassiopeia A, astronomers using NASA's NuSTAR X-ray telescope have for the first time mapped out radioactive titanium in a supernova. Charting this astrophysical frontier, as described in the journal Nature, will help scientists understand what happens when a massive star explodes.
Supernova remnants are the leftover shells of gas and dust forged from within the exploding star. The beautiful video above shows how that explosion develops over roughly 150 milliseconds -- less than two blinks of an eye -- and took millions of computer hours to simulate, scientists said.
Going supernova is a fate reserved for stars at least eight times the mass of our sun. At the end of its life, having exhausted its fuel and left with a dense iron core, the star collapses and sheds its outer layers, leaving behind a dense neutron star in its wake. And the elements that are created in this pressure-cooker environment are what seeded the rest of the universe with heavier elements. Without dead stars, the universe would probably still be made of just hydrogen and helium gas.
"This giant explosion that we’re trying to understand forges and distributes the elements that make up our bodies, the calcium in your bones and teeth, the gold in your wedding ring," said NuSTAR principal investigator and Caltech astrophysicist Fiona Harrison.
So supernova are integral to our own origins – and yet scientists have little idea how they happen. They see the star before it explodes, and they see its remnants afterward. They know the supernovae look uneven and lopsided, even though they came from spherical, symmetrical stars. And thus far, they haven’t been able to agree on computer simulations that explain how this happens.
"We know that stars explode but in general haven’t been able to make them explode in our computers," said lead author and Caltech research scientist Brian Grefenstette.
Part of the problem is that astronomers can’t see the entire supernova remnant; telescopes like the Chandra X-ray Observatory can only see low-energy X-ray light. But NuSTAR (short for Nuclear Spectroscopic Telescope Array) can see extremely high-energy X-rays, even though it’s one-fifteenth the size of Chandra.
So the scientists set out to test two main theories: whether the supernova was caused in part by two narrow jets of material streaming out of either end of a rotating star, or whether it was the result of stuff "sloshing" around inside, leaving behind a lumpy shape.
Chandra had seen hot elements like iron and silicon and magnesium in the supernova cloud, and the shape of some of the material seemed to support the double-jets theory, vaguely following where the beams would be.
But NuSTAR was able to pick up signs of titanium-44, which gives off high-energy X-rays as it decays into calcium – X-ray light that can’t be imaged by Chandra or other such telescopes. And that titanium was lumpy-looking, supporting the "sloshing" theory.
The researchers think that, as the iron core of the dying star collapsed, it gave off neutrinos that heated the matter behind the shock wave, causing bubbles to rise (rather like they would from the bottom of a pot of boiling water, Grefenstette said) and causing material to slosh around. Once those break through the surrounding material, "it’s like blowing the top off a pressure cooker," he said.
"We think that these large bubbles, which were formed in the first fraction of a second as the star collapsed, have been preserved for hundreds of years like a fossil record in the radioactive ash of the explosion," Grefenstette said, referring to the titanium cloud.
But the research raised another question. The scientists aren’t sure why the pattern of iron detected by Chandra and the titanium cloud detected by NuSTAR don’t match up; after all, they should have both been created in the same location inside the star.
Chandra can only detect the iron if it’s hot, so perhaps much of the iron from the supernova remnant is cold and thus hidden from the telescope’s sight. That would be strange, because there’s no clear reason some of the iron should be hot and some of it cold, the scientists said.
It seems that there are far more mysteries still left to investigate about the death throes of these stars, researchers said.
"I think it’s a terrific piece of work … NuSTAR is pioneering science," said Robert Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics who was not involved in the study.