We Still Don't Know How Stars Explode

We Still Don't Know How Stars Explode
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Stars explode. This we know. But it may surprise you to learn that we still don't exactly know how.

Stellar explosions -- supernovae -- are well known to scientists who gaze out into the cosmos. It is the destiny of stars that are roughly eight to twenty-five times the mass of our sun to go out in a fiery, yet silent bang: a type II supernova. When such stars use up the fuel that drives stellar fusion -- primarily hydrogen and helium -- the core progressively fuses into heavier and heavier elements until it turns into iron. At this point, it ceases to emit radiation and no longer generates outward pressure. As a result, the star begins to collapse under its crushing gravity, forming a compact neutron star just fourteen miles in diameter. But suddenly, in the midst of this collapse, powerful shocks begin and resonate throughout the receding outer layers, until finally, in a luminous explosion, those layers are blasted away, spewing out elemental remnants that fertilize the surrounding interstellar medium. In this enriched bath of starstuff, planets may form which could one day sprout life.

The prior description forms a satisfying and seemingly complete tale of life, death, and rebirth, but there's actually a crucial detail that for years has remained an incredible mystery: Why would a star that's imploding under the force of gravity suddenly and rapidly reverse course and instead spectacularly explode outward? Intuition suggests that the collapse would continue until the star fizzles out, and this is exactly what happens when massive computers are programmed to simulate supernova!

“In a huge failure of theory, most computers can’t actually make a star explode," Dr. Fiona Harrison, a Professor of Physics and Astronomy at the California Institute of Technology, revealed in a NASA presentation at the Smithsonian National Air and Space Museum held September 30th in Washington, D.C. "The explosion halts until an ad-hoc mechanism – the sloshing around of the central part of the star – is put in by theorists to make the star fly apart."

But is this computational sleight of hand what actually happens? Another idea is that the fading stellar core starts rapidly rotating, launching streams of gas that spur the stellar blast.

As you can imagine, it has been very difficult for scientists to determine the exact explanation. Not only are supernovae exceedingly rare on human timescales, but their immense brightness -- occasionally equivalent to the light output of an entire galaxy -- makes it incredibly tough for astronomers to peer into heart of the explosion and observe what's really going on.

However, with the launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) in 2012, the tides of discovery sharply turned. The telescope (seen above), the first capable of focusing light in the high energy X-ray region of the electromagnetic spectrum, turned its gaze toward a supernova remnant called Cassiopeia A, whose light started reaching Earth a mere 300 years ago. Back on Earth, astronomers were able to pierce the dense outer layers of the stellar corpse and stare straight into the heart of the supernova remnants. And what did they see?

“The shape of the explosion was bubbly, like what you would expect if that sloshing mechanism theorists predicted really happens in life,“ Harrison said.

The observation offers strong support to prevailing theory, but as it is just a single data point, more evidence is needed before we can once and for all lay to rest the tantalizing mystery of how stars explode.

(Images: NASA/JPL-Caltech)

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