Image courtesy Thomas Janka
Published April 30, 2010
The mysterious first moments of a supernova have now been modeled in 3-D—showing what happens in a dying star's heart from half a second to about two hours after the blast begins.
The development could help scientists eventually "rewind" the leftovers of real cataclysmic star explosions to find out how they get started and why their leftovers assume a variety of shapes. (See supernova pictures.)
"These are the first three-dimensional models linking the beginning of the explosion to the supernova structure we see hours later," said study co-author Hans-Thomas Janka, an astrophysicist at the Max Planck Institute for Astronomy in Garching, Germany.
The new simulations also give the best peek yet at the prime suspect in the mystery of what kills big stars from the inside out: a torrential spasm of ghostly subatomic particles called neutrinos.
Supernova's Hidden Heart
Stars fuse hydrogen into helium deep down at their hearts. Very massive stars have enough oomph to continue fusing helium into progressively heavier elements, a process that results in onionlike layers, from lighter hydrogen at the surface to dense iron at the core.
When these stars run out of fuel, their cores collapse, and they explode in what are known as Type II supernovae. The blasts are so powerful that the supernovae outshine their entire host galaxies for a few days.
But the physics of supernova triggers are poorly understood, because we can't see that deeply into a star.
"The first time you can see the explosion is when it reaches the surface of the star," said Armin Rest, an astrophysicist at Harvard University who was not involved in the work. (See: "Supernova Caught Starting to Explode for First Time.")
Our sun won't go supernova—that requires a star about nine times heftier. But if it did, Rest said, "we wouldn't know for the first hour," because hot plasma around the star's core would absorb evidence of the turmoil below.
The new 3-D model, to be described in the May 10 issue of The Astrophysical Journal, is based on theoretical physics derived from observations of real supernovae. According to Rest, the simulation offers unprecedented detail about what's happening in a dying star's interior.
"I think this work is really exciting," Rest said, adding that Janka and his team's model "is at the forefront" of supernova simulations.
Dying Star's Inner Turmoil
With a little more computer crunching, Janka and others in the field hope to use the new model like a forensic investigation tool, examining the shape of debris left after a real star explosion to trace its exact trigger.
As seen from Earth, the remnants of Type II supernovae range from lopsided "puffballs" like Cassiopeia A to spindly structures like the Crab Nebula. The remnants often feature "bullets" of nickel, iron, and other heavy elements that must have punched through the stars' outer layers during the explosion. (See pictures of supernova remnants.)
The most widely accepted explanation for what sets off a supernova—and leads to asymmetric remnants—is a storm of neutrinos in the dying star's core, said Stan Woosley, an astrophysicist at University of California, Santa Cruz, who also wasn't part of the new work.
Stars keep their spherical shapes because the pull of gravity holding their matter together is counterbalanced by an outward push coming from the energy released as elements go through fusion inside.
At the start of a Type II supernova, the theory goes, a star's iron core gets too massive for its own good, and it gives in to gravity's pull.
The iron core instantly squashes itself into a superdense wad, smashing together protons and electrons to form neutrons—spewing out a horde of neutrinos. (Related: "Particles Larger Than Galaxies Fill the Universe?")
The layer of matter directly above the core falls suddenly into the core, triggering a cascade in which the star's layers gradually fall inward. Infalling matter smacks into the dense core and rebounds, sending a rush of neutrinos to collide with the falling layers above.
These hard-to-detect particles are thought to "boil" any matter in their way, mixing the matter into an uneven brew. The resulting chaos shoots "bullets" of heavy materials out through the collapsing layers.
Ultimately, the shockwave from the core's collapse causes the mixed-up star to explode, leading to oddball remnants. (Related pictures: "'Fossil' Fireballs Found in Supernova Debris.")
Supercomputers to Solve Supernova Mysteries
Previous 2-D models were able to show neutrinos triggering supernovae, but the 3-D simulation literally adds a whole new dimension—making for the most realistic-looking supernova simulations to date.
Still, the physics aren't yet detailed enough to be completely realistic, Woosley said.
"This is where the big money is," he said of the computing costs, noting that crunching supernovae models is one of the biggest uses of supercomputer time in the world.
If supernovae theorists like Janka could make a computing wish, what would they ask for?
"Three or four months of nonstop computing time on about 50,000 processors," Janka said—equal to the power of 25,000 of the best dual-core desktop computers. NASA's Pleiades 973-teraflop supercomputer might meet such demands, but it's shared by 1,500 scientists.
"We're getting there," UCSC's Woosley added. "In a few years … I think we'll have this licked."
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